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Ngā Whetū Resources




Glossary on Astronomy A - Z

Last updated 17 May 2009. © Nga Whetu Resources

The process in which electromagnetic radiation gives up its energy to an atom or molecule in a substance.

Absorption line spectrum
A spectrum showing narrow dark lines at some specific wavelengths. The lines are formed by atoms absorbing light, which lifts their electrons to a higher energy level. If the electrons go down in energy level they will emit a specific photon energy that produces an emission line spectrum.

The term abundant means present in great quantity or similar meanings dependent on its application. In astronomy the term is used to express large amounts of a certain substance that are present in a certain situation.
As an example, Hydrogen is the most abundant chemical element in the Universe. Hydrogen amounts to about 75% of all ordinary matter in the Universe.

Accumulation of dust and gas onto larger bodies such as stars, planets and moons under influence of gravity.

Accretion disk
A relatively flat sheet of gas and dust surrounding a rotating newborn star, a black hole, or any massive object growing in size by attracting material.

Active Galactic Nuclei (AGN)
A class of galaxies which spew massive amounts of energy from their centres, far more than ordinary galaxies. Many astronomers believe super massive black holes may lie at the centre of these galaxies and power their explosive energy output.

In general the word altitude indicates “height above a certain level”. In astronomy altitude refers to the vertical angle from the horizon to a celestial object. If the object is in the zenith, the altitude is 90 degrees.

Local co-ordinates.
Image www.srrb.noaa.gov

Ammonia (NH3)
a compound occurring in pure form on Earth as a gas with a very strong odour. In contact with acids it can form salt such as ammonium chloride and ammonium nitrate. Ammonia dissolves readily in water, forming ions NH4+ and OH-. Water-ammonia ionic oceans are believed to exist under the cloud cover of Uranus and Neptune

Angstrom (Å)
A unit of length equal to 0.0000000001 metres. This may also be written as 1 x 10-10 m (see scientific notation). It is traditionally used to express wavelength of light, although the SI unit for that is nanometre (nm). 1 Angstrom = 0.1 nm. Formally it should be written with Swedish characters as "Ångström" and pronounced /ˈɔːŋstrəm/.

Angular momentum
In astronomy it is the quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. Thus objects in elliptical orbits travel faster at Perihelion and more slowly at Aphelion. A spinning body also possesses spin angular momentum, that is dependent on the speed of rotation and its mass distribution..

Antarctic literally means “opposite of Arctic” and refers to the region around the Earth’s South Pole. The continent around the South Pole is therefore called Antarctica.

The point in an elliptical orbit where a celestial object is farthest from the Sun. Opposite of Perihelion.

The point in an elliptical orbit where the orbiting object is farthest away from the centre of attraction. Special names are given to this orbital point for commonly used systems. For an object orbiting the Sun see Aphelion, and for an Earth satellite see Apogee.

The point in its orbit where an Earth satellite is farthest from Earth. Opposite of Perigee.

Apollo (missions)
NASA Space program to land humans on the Moon and to collect samples from the Moon surface. 1963 - 1972. Apollo 11 was the first successful landing with astronauts. The Lander "Eagle" touched down in Mare Tranquillitatis on 20 July 1969. Five more manned missions landed on the Moon and returned home safely. More...

Neil Armstrong:
"A small step for man..."
July 1969

In astronomy the term apparent refers to how we see something from Earth.
Example: Apparent magnitude is the brightness as we see a star from Earth expressed on the magnitude scale. This is opposed to absolute magnitude, that would be the brightness as we would see it, when the star was at a standard distance of 10 Parsec.

Arc minute
An angular measurement equal to 1/60th of a degree.

Arc second
An angular measurement equal to 1/60th of an arc minute or 1/3600th of a degree.

Arctic refers to the region around the Earth’s North Pole. The word Arctic coes from the Greek a??t???? (arktikos), "near the Bear”. The name refers to the constellation Ursa Major, the "Great Bear", which is prominent in the northern sky.

Arctic Circle
The Arctic circle is a parallel (circle of equal latitude) on Earth at a latitude of about 66.5 degrees North. This is the difference between 90 and 23.5 degrees, the latter being the obliquity of the Earth’s axis. This means that above the Arctic Circle the Sun is circumpolar around mid-summer and never rises around mid-winter on the Northern Hemisphere. The equivalent parallel on the Southern Hemisphere is called “Antarctic circle”.

Argon (Ar)
a chemical element with atomic number 18, and one of the noble gases. Noble gases are very stable and hard to chemically bond with other elements. There is about 0.93% Argon present in Earth's atmosphere, making it the most abundant noble gas on Earth. The atmosphere on Mars contains 1.6% Argon, and Mercury's very thin atmosphere contains up to 70% Argon, believed to result from releases of the gas as a decay product from radioactive materials on the planet

Aristotle (384 - 322 BC)
Greek philosopher, a student of Plato and teacher of Alexander the Great. Aristotle (together with Plato and Socrates, Plato's teacher) is one of the most important founding figures in Western philosophy and science. He was the first to create a comprehensive system of philosophy, encompassing morality and aesthetics, logic and science, politics and metaphysics.

Astrology is a group of belief systems that relate the movements and relative positions of celestial bodies to events experienced in human life. Before the Renaissance (14th – 17th century), Astrology and Astronomy were one and the same philosophy, with the primary motivation of prediction of events in a divine or practical context.

Astrometry is the branch of astronomy that deals with the determination of the positions and motions of stars and solar system objects and the establishment of celestial reference frames for these positions and motions.

Astronomical Unit (AU)
A unit of distance used to express distances within the Solar system. The official definition is “The distance from the centre of the Sun, at which a massless particle has an orbital period of 365.2568983 days, which is 149,597,870,691 ± 30 metres". Hence it is the average distance of Earth from the Sun. A good approximation is 150 million kilometres or more precise 149,597,871 km.

The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.

The part of astronomy that deals principally with the physics of the universe, including nucleosynthesis, luminosity, density, temperature, and the chemical composition of stars, galaxies, and the interstellar medium.

The gas that surrounds a planet or other celestial object. Planets have very different pressure and composition in their atmosphere. Earth's atmosphere is made up of mostly nitrogen and oxygen.

The smallest component of matter that retains its chemical properties. An atom consists of a nucleus and at least one electron. The nucleus is made up of protons and neutrons. The number of protons in the nucleus (atomic number) uniquely determines the element.

AuroraAurora (plural aurorae)
Light radiated by ions in the Earth's atmosphere, mainly near the geomagnetic poles, stimulated by bombardment with energetic particles ejected from the Sun (see Stellar Wind). The Aurora Borealis is seen in the north on the Northern hemisphere; the Aurora Australis in the south on the Southern hemisphere.

Aurora Australis

Azimuth is a horizontal angle measured from a reference direction. In astronomy azimuth is measured from North towards East (clockwise when looking down) to the point on the horizon, directly below a celestial object. Thus the combination of azimuth and elevation or altitude, uniquely define an objects position in the local sky.

Local co-ordinates.
Image www.srrb.noaa.gov

Balmer lines
Emission or absorption lines in the spectrum of hydrogen that arise from transitions between the second (or first excited) state and higher energy states of the hydrogen atom. They were discovered by Swiss physicist J. J. Balmer (1825 – 1898).

A unit of pressure. 1 bar = 100 kPa (kilopascal). The bar is not an SI unit, but is still frequently used because it is close to average atmospheric pressure on Earth's surface.

Barringer meteor crater
Barringer meteor craterA meteor impact crater, located near Winslow in northern Arizona, United States. It is named as Barringer Crater in honour of Daniel Barringer who was first to suggest that it was produced by a meteorite impact. The crater is about 1,200 m in diameter, and 170 m deep. It is surrounded by a rim that rises 45 m above the surrounding plains. The crater was created about 50,000 years ago by a nickel-iron meteorite about 50 meters across, which impacted the area at a speed of several kilometres per second.

Barringer meteor crater

Any of the subatomic particles which interact via the strong nuclear force. Most commonly, these are protons and neutrons. Their presence in the universe is determined through their gravitational and electromagnetic interactions. Baryonic matter is the ordinary matter we see around us every day and is opposed to anti-baryonic matter, such as the not fully understood dark matter, making up the majority of the Universe according to present cosmological models.

BepiColombo (mission)
Planned ESA mission to Mercury, to be launched in 2013. It will enter into a polar orbit around the planet in 2019. More.

Big Bang
A theory of cosmology in which the expansion of the universe is presumed to have begun with a primeval explosion (referred to as the "Big Bang").

Eclipsing BinaryBinary (star system)
A stellar system consisting of two stars orbiting around their common centre of mass. For each star, the other is its companion star. Recent research suggests that a large percentage of stars are part of systems with at least two stars. Binary star systems are very important in astrophysics, because observing their mutual orbits allows their mass to be determined. The period of the orbits can be determined when one star temporarily obscures (occults) the other star in the line of sight from Earth.

Occulting Binary system

Black body
A Black Body in physics, is an idealised body that absorbs all EM-radiation that strikes it. A black body emits a temperature-dependent spectrum of light, which is called blackbody-radiation. By observing the wavelength for which the light of a star is at maximum intensity (the colour of a star), astronomers can find the star's photosphere temperature from the theoretical blackbody-radiation diagram.

Blackbody-radiation diagram better
known as Wien's displacement diagram.

Black dwarf
A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.

Black hole
An object whose gravity is so strong that not even light can escape from it.

Blue shift
An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them. See also Doppler effect.

Bolometric luminosity
The total energy radiated by an object at all wavelengths, usually given in Joules per second (or Watt).

Boltzmann constant; k
A constant named after L. Boltzmann (1844 – 1906), which describes the relationship between temperature and kinetic energy for molecules in an ideal gas. It is equal to 1.380622 x 10-23 Joules / oK (see scientific notation).

Brahe, Tycho (1546 - 1601)
Danish astronomer whose accurate astronomical observations of Mars in the last quarter of the 16th century formed the basis for Kepler's laws of planetary motion.

"Braking radiation", is electromagnetic radiation produced by the deceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus.

Brightness indicates the visual perception of radiation when looking at a radiating object, in astronomy of course a celestial object, such as a star or planet. More accurate terms for brightness are luminosity, that indicates energy output, or visual magnitude that indicates measured luminosity expressed on the magnitude scale.

The process of establishing the relationship between a measuring device and the units of measure. This is done by comparing a device or the output of an instrument to a standard having known measurement characteristics. For example the length of a stick can be calibrated by comparing it to a standard that has a known length. Once the relationship of the stick to the standard is known, the stick is calibrated and can be used to measure the length of other things. Calibration should include an evaluation of the uncertainty of the measurement.

Cannon, Annie J. (1863-1941)
American astronomer who laid the foundation for classification of stars according to temperature derived from their spectrum. She catalogued more than 200,000 stars herself and established the Henry Draper Catalogue and helped to define the Harvard Classification Scheme.


Annie Jump Cannon

Carbon (C)
a chemical element with atomic number 6. It is the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is present in all known life forms. In the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen. This abundance, together with the unique diversity of organic compounds and their unusual polymer-forming ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life

Carbon cycle
The cycle by which the element carbon is exchanged between the many forms of carbon in the atmosphere, oceans, land mass and life forms on Earth. Carbon is very abundant on Earth and is the chemical basis for all life forms. It appears in numerous compounds, e.g. as carbon dioxide and many carbohydrates.

Carbon dioxide (CO2)
a chemical compound made up of one carbon atom bonded to two oxygen atoms. It is a gas under normal conditions on Earth, but freezes below -78 °C to form "dry ice". Carbon dioxide is an important greenhouse gas because it transmits visible light but absorbs strongly in the infrared and near-infrared. Carbon dioxide is produced by all animals, plants, fungi and micro organisms during respiration and is used by plants during photosynthesis. This is to make sugars which may either be consumed again in respiration or used as the raw material for plant growth. It is, therefore, a major component of the carbon cycle. Carbon dioxide is generated as a by-product of the combustion of fossil fuels or vegetable matter, among other chemical processes. Some carbon dioxide is output by volcanoes and other geothermal processes such as hot springs

Cassini-Huygens (mission)
Dedicated NASA / ESA mission to Saturn and its moons. Launched October 1997 and arrived at Saturn June 2004. The Huygens probe landed on Saturn's moon Titan in January 2005. Cassini is still orbiting Saturn and operational. More.

CCD's or charge-coupled devices are most widely used in arrays of photoelectric light sensors to serialize parallel analogue signals. As such the name CCD often refers to the entire light-sensitive device. In this form they are thus used for digital imaging, in e.g. digital photography, spectroscopy, electron microscopes, etc. In astronomy they are very useful in photometry, photography and spectroscopy.

Celestial is a term derived from the Latin caelum which means "sky" or "heaven". In astronomy celestial refers to the sky or to space.

Celestial Equator
The celestial equator is the projection onto the Celestial Sphere of the Earth's equator. It thus lies in the same plane as the Earth’s equator. Right-Ascension is measured along the Celestial Equator.

Celestial Pole
The celestial poles are the two points where the extended rotation axis of the Earth intersects the Celestial Sphere. The points are referred to as North Celestial Pole (NCP) and South Celestial Pole (SCP).

Celestial Sphere
The celestial sphere is an imaginary and very large sphere with the earth located at its centre. Stars and other celestial bodies are thought to be projected onto this sphere. It is useful for defining stellar positions in Declination and Right-Ascension that are linked to the equator and ecliptic.

Celestial Sphere. Image www.astro.columbia.edu

Temperature scale. From 1744 until 1954, 0 oC on the Celsius scale was defined as the freezing point of water and 100 oC was defined as the boiling point of water under a pressure of one standard atmosphere. However, the Celsius scale is currently, by international agreement related to the Kelvin (K) scale, which is the SI unit of temperature. Absolute zero-the temperature at which no energy remains in a substance-is defined as being precisely 0 oK and -273.15 oC. Hence 0 oC is 273.15 oK.

Cepheid Variable
A type of variable star which exhibits a regular pattern of changing brightness as a function of time. The period of the pulsation pattern is directly related to the star's intrinsic brightness. Thus, Cepheid variables are a powerful tool for determining distances in modern astronomy.

Chlorofluorocarbons (CFC's) are a family of chemical compounds developed in the 1930's as a safe, non-toxic, non-flammable alternative to dangerous substances like ammonia, for purposes of refrigeration and spray can propellants. Their usage grew enormously over the years. Very little chlorine exists naturally in the atmosphere, but it turns out that CFC's are an excellent way of introducing chlorine into the ozone layer. The ultraviolet radiation at this altitude breaks down CFC's, freeing the chlorine. Under the proper conditions, this chlorine has the potential to destroy large amounts of ozone, which has indeed been observed, especially over Antarctica. As a consequence, levels of genetically harmful ultraviolet radiation at Earth's surface have increased. (Source: www.theozonehole.com).

CGRO (mission)
The Compton Gamma Ray (space) Observatory (1991 - 2000). The Observatory was named in honour of Dr. Arthur Holly Compton, who won the Nobel prize in physics for work on scattering of high-energy photons by electrons - a process which is central to the technique of gamma-ray detection. More...

Chandra X-ray Observatory (CXO)
One of NASA's Great Observatories in Earth orbit, launched in July 1999, and named after S. Chandrasekhar. It was previously named the Advanced X-ray Astrophysics Facility (AXAF). More...

Chandrasekhar limit
A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star.

Chandrasekhar S. (1910 - 1995)
Indian astrophysicist renowned for creating theoretical models of white dwarf stars, among other achievements. His equations explained the underlying physics behind the creation of white dwarfs, neutron stars and other extremely compact objects.

Charged particle
A particle (e.g. an electron or proton) carrying a charge. Charge is the fundamental property of a particle that causes it to be affected by the electromagnetic force.

Rocky meteorites that have not been modified due to melting or major collisions, after they formed in the early Solar system. The parent bodies of chondrites are small to medium sized asteroids that were never part of any body large enough to undergo melting or planetary formation. These bodies formed shortly after the beginning of the Solar System's history, about 4.55 billion years ago. Chondrites are the most common form of meteorites found on Earth and give important information about the early Solar system.

In astronomy, a circumpolar star is a star that never rises or sets as seen from a given position. More precisely, a circumpolar star has a declination larger than or equal to 90 degrees minus the latitude of the position.

Climate change
Any long-term significant change in the average weather that a given region experiences. Average weather may include temperature, precipitation and wind patterns. It involves changes over decades to millions of years. These changes can be caused by dynamic processes on Earth, external forces including variations in sunlight intensity, and more recently by human activities.

Cluster of galaxies
A system of galaxies containing from a few to a few thousand member galaxies which are all gravitationally bound to each other.

CNO cycle

A series of thermonuclear fusion reactions by which stars significantly larger than the Sun
fuse hydrogen into helium. The Sun produces less than 2% of its energy in this way.
This process requires the presence of nuclei of carbon, nitrogen and oxygen.
Hence its name CNO-cycle

CNO cycle.
Credit: Wikipedia

Collecting area
The amount of area a telescope has that is capable of collecting electromagnetic radiation. Collecting area is important for a telescope's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.

As opposed to a pure element, a chemical compound is a substance consisting of two or more different chemical elements, bonded together in a fixed proportion.

Compton effect
An effect named after A. H. Compton (1892 - 1962), that demonstrates that photons (the quantum of electromagnetic radiation) have momentum. A photon fired at a stationary particle, such as an electron, will impart momentum to the electron and, since its energy has been decreased, will experience a corresponding decrease in frequency.

The ability of a material to conduct an electric current, usually because it has free electrons that can carry charge through the material when an electric potential is placed across the material.

A position of a celestial object in the direction of the Sun as seen from Earth. The planets Mercury and Venus, being closer to the Sun than Earth, have two possible conjunctions: between Earth and the Sun (inferior conjunction) and behind the Sun as seen from Earth (superior conjunction). At inferior conjunction the planet is closest to Earth. Planets further from the Sun than Earth can only be behind the Sun during conjunction. Those planets can also be in opposition, when Earth is between the planet and the Sun.

Inner planet (left) and Outer planet

A grouping of stars that appear to form a particular configuration in the sky, usually related to mythology. There are 88 constellations on modern star charts, most of which date back to the ancient Greeks. More…

One of the constellations in the Zodiac: Gemini
Image www.astrologyweekly.co

The transfer of energy by flow of material in a liquid or gas, driven by temperature differences. For example, in an electric jug the heating element raises the temperature of the water next to it, which expands and rises. Colder water then flows in beneath it, setting up a convection current.
In the Sun (and possibly other stars) convection transfers heat from the radiation zone to the photosphere, and produces the very powerful magnetic fields associated with sunspots and other solar activity

Cook, Captain James (1728-1779)
English explorer, navigator and cartographer. Cook was the first to map Newfoundland prior to making three voyages to the Pacific Ocean during which he achieved the first European contact with the eastern coastline of Australia and the Hawaiian Islands as well as the first recorded circumnavigation of New Zealand. In astronomy Cook is also famous for observing the 3 June 1769 transit of Venus in Tahiti and the 9 November 1769 transit of Mercury in New Zealand.

Captain James Cook

Copernicus (mission)
NASA ultraviolet/ X-ray mission, also known as OAO-3. More...

Copernicus, Nicolaus (1473 - 1543)
Polish astronomer who advanced the theory that the Earth and other planets revolve around the Sun (the "heliocentric" theory). This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was the centre of the Universe, and all objects, including the sun, circle it. The Ptolemaic model had been widely accepted in Europe for 1000 years when Copernicus proposed his model. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century B.C., a fact known to Copernicus but long ignored by others prior to him.

Nicolaus Copernicus

The uppermost level of a star's atmosphere. In the sun, the corona is characterized by low density and very high temperature (> 1,000,000 oK)

COS-B (mission)
A satellite launched in August 1975 to study extraterrestrial sources of gamma-ray emission. More...

Cosmic Microwave Background radiation (CMB)
The background of radiation mostly in the frequency range 3 x 108 to 3 x 1011 Hz (see scientific notation) discovered in space in 1965. It is believed to be the cosmologically red shifted radiation released by the Big Bang itself.

Cosmic rays
Atomic nuclei (protons, alpha particles and electrons) that strike Earth's atmosphere with high energies. They originate from the Sun as well as from objects as far as the edges of the observable Universe.

Cosmological constant; Λ
A "constant" factor which Einstein added to his general theory of relativity in the mistaken belief that the Universe was neither expanding nor contracting. The cosmological constant was found to be unnecessary once observations indicated that the Universe was expanding. Had Einstein believed what his equations were telling him, he could have claimed the expansion of the Universe as perhaps the greatest and most convincing prediction of general relativity; he called this the "greatest blunder of his life".

Cosmological distance
A distance far beyond the boundaries of our Galaxy. When viewing objects at cosmological distances, the curved nature of space time could become apparent. Possible cosmological effects include time dilation and Cosmological redshift.

Cosmological Model
A cosmological model is a mathematical description of the Universe, which tries to explain its origin, evolution and ultimate fate and thus forms the basis of modern cosmology. Historically the term cosmological Model refers to the classical Earth-centred Ptolemaic model where the Earth is the immovable centre of the Universe (Solar System) that in the 17th century gradually was replaced by the Copernican Sun-centred model.

Cosmological redshift
An effect where light emitted from a distant source appears red shifted because of the expansion of space time itself. This is different from the Doppler effect, that is due to a relative motion between light source and observer.

The astrophysical study of the history, structure, and dynamics of the universe.

Dark matter
Name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves but which until now, has escaped all detections. There are many theories on what dark matter could be. Not one, at the moment is convincing enough and the question is still a mystery.

de Broglie wavelength
The quantum mechanical "wavelength" associated with a particle, named after the scientist L. de Broglie (1892 - 1987) who discovered it. In quantum mechanics, all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant.

A co-ordinate which, along with Right Ascension, may be used to locate any position in the sky. Declination is analogous to latitude for locating positions on Earth, and ranges from +90 to -90 degrees.

An image processing technique that removes features in an image that are caused by the telescope itself rather than by actual light coming from the sky. For example, the optical analogue would be to remove the spikes and halos which often appear on images of bright stars because of light scattered by the telescope's internal supports.

The ratio between the mass of an object and its volume. In the metric system, density is measured in grams per cubic centimetre (or kilograms per litre); the density of water is 1.0 gm/cm3; iron is 7.9 gm/cm3.; lead is 11.3 gm/cm3.

Deuterium (2H)
a stable isotope of hydrogen in which the nucleus consists of one proton and one neutron. Deuterium is very abundant in Earth's oceans in a ratio of 1 : 6500 to ordinary hydrogen.

Diffraction is the bending of light when it passed along a sharp edge or through a narrow slit. This effect can be clearly demonstrated in a ripple tank with water waves (right hand image). In principle diffraction is dependent on wavelength and therefore a slit as shown in the image can work as a dispersive element, separating different wavelengths.

When light responds to a material in a way that depends on wavelength, the material is said to be dispersive. Light reflecting off e.g. a CD or DVD surface shows dispersion in that it shows "rainbow" colours. Actually, rainbows do result from dispersion of sunlight in water droplets. In optics, dispersion is used to spatially separate different wavelengths as shown in the figure. The prism refracts the light twice but in doing so is dispersive. In astronomy, the same principle is used in spectrographs to study the spectrum of celestial objects.

Doppler effect
The apparent change in wavelength (or frequency) of sound or light waves by the relative motion between source and observer, named after C. J. Doppler. Waves emitted by a moving object as received by an observer will be blue shifted (compressed) if approaching, red shifted (elongated) if receding. It occurs both in sound and light. See also Cosmological redshift.

Doppler, Christian (1803-1853)
Austrian physicist who first described in 1842 how the observed frequency of sound waves and light is affected by the relative motion between the source and the observer. This phenomenon became known as the Doppler effect. This laid the foundation for the important use of this effect for measuring velocity in astronomy.

Christian, Andreas Doppler

In astronomy, irregularly shaped grains of carbon and/or silicates measuring a fraction of a micron across, which are found in inter stellar space. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.

Dust tail
A stream of dust particles emitted from the nucleus of a comet.

The term Dwarf star generally refers to any star in the main sequence of the HR-diagram, like our Sun, to contrast them with the Giants and Super Giants. Other categories of Dwarf stars are White Dwarfs, and other sub-classes such is Brown Dwarfs and Red Dwarfs. More...

Non-circular; elliptical (applied to an orbit).

A value that defines the shape of an ellipse or elliptical orbit. The eccentricity of an ellipse is the ratio of the distance between the foci and the length of the major axis: e = 2ea / 2a.
If the eccentricity = 0 the ellipse becomes a circle. Highly elliptical orbits, e.g. of long period comets, have an eccentricity close to one. The orbit of Halley's comet for instance has an eccentricity of 0.967. If the eccentricity = 1, the ellipse opens up to a parabola. For a comet that would mean that it doesn't come back, but disappears into interstellar space after once going around the Sun.

The passage of one celestial body in front of another, cutting off the light from the second body (e.g. an eclipse of the sun by the moon, or one star in a binary system eclipsing the other). It may also be the passage of all or part of one body through the shadow of another (e.g. a lunar eclipse in which the moon passes through the Earth's shadow).

The plane of Earth's orbit about the Sun.

Einstein Observatory
The first fully imaging x-ray telescope in space, launched by NASA in 1978. Originally named "HEAO-2" (High Energy Astrophysics Observatory 2), it was renamed for Albert Einstein upon launch.

Einstein, Albert (1879 - 1955)
German-American physicist; developed the Special and General Theories of Relativity which along with Quantum Mechanics are the foundation of modern physics.

Albert Einstein

Material that is ejected. Used mostly to describe the content of a massive star that is propelled outward in a supernova explosion. Also used to describe the material that is blown radially outward in a meteor impact on the surface of a planet or moon.

Electromagnetic spectrum
The full range of frequencies of Electro Magnetic waves, from radio waves to gamma rays.

Electromagnetic waves (radiation)
Another term for light. Light waves are fluctuations of electric and magnetic fields in space.

A negatively charged fundamental particle commonly found in the outer layers of atoms. The electron has only 0.0005 times the mass of the proton.

Electron volt (eV)
The change of potential energy experienced by an electron moving from a place where the potential has a value of V to a place where it has a value of (V+1 volt). This is a convenient energy unit when dealing with the motions of electrons and ions in electric fields; the unit is also the one used to describe the energy of X-rays and gamma rays.

The fundamental kind of atoms that make up the building blocks of matter, which are each shown on the periodic table of the elements. The most abundant elements in the universe are hydrogen and helium. These two elements make up about 80% and 20% of all the baryonic matter in the universe respectively.
The remaining heavy elements, despite comprising only a very small fraction of the universe, can greatly influence astronomical phenomena.

In astronomy elevation refers to the vertical angle from the horizon to a celestial object. As such it is identical with altitude.

Local co-ordinates.
Image www.srrb.noaa.gov

Oval. That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Kepler the careful observations by Tycho Brahe

The production of light, or more generally, electromagnetic radiation by an atom or other object.

Emission line spectrum
A spectrum consisting of bright lines at certain wavelengths separated by darker regions. It is caused by electrons in a substance that go down in energy level and thus emit photon energy.

Energy is a physical quantity that describes the amount of work that can be performed by a force. Several different forms of energy exist to explain all known natural phenomena. These forms of energy are often named after a related force. Historically energy has been expressed in several units such as ergs and calories. At present, the SI unit of energy, the Joule. For very small quantities of energy, such as in atomic physics, the unit Electron Volt (eV) is used. 1 eV = 1.60217653×10-19 Joule.

The curve on the surface of a rotating body formed by the intersection with the plane passing through the centre of the body, perpendicular to the axis of rotation. This plane is called the equatorial plane.

Equatorial diameter
Diameter of a celestial body measured across the equatorial plane

Equatorial Mount
The classic type of telescope mount with one axis parallel to the Earth's polar axis (i.e. pointing at the celestial pole) and the other axis at right angles.
Once the object is located, only the polar axis need to be driven by a motor to counteract Earth's rotation

Occurs twice a year, when the centre of the Sun can be observed to be directly above Earth's equator. This happens around 21 March and 22 September

A form of the metric unit for power. It is equal to 10-7 Watt (see scientific notation).

European Space Agency, established in 1975. ESA is an intergovernmental organisation dedicated to the exploration of space, currently with 17 European member states

Escape velocity
The speed at which an object can leave another object behind, without being recalled by its gravitational force. The escape velocity of Earth - which must, for instance, be attained by a spacecraft if it is to reach another planet - is about 40,000 km per hour or 11 km per second

Event horizon
A surface around a black hole within which nothing can escape. In addition, nothing can prevent a particle from hitting the singularity in a very short amount of proper time once it has entered the horizon. In this sense, the event horizon is a "point of no return". See Schwarzschild radius.

Evolved star
A star near the end of its lifetime when most of its fuel has been used up. This period of the star's life is characterized by loss of mass from its surface in the form of a stellar wind.

Exomars rover (mission)
Approved ESA mission to Mars in search for evidence of (fossil) life on the planet. It will involve an Orbiter and Lander. More.

EXOSAT (mission)
European Space Agency's X-ray Observatory that completed its mission in 1986. It was the first spacecraft designed to observe X-rays in space. More.

Explorer (missions)
Large number of NASA space missions with various scientific goals. Explorer 1 was launched in 1958 and the latest Explorer 78 in 2000, although there is some confusion about which space missions actually belonged to the Explorer programme. More.

Extinction (atmospheric)
Atmospheric extinction is the combined effect of absorption and scattering of light as it passes through the atmosphere. It results in dimmer objects (less light) than without this effect.

Extra galactic
Outside of, or beyond, our own galaxy.

In the Fahrenheit temperature scale, the freezing point of water is 32 oF and the boiling point 212 oF. Hence the boiling and freezing points of water are exactly 180 oF apart. A temperature in Fahrenheit can be calculated from Celsius by [oF] = [oC] × 9/5 + 32. Conversely [oC] = ([oF] - 32) × 5/9

Fibre optics
An optical fiber is a glass or plastic fiber that carries light along its length. Light is kept in the core of the optical fiber by total internal reflection. This causes the fiber to act as a wave guide.
Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference.

Five elements
In traditional Chinese philosophy, natural phenomena can be classified into the Wu Xing (五行) , or the Five Phases, usually quite poorly translated as Five Elements, Five Movements or Five Steps. These elements and some of their cosmological interpretations are:

Heavenly Creature
Azure Dragon
Vermillion Bird
Yellow Dragon
White Tiger
Black Tortoise
Blue or Black
Centre / Zenith

Flamsteed, John (1646 - 1719)
English astronomer. Flamsteed accurately calculated the solar eclipses of 1666 and 1668. He was responsible for several of the earliest recorded sightings of the planet Uranus, which he mistook for a star and catalogued as 34 Tauri. In 1725 Flamsteed's Historia Coelestis Britannica was published. This contained a catalogue of 2,935 stars to much greater accuracy than any prior work

A lighting effect in which an object absorbs a photon that triggers the emission of another photon, usually with a longer wavelength. Often the absorbed light is UV-radiation and the emitted light is in the visible spectrum.

A measure of the amount of energy given off by an astronomical object over a fixed amount of time and area. Because the energy is measured per time and area. Flux measurements make it easier for astronomers to compare the relative energy output of objects with very different sizes or ages.

Fly by
A manoeuvre of a spacecraft involving a gravitational slingshot or gravity assist, using the relative movement and gravity of a planet or other celestial body to alter the path and speed of the spacecraft, typically in order to save fuel, but often at the expense of longer travel time. Gravity assist can be used to decelerate a spacecraft (useful when traveling to an inner planet) or accelerate a spacecraft (useful when traveling to an outer planet). Jupiter with its large mass, is a favourable object for the latter.

Fly by principleExplanation: Suppose that you are a "stationary" observer and that you see a planet moving left at speed U, and a spaceship moving right at speed v. The spaceship will pass close to the planet, moving at speed U + v relative to the planet because the planet is moving in the opposite direction at speed U. When the spaceship leaves orbit, it is still moving at a speed of U + v relative to the planet's surface but in the opposite direction, to the left. Since the planet is still moving left at speed U, the spaceship is now moving left at speed 2U + v from your point of view. Hence its speed has increased by 2U, twice the speed at which the planet is moving.

The energy and momentum that the spacecraft has gained, comes from an equal loss by the planet. Because the ratio of mass between spacecraft and planet is so small, the effect on the planet's orbit is immeasurably small.

Focal point (plural: foci)
In mathematics, a pair of special points in conic sections (ellipse, parabola, hyperbola). In an ellipse, the sum of the distances from any point on the ellipse to the two foci is a constant (which is always the length of the major axis of the ellipse). In a circle, there is only one focus, the centre of the circle, and all the points of the circle have the same distance to it. In an elliptic orbit, the central body is located in any one of the foci of the ellipse

Fossil fuel
Fossil source fuels, in the form of hydrocarbons found within the top layer of the Earth's crust. Fossil fuels are non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being formed. Concern about fossil fuel supplies is one of the causes of regional and global conflicts. The exploration and use of fossil fuels raise environmental concerns. A global movement toward the generation of renewable energy is therefore under way to help meet increased energy needs

A property of a wave that describes how many wave patterns or cycles pass by in a period of time. Frequency is often measured in Hertz (Hz), where a wave with a frequency of 1 Hz will pass by at 1 cycle per second.

See Nuclear fusion

Yuri GagarinGagarin, Yuri (1934-1968)
Russian astronaut. Gagarin became the first human to travel into space on 12 April 1961, in a Vostok 3KA-2 spacecraft and returned safely.
On 27 March 1968, he and his instructor died when their MiG-15UTI crashed on a routine training flight.

First Astronaut: Yuri Gagarin

Galactic halo
A spherical region surrounding the centre of a galaxy. This region may extend beyond the luminous boundaries of the galaxy and contain a significant fraction of the galaxy's mass.

Galactic Plane
The Galactic Plane is part of the Galactic co-ordinate system used in astronomy (More…). More loosely it can be defined as the plane through the Sun and the centre of the Milky Way, parallel to the disk of the galaxy.

A component of our universe made up of gas and a large number (usually more than a million) of stars held together by gravity. An example is our own Milky Way Galaxy.

Galilei, Galileo (1564 - 1642)
An Italian scientist, Galileo was renowned for his fundamental contribution to physics, astronomy, and philosophy of science in general. He is regarded as the chief founder of modern science. He developed the telescope, with which he found craters on the Moon and discovered the largest moons of Jupiter. Galileo was condemned by the Catholic Church for his heliocentric view of the cosmos based on the theory of Copernicus.

Galileo Galilei

Galileo (mission)
Dedicated and successful mission to Jupiter launched by NASA in 1989. The mission ended in 2003 when the spacecraft was steered into the giant planet to avoid contamination of any of the Galilean Moons, on which life forms may exists below the ice sheet. More.

Gamma ray
The highest energy, shortest wavelength of electromagnetic radiation.

Gamma Ray Imaging Platform (GRIP)
A balloon-borne gamma-ray telescope made by a group at the California Institute of Technology. It has had many successful flights. More...

Gamma Ray Imaging Spectrometer (GRIS)
A balloon-borne instrument, flown from 1988, which uses germanium detectors for high resolution gamma-ray spectroscopy. More...

Gamma-Ray Burst (GRB)
Plural is GRB's. A burst of gamma rays from space lasting from a fraction of a second to many minutes. There is no clear scientific consensus as to their cause. Recently, their distances were determined to be large, placing the origins of the bursts in other galaxies.

One of the three states of matter, in which atoms, molecules, or ions move freely and are not bound to each other.
In astronomy, it usually refers to hydrogen or helium.

Gas Planets
The four planets in our Solar System that are furthest from the Sun: Jupiter, Saturn, Uranus and Neptune. They are also called the "Jovian" planets which means Jupiter-like or alternatively "Gas Giants". They consist mostly of gas and do not have a clearly defined surface. Deeper below the cloud tops, the gases become liquid or take on other physical states such as Metallic Hydrogen. Jupiter and Saturn are mostly composed of hydrogen and helium, just like stars and could be called "failed stars", because they are too small to maintain hydrogen fusion in their cores. Uranus and Neptune are distinctly different and contain large volumes of water and ammonia and are therefore also called "Ice Giants". They have a predominantly blue colour due to methane in the upper layers.

General relativity
The geometric theory of gravitation developed by Albert Einstein, incorporating and extending the theory of special relativity to accelerated frames of reference and introducing the principle that gravitational and inertial forces are equivalent. The theory has consequences for the bending of light by massive objects, the nature of black holes, and the fabric of space and time.

Geologic Timescale
A time scheme that describes geological events during the history of the Earth over its entire 4.5 billion year existence. Sections in this time scheme are of irregular time span and are generally defined by major geological events such as mass extinctions. More...

A giant star has a radius between 10 and 100 of the Solar radius Rsun and a luminosity between 10 and 1,000 times that of the Sun. Giant stars are found above the main sequence in the HR-diagram and below the Super Giants.

Giant Molecular Cloud (GMC)
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules (two hydrogen atoms bound together), though also containing other molecules observable by radio telescopes. These clouds can contain enough mass to make several million stars like our Sun and are often the sites of star formation.

Ginga (mission)
The third Japanese X-ray mission, also known as Astro-C. More...

Gamma-ray Large Area Space Telescope (GLAST)
An international mission launched 11 June 2008. GLAST will study the universe in the energy range 10 keV - 300 Gev. More...

Global warming
The increase in the average temperature of Earth's near-surface air and oceans since the mid-twentieth century, and its projected continuation. Global warming is an aspect of Climate change. The concept of Global Warming can also be applied when studying weather systems on other planets, such as Venus.

Globular cluster
A spherically symmetric collection of stars which share a common origin. The cluster may contain up to millions of stars spanning up to 50 parsecs.

Gold (Au)
a chemical element with atomic number 79 and of the most sought after precious metals. As one of the neutron-rich elements, gold is naturally created in supernova explosions or even from collisions of neutron stars. Its presence in the Solar system suggest that our Sun is a third generation star

The mottled appearance of the solar photosphere, caused by gases rising from the interior of the Sun (see granules)

Convective cells (about 1000 km in diameter) in the Solar photosphere. Each granule lasts about 5 minutes on the average and represents a temperature roughly 300o higher than the surrounding dark areas.

Gravitational collapse
When a massive body collapses under its own weight. For example, interstellar clouds collapse to become stars until the onset of nuclear fusion stops the collapse.

Gravitational radius
See Event horizon

Gravitational waves
Ripples in space-time caused by the motion of objects in the universe. The most notable sources are orbiting neutron stars, merging black holes, and collapsing stars. Gravitational waves are also thought to emanate from the Big Bang.

Gravitationally bound
Objects held in orbit about each other by their gravitational attraction. For example, satellites in orbit around Earth are gravitationally bound to Earth since they can't escape Earth's gravity. By contrast, the Voyager spacecraft, which explored the outer solar system, was launched with enough energy to escape Earth's gravity altogether, and hence it is not gravitationally bound.

A mutual physical force attracting two bodies that have mass.

Gravity field
A description of the strength of the force of gravity through space, exerted by a celestial body like Earth or the Sun. In its simplest form a gravity field can be described by Newton's law of gravity. In the modern concept of a gravity field, gravity is not a force, but rather the curvature of space-time, "forcing" objects that move, to follow that curvature.

Goddard Space Flight Centre, one of the centres operated by NASA.

Hard x-ray
High energy x-rays, often from about 10 keV to nearly 1000 keV. The dividing line between hard and soft x-rays is not well defined and can depend on the context.

Hawking radiation
A theory first proposed by British physicist Stephen Hawking (1942 - ), that due to a combination of properties of quantum mechanics and gravity, under certain conditions black holes can seem to emit radiation.

Hawking temperature
The temperature inferred for a black hole based on the Hawking radiation detected from it.

A "bubble" in space around the Sun with a diameter of about 150 AU. It ends with the heliopause, where the Solar wind and the interstellar medium balance each other, and the influence of the Sun effectively stops.

Helium (He)
The second lightest and second most abundant chemical element with atomic number 2. The typical helium atom consists of a nucleus of two protons and two neutrons surrounded by two electrons. Helium was first discovered in our Sun. Roughly 25% of the mass of our Sun is helium.

Literally "half of a sphere". On Earth or other celestial bodies it refers to the area north of the equator (northern hemisphere) or south of the equator (southernhemisphere)

William Herschel

Herschel, Sir William (1738 -1822)
British astronomer famous for discovering Uranus. He also discovered infrared radiation and made many other discoveries in astronomy.

Sir William Herschel

Hertz (Hz) (unit)
The derived SI unit of frequency, defined as a frequency of 1 cycle per second.

Hertz, Heinrich (1857 - 1894)
A German physics professor who did the first experiments with generating and receiving electromagnetic waves, in particular radio waves. In his honour, the units associated with measuring the frequency or cycles per second of waves is called the Hertz (Hz).

Hertzsprung, Ejnar (1873-1967)
A Danish astronomer most famous for his work on classification of stars according to spectral type and luminosity. Henri Russell did the same independently, which lead to the Hertzsprung-Russell diagram.



Ejnar Hertzsprung

Hipparchus (190 BC-120 BC)
Hipparchus is considered the greatest ancient astronomical observer and, by some, the greatest overall astronomer of antiquity. He developed accurate models for the motion of the Sun and Moon . He developed trigonometry and constructed trigonometric tables, and he has solved several problems of spherical trigonometry. With his solar and lunar theories and his trigonometry, he may have been the first to develop a reliable method to predict solar eclipses. His other reputed achievements include the discovery of precession, the compilation of the first comprehensive star catalog of the western world. The work in astronomy by Ptolemaeus three centuries later, has been largely based upon Hipparchus' legacy.

The Hertzsprung-Russell diagram (usually referred to as H-R diagram) shows the relationship between luminosity, and stellar class or temperature of stars. The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell and represented a huge leap forward in understanding the evolution of stars.

Hubble Space TelescopeHST
Hubble Space Telescope that was carried into orbit by a Space Shuttle in April 1990. It is named for American astronomer Edwin Hubble. Although not the first space telescope, the HST is one of the largest and most versatile, and is well known as both a vital research tool and a public relations boon for astronomy. The HST is a collaboration between NASA and the ESA, and is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.

More on HST...

Hubble constant Ho; Hubble's Law
The constant which determines the linear relationship between the distance to a galaxy and its velocity of recession due to the expansion of the Universe. This relationship is called Hubble's Law. The latest observed value of H0 has been determined at 72 km/s/Mpc ± 8 km/s/Mpc by the Hubble H0 Key Project team. More...

Hubble, Edwin P. (1889 - 1953)
American astronomer whose observations proved that galaxies are "island universes", not nebulae inside our own galaxy. His greatest discovery, called "Hubble's Law", was the linear relationship between a galaxy's distance and the speed with which it seems to be moving away from us (Cosmological Redshift). The Hubble Space Telescope is named in his honour.

Huygens probe
See Cassini Huygens mission

Huygens, Christiaan (1629 - 1695)
A Dutch physicist who was the leading proponent of the wave theory of light. He also made important contributions to mechanics, stating that in a collision between bodies, neither loses nor gains ``motion'' (his term for momentum). In astronomy, he discovered Titan (Saturn's largest moon) and was the first to correctly identify the observed elongation of Saturn as the presence of Saturn's rings.

compounds consisting entirely of carbon and hydrogen. On Earth, hydrocarbons can be gases, liquids or low melting solids such as waxes. Methane (CH4) and Ethane (C2H6) are well known forms of hydrocarbons. Hydrocarbons are one of Earth's most important resources of energy in the form of combustible fuels, chemically reacting with oxygen and forming carbon dioxide.

Hydrogen (H)
The lightest and most abundant chemical element in the Universe with atomic number 1. A hydrogen atom consists of one proton and one electron. Hydrogen composes about 75% of the mass of the Sun.

International Astronomical Union, that was founded in 1919. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. Among the tasks of the IAU are the definition of fundamental astronomical and physical constants; unambiguous definition of astronomical names and terms; promotion of educational activities in astronomy; and informal discussions on the possibilities for future international large-scale facilities. The IAU serves as the internationally recognized authority for assigning names to celestial bodies and surface features on them

The Space Research Institute in Russia. It is the equivalent of NASA in the U.S.

A violent inward collapse. An inward explosion.

The inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its planet's equator.

Infrared (IR)
Electromagnetic radiation at wavelengths longer than the red end of visible light and shorter than microwaves (roughly between 1 and 100 microns). Almost none of the infrared portion of the electromagnetic spectrum can reach the surface of Earth, although some portions can be observed by high-altitude aircraft or by telescopes on high mountain tops.

Inter Galactic Space
The space between galaxies that generally is very close to a vacuum In the vicinity of galaxies, it contains a plasma of electrons and protons: the inter galactic medium (IGM). Even stars are found in inter galactic space that do not belong to a particular galaxy.

Inter Planetary Medium
The space between planets, Moons and other bodies is filled with the Inter Planetary Medium that consists of dust and hot plasma that originates from the Sun. The plasma is highly conductive and and carries the Sun's magnetic field all the way to the outskirts of the Solar System up to a distance of 160 AU.

Inter planetary space
Interplanetary Space is the space in the Solar System in between the Sun, planets, moons and other bodies. It extends throughout the heliosphere, until the boundary of the Solar System at about 100 AU from the Sun. Inter planetary space is not empty but filled with dust and hot plasma from the Solar wind.

Inter stellar medium
The gas and dust between stars, which fills the plane of the Galaxy much like air fills the world we live in. For centuries, scientists believed that the space between the stars was empty. It wasn't until the eighteenth century, when William Herschel observed nebulous patches of sky through his telescope, that serious consideration was given to the notion that interstellar space was something to study. It was only in the last century that observations of inter stellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure.

Inter stellar space
All the space within a galaxy not occupied by stars or their planetary (solar) systems. It is filled with the inter stellar medium of gas and dust. The space between galaxies is referred to as inter galactic space.

Ionic (or ionised) gas
Gas whose atoms have lost or gained electrons, causing them to be electrically charged. In astronomy, this term is most often used to describe the gas around hot stars, where the high temperature causes atoms to lose electrons.

Ionisation is the process in which electrons are added or subtracted from atoms. This results in ions, that are electrically charged atoms, positive when electrons have been lost, or negative when electrons have been added.

An atom or molecule with one or more electrons stripped off or added, giving it a net positive or negative charge respectively.

Iron (Fe)
a chemical element with atomic number 26. Iron and nickel are notable for being the final elements produced by stellar nucleo synthesis, and are therefore the heaviest elements which do not require a supernova explosion for formation. Iron and nickel are therefore the most abundant metals in metallic meteorites and in the dense, metal cores of planets such as Earth.

International Ultraviolet Explorer, an ultraviolet space observatory launched in 1978. Originally designed for a 3 year mission, IUE exceeded all expectations and functioned for over 18 years, finally ceasing operation in September 1996. More...

Beams of particles, usually coming from an active galactic nucleus or a pulsar. Unlike a jet airplane, when the stream of gas is in one direction, astrophysical jets come in pairs with each jet aiming in opposite directions.

The SI unit of energy. It is equal to 1 Newton metre (1 Nm).

Juno (mission)
Planned mission to Jupiter in NASA's New Frontiers Programme. Expected launch in 2011 and insertion onto a polar orbit around Jupiter in 2016. More.

Kelvin (unit)
The fundamental SI unit of thermodynamic temperature named after Lord Kelvin, defined as 1/273.16 of the thermodynamic temperature of the triple point of water. More practically speaking, the Kelvin temperature scale measures an object's temperature above absolute zero, the theoretical coldest possible temperature. On the Kelvin scale the freezing point of water is 273.15 oK. The Kelvin temperature scale is often used in science.

Kelvin, Lord William T. (1824 - 1907)
Scottish mathematician and physicist who contributed to many branches of physics. Most known for his contributions to Thermodynamics and the unit of absolute temperature (Kelvin) named after him.

Kepler's laws
Kepler's first law
A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler's second law
A line directed from the Sun to a planet sweeps out equal areas in equal times as the planet orbits the Sun.

Kepler's third law
The square of the period of a planet's orbit is proportional to the cube of that planet's semi major axis; the constant of proportionality is the same for all planets.

Kepler, Johannes (1571 - 1630)
German astronomer and mathematician. Considered a founder of modern astronomy, he formulated the famous three laws of planetary motion. They comprise a quantitative formulation of Copernicus's theory that the planets orbit around the Sun.

Kilogram (kg)
The fundamental SI unit of mass. The kilogram is the only SI unit still maintained by a physical artifact (a platinum-iridium bar) kept in the International Bureau of Weights and Measures at Sèvres, France. One kilogram is equivalent to 1,000 grams or about 2.2 pounds; the mass of a litre of water.

Branch of mechanics that studies motion of an object, without studying the force causing the motion. Kepler's laws of elliptic motion are examples.

Kuiper, Gerard Peter (1905-1973)
Dutch American astronomer. Lived most of his working life in the United States. Famous for e.g. suggesting a disk of objects beyond Neptune's orbit in the outer Solar system, later called Kuiper Belt

A representation of the luminosity of an object in terms of Solar luminosity. The average luminosity of the Sun is about 3.8 x 1026 Watt. Astronomers often express luminosity of other stars in terms of solar units, which makes the resulting numbers smaller and easier to compare to the Sun.

Lagrange pointsLagrange points
Points in the vicinity of two massive bodies (such as the Sun and Earth) where each others' respective gravities balance. An object in any of these points is effectively stationary with respect to the secondary body.

There are five Lagrange points,
labeled L1 through L5.

L1, L2, and L3 lie along the centerline between the centres of mass between the two masses; L1 is on the inward side of the secondary, L2 is on the outward side of the secondary; and L3 is on the outward side of the primary. L4 and L5, the so-called Trojan points, lie along the orbit of the secondary around the primary, sixty degrees ahead and behind of the secondary. L1 through L3 are points of unstable equilibrium; any disturbance will move a test particle there out of the Lagrange point. L4 and L5 are points of stable equilibrium, provided that the mass of the secondary is less than about 1/25 the mass of the primary.

Lagrange, Joseph (1736 - 1813)
A French mathematician of the eighteenth century. His work Mecanique Analytique (Analytical Mechanics; 1788) was a mathematical masterpiece. It contained clear, symmetrical notation and covered almost every area of pure mathematics. Lagrange developed the calculus of variations, established the theory of differential equations, and provided many new solutions and theorems in number theory. His classic Theorie des fonctions analytiques laid some of the foundations of group theory. Lagrange also invented the method of solving differential equations known as variation of parameters.

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It's a device that produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher levels so that when they return to lower energy levels they emit energy. Therefore Lasers emit radiation of one or more precise wavelengths.

A co-ordinate that defines a location on Earth or other celestial bodies north or south of the equator. Lines of equal latitude are horizontal lines shown running east-to-west on maps or small circles parallel to the equator on a globe

Leavitt, Henrietta (1868 - 1921)
An American astronomer who in 1912, discovered the period-luminosity relationship of Cepheid variables in the Magellanic Clouds. With this discovery she provided a major contribution to the problem of measuring distances in the Universe.

Henrietta Swan Leavitt

Generally used to identify the Moon’s swaying back and forth about a point of equilibrium in its tidal locking to Earth. Because the Moon is tidally locked to Earth, it always shows the same face to Earth, but due to libration we can see about 59% of the Moon’s surface. Also used to describe this swaying motion for other celestial bodies that are tidally locked. More…

Image NASA

The common term for electromagnetic radiation, usually referring to that portion visible to the human eye. However, other bands of the e-m spectrum are also often referred to as different forms of light.

Light curve
A graph showing how the radiation from an object varies over time.

Light year
A unit of length used in astronomy which equals the distance light travels in a year. At the rate of 300,000 kilometres per second, 1 light-year is equivalent to 9.46053 x 1012 km or 63,240 AU (see scientific notation).

The outer edge of the apparent disk of a celestial body.

In mathematics a linear function between variables is described by an algebraic equation in which each term is either a constant or the product of a constant and a single variable. The graph of a linear function is a straight line in 2 D and a (hyper)plane in more dimensional space.

LISA (Laser Interferometer Space Antenna)
A proposed NASA / ESA mission which will detect gravitational waves. LISA will consist of three satellites that use laser interferometry to monitor their positions relative to each other. Gravitational waves passing by the satellites cause small changes in the distances between the satellites. More...

Lithium (Li)
a chemical element with atomic number 3. It is a soft alkali metal with a silver-white colour. It is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly reactive, corroding quickly in moist air to form a black tarnish. According to cosmology, lithium was one of the few elements created in the Big Bang, although its quantity in the Universe has vastly decreased

Logarithmic scale
A graph scale that uses the logarithm of a physical quantity instead of the quantity itself in a graph. This scale is often used when the data covers a large range of values. Often the ten-base logarithm is used, where fixed tic intervals cover the range of a power of ten, i.e. 100=1, 101=10, 102=100, 103=1000, etc. (give example from astronomy).

A co-ordinate that defines a location on Earth or other celestial bodies east or west of a reference meridian. On Earth that is the meridian of Greenwich. Lines of equal longitude are vertical lines shown running north-to-south on maps or great circles passing through both poles on a globe

The rate at which a star or other object emits energy. It is expressed in a unit of energy per time period, either in Watt or in units of 3.846×1026 W, denoted LSun , being the luminosity of the Sun.

Luna (missions)
Luna is a series of 24 Lunar missions by the Soviet Union in the period 1959 -1976. Of specific historic significance is the Luna 3 mission, that returned the first image of the far side of the Moon in history, taken in October, 1959. More…

First image of the far side of the Moon
taken by Luna 3 in 1959.

Lunation or Lunar Cycle is the mean period of the Moon’s orbit around the Earth. Also called the Synodic period. It is on average 29.530589 days.

A representation of the mass of an object in terms of Solar mass. The average mass of the Sun is about 2x1030 kg. Astronomers often express the mass of other objects in terms of solar mass, since it makes the resulting numbers smaller and easier to compare.

Magellan (mission)
Successful radar mapping mission to Venus, 1989 - 1994. This NASA mission provided detailed maps of the Venus topography, observed through the dense, opaque atmosphere. More...

Magnesium (Mg)
A chemical element and alkaline metal with atomic number 12. It is an essential element for all living cells and constitutes about 2% of the Earth's crust. It is the ninth most abundant element in the Universe.

Magnetic field
A description of the strength of the magnetic force exerted by an object in space. Bar magnets have "dipolar" fields, as the force is exerted from the two ends of the bar. In simple terms, Earth and most other planets, the Sun, stars, and pulsars all have dipolar magnetic fields.

Magnetic pole
Either of two limited regions in a magnetic object at which the magnetic field is most intense. The two regions have opposing polarities, which are labeled "North" and "South".

Magneto sphere
The region of space in which the magnetic field of an object (e.g., a star or planet) dominates the radiation pressure of the stellar wind to which it is exposed.

Magneto tail
The portion of a planetary magnetosphere behind the planet as seen from the Sun, which is stretched in the direction of the solar wind.

The degree of brightness of a celestial body as we see it from Earth, designated on a numerical scale, on which the star Vega has magnitude 0 and the faintest star visible without a telescope has magnitude +6. A negative magnitude means a brighter object. The Sun's magnitude is -26.73. A decrease of one magnitude represents an increase in apparent brightness by a factor of 2.512.

Main Sequence
The Main Sequence is a significant diagonal band of stars in the Hertzsprung-Russell diagram. These stars are all fusing Hydrogen into Helium and are in hydrostatic equilibrium, meaning that they are in a stable equilibrium between outward pressure from the nuclear fusion and inward pull by gravity. The position of a star in the Main Sequence is primarily determined by its mass and temperature.

Mariner (mission)
A suite of 10 NASA missions to various planets from 1962 to 1973. Seven missions were successful, flying by Mars, Venus and Mercury. Mariner 10 was the first mission to Mercury. It took 30 years before Mercury was visited for the second time by MESSENGER

Mars (mission)
A series of Soviet missions to Mars from 1971 to 1973. Landing attempts of Mars 2 and 3 and Mars 6 and 7 failed or were only very short lived. More.

Mars Express (mission)
First ESA Orbiter and Lander mission to Mars launched in 2003. The Lander "Beagle 2" failed, but the Orbiter was successful and witnessed the landing of Phoenix in 2008. More.

Mars Global Surveyor (mission)
Mars Orbiter mission by NASA, launched in 1997. In spite of technical problems the missions returned successful data until 2003. More.

Mars Observer (mission)
Attempted Mars Orbiter mission by NASA in 1992. More.

Mars Odyssey (mission)
Successful NASA Orbiter mission to Mars 2001 - 2006. The Orbiter served as a communications platform for the Mars Exploration Rovers (MER). More.

Mars Pathfinder (mission)
Low cost and successful NASA mission to Mars with a Lander and rover vehicle in 1996 - 1997. More.

A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.
The SI unit of mass is the kilogram (kg).

A word used for any kind of substance that has mass and takes up space. It is somewhat loosely defined and not really a scientific term.

Maxwell, James,C. (1831-1879)
A Scottish theoretical physicist and mathematician. His most significant achievement was that he unified all equations of electricity, magnetism and even optics into one consistent theory. His set of equations—Maxwell's equations—demonstrated that electricity, magnetism and light are all manifestations of the same phenomenon: the electromagnetic field.


James Clerk Maxwell

A unit of energy used to describe large amounts of explosive energy. 1 Mega ton is 1 million tons of TNT and is equal to 4.184 x 1015 Joule.

MER (missions)
Two identical NASA Mars Exploration Missions, launched in 2003. The rover "Spirit" and "Opportunity" have explored the Martian surface for more than four years and are still operational, vastly exceeding their planned mission duration. MER1. and MER2...

A great circle on a globe or sphere, passing through both poles. In astronomy also an imaginary great circle on the celestial sphere, running through north and south points on the horizon and the zenith. The meridian also passes through the North Celestial Pole for an observer on the northern hemisphere or through the South Celestial Pole on the southern hemisphere

MESSENGER (mission)
Second mission to Mercury and the first one to enter into orbit around the planet. This NASA mission was launched in 2004, it had its second flyby of Mercury in October 2008 and is due to enter orbit in 2011. More.

Messier, Charles (1730 - 1817)
The 18th century French astronomer who compiled a list of approximately 100 fuzzy, diffuse looking objects which appeared at fixed positions in the sky. Being a comet-hunter, Messier compiled this list of objects which he knew were not comets. His list is now well known to professional and amateur astronomers as containing the brightest and most striking nebulae, star clusters, and galaxies in the sky.

In astronomy the term metal refers to any element other than hydrogen or helium, that thus does not originate from the Big Bang. These metals are formed from nuclear processes in stars or supernovae. The amount of metal in a star (metallicity) is an indication for the age of the star, where younger stars are formed from material that has been going through a previous stellar life cycle and thus contain more metals.

Metallic hydrogen
A degenerate state of hydrogen that occurs under very high pressure, effectively creating a liquid of free protons and electrons. Liquid metallic hydrogen is thought to be present in very large volumes inside the Gas planets Jupiter and Saturn.

Methane (CH4)
a chemical compound that is the principal component of natural gas. Just like Carbon dioxide, methane appears in Earth's atmosphere and is a greenhouse gas with a high global warming potential. Methane in the atmosphere is eventually oxidized, producing carbon dioxide and water. In addition, there is a large, but unknown, amount of methane chemically bound in the ocean floors. Global warming could release this methane, which could cause a further sharp rise in global temperatures. Such releases of methane may have been a factor in previous major extinction events. The Earth's crust also contains huge amounts of methane.

MetNet (mission)
Russian-Finnish mission to Mars in early stage of development. More.

Metre; m
The fundamental SI unit of length, defined as the length of the path traveled by EM-radiation in vacuum during a period of 1/299 792 458 s. A unit of length equal to about 39 inches. A kilometre is equal to 1000 meters.

Metric system
See SI.

Micro quasar
Micro quasars are stellar mass black holes, that display characteristics of the super massive black holes found at the centres of some galaxies. For instance, they have radio jets - something not every black hole has.

Electromagnetic radiation which has a longer wavelength (between 1 mm and 30 cm) than visible light. Microwaves can be used to study the Universe, communicate with satellites in Earth orbit, and cook popcorn and other food.

A material is in molecular form when it consists of molecules (a stable group of two or more atoms) rather than of individual atoms. E.g. molecular hydrogen is H2, where hydrogen molecules are formed out of two hydrogen atoms.

A stable, electrically neutral group of at least two atoms in a definite arrangement, held together by strong chemical bonds.

Mars Reconnaissance Orbiter
Next generation Mars Orbiter mission launched by NASA in 2005, accumulating high resolution imagery of the Martian surface in search for potential landing sites for manned missions. More.

MSL (mission)
Next generation rover mission by NASA to be launched in 2009. More.

The National Aeronautics and Space Administration, founded in 1958 as the successor to the National Advisory Committee for Aeronautics.

Nebula (pl. nebulae)
A diffuse mass of interstellar dust and gas. A reflection nebula shines by light reflected from nearby stars. An emission nebula shines by emitting light as electrons recombine with protons to form hydrogen. The electrons were made free by the ultraviolet light of a nearby star shining on a cloud of hydrogen gas. A planetary nebula results from the explosion of a solar-like type star.

Neon (Ne)
A chemical element with atomic number 10. It is an odourless, inert, noble gas that can be found in the Earth's atmosphere in trace amounts, although it is the fifth most abundant element in the Universe.

Elementary particle that travels close to the speed of light, has no charge and hardly any mass, and is able to pass through ordinary matter almost undisturbed and unnoticed. Neutrinos are therefore extremely difficult to detect. They are usually denoted by the Greek letter ν (nu).

A particle with approximately the mass of a proton, but zero charge, commonly found in the nucleus of atoms.

Neutron star
The imploded core of a massive star resulting from a supernova explosion. The original star must have a mass above the Chandrasekhar limit, but less than two times the Solar mass. A Neutron star has a typical diameter of only 30 km and the mass of up to two times the mass of the Sun. Rapidly rotating Neutron stars can be observed as pulsars.

New Horizons (mission)
Mission to Pluto and the Kuiper Belt region, launched January 2006. Due to arrive at Pluto-Charon in 2015. More.

Newton (unit)
The newton is the SI unit of force; it is equal to the amount of force required to accelerate a mass of one kilogram at a rate of one metre per second per second.

Newton's law of universal gravitation
Two bodies attract each other with equal and opposite forces. The magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centres of mass of the two bodies.

Newton's laws of motion
Newton's first law of motion
A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

Newton's second law of motion
For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

Newton's third law of motion
In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction

Newton, Sir Isaac (1643 - 1727)
English physicist, mathematician, astronomer, natural philosopher, alchemist and theologian. His Philosophiæ Naturalis Principia Mathematica, published in 1687, is said to be the greatest single work in the history of science. In this work, Newton described universal gravitation and the three laws of motion, laying the groundwork for classical mechanics, which dominated the scientific view of the physical universe for the next three centuries and is the basis for modern engineering.

Nickel (Ni)
a chemical element with atomic number 28. See also Iron

Nitrogen (N)
a chemical element with atomic number 7. Nitrogen is a colourless, odourless, tasteless and mostly inert gas, constituting 78.08% by volume of Earth's atmosphere. Nitrogen occurs in all living organisms - it is a part of amino acids and thus of proteins, and of nucleic acids (DNA and RNA), and resides in the chemical structure of almost all neurotransmitters

The random fluctuations that are always associated with a measurement when it is repeated. Noise appears in astronomical images as fluctuations in the image background. These fluctuations do not represent any real sources of light in the sky, but rather are caused by the imperfections of the telescope. If the noise is too high, it may obscure the dimmest objects within the field of view.

Normal scales
Refers to the use of scales on Earth. These scales read out in units of mass (m) for which the SI unit is kg. However these devices actually measure the weight force (F) which should be expressed in Newton N. The point is that "normal" scales are calibrated to apply the average acceleration of gravity at Earth's surface (about 9.8 m s-2) to convert force into mass using Newton's second law: F = m a or m = F / 9.81. If you take these scales to a place where the acceleration of gravity is different (a mountain top or another planet) they will still measure the force correctly, but display a mass that is incorrect

Nova (plural: novae)
A star that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by hundreds to thousands of times before fading back to its original luminosity.

Nuclear fission
The splitting of the atomic nucleus of a heavy element, resulting in the emission of nuclear energy and possibly causing a chain reaction within a mass of the element

Nuclear fusion
A nuclear process requiring very high temperature and pressure, whereby small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy. This is the common process in stars.

Nucleo synthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons (protons and neutrons). Stellar nucleosynthesis occurs inside stars and the heaviest elements are produced in explosive nucleosynthesis, such as in supernova explosions.

Nucleus (plural nuclei)
The core of an atom without the electrons. A nucleus is generally made up of protons and neutrons. The number of protons (atomic number) determines which chemical element the nucleus represents.

Obliquity (axial tilt)
In general the angle between the equatorial and orbital planes of a body. For planets the obliquity is the angle between the planes of the equator and the ecliptic.

The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star.

Oort, Jan Hendrik (1900 -1992)

Oort Cloud
A spherical cloud of trillions of icy objects that can form comets when they are forced into an orbit about the Sun. The Oort Cloud was suggested by the Dutch astronomer Jan Oort, and extends up to about 1 light year from the Sun. The end of the Oort cloud defines the boundary of the gravitional influence of the Sun.

Impression of the Oort Cloud
Image Wikipedia

J.H. Oort

Oort, Jan Hendrik (1900 -1992)
Dutch astronomer. He promoted the technique of radio astronomy. The Oort cloud of comets bears his name.

J.H. Oort

A property of matter that prevents light from passing through it. The opacity or opaqueness of something depends on the frequency of the light. For instance, the atmosphere of Venus is transparent to ultraviolet light, but is opaque to visible light.

Open Cluster
A group of stars that are loosely bound by gravity. They have formed from the same Giant Molecular Cloud and are generally young stars. Famous examples of Open Clusters that are visible with the naked eye are the Pleiades and the Hyades clusters.


The position of an outer planet (being at a greater distance to the Sun than Earth) when the planet is on the line between Earth and the Sun. At opposition the planet is closest to Earth.

The path of an object that is moving around a second object under influence of mutual gravity.

Orbital ResonanceOrbital resonance
A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5. Example: Io is in a 2:1 orbital resonance with Europa and a 4:1 orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede.

Oxygen (O)
a chemical element with atomic number 8. All major classes of structural molecules in living organisms, such as proteins, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that comprise animal shells, teeth, and bone. Molecular Oxygen in the form of O2 is produced from water by cyano-bacteria, algae and plants during photosynthesis and is used in cellular respiration for all complex life. Oxygen is toxic to anaerobic organisms, which were the dominant form of early life on Earth until O2 began to accumulate in the atmosphere 2.5 billion years ago.

Another form of molecular oxygen, ozone (O3), helps protect the biosphere from ultraviolet radiation with the high-altitude ozone layer. See also Oxygen.

Ozone layer
A layer in the lower part of Earth's stratosphere (about 20-60 km above sea level) where the greatest concentration of ozone (03) appears. This is the layer responsible for the absorption of ultraviolet radiation.

Pair production
The physical process whereby a gamma-ray photon, usually through an interaction with the electromagnetic field of a nucleus, produces an electron and an anti-electron (positron). The original photon no longer exists, its energy having gone to the two resulting particles. The inverse process, pair annihilation, creates two gamma-ray photons from the mutual destruction of an electron/positron pair.

The history of the magnetic field on Earth or other celestial body, as preserved in records of magnetisation in rocks and magnetic minerals. On Earth, paleomagnetism has shown that both direction and intensity of the magnetic field has changed considerably over millions of years. Paleomagnetism is also important in research of other planets or moons.

The apparent motion of a relatively close object compared to a more distant background as the location of the observer changes. Astronomically, it is half the angle which a star appears to move as the earth moves from one side of the sun to the other.

In geography a parallel is a small circle around Earth that is parallel to the equator. A parallel therefore is a location of equal latitude. On a spherical Earth a parallel is always perpendicular to (at 90 degrees angle with) a meridian.

Unit of distance used in astronomy. It is equal to the distance to an object which has a parallax of one arc second. It is equal to 3.26 light year, or 3.1 x 1016 metre (see scientific notation). A kiloparsec (kpc) is equal to 1000 parsec. A megaparsec (Mpc) is equal to a million (106) parsec.

Pascal (unit)
The SI unit for pressure is the Pascal (Pa), and is equal to one Newton per square metre (N m-1). Atmospheric pressure on Earth is usually expressed in hPa (hectoPascal) that is 100 Pa. 1 hPa is equal to 1 mBar, the historical unit for atmospheric pressure. The unit is named after Blaise Pascal (1623 -1662), a French mathematician, physicist, and religious philosopher.

The point in an elliptical orbit where the orbiting object is closest to the centre of attraction. Special names are given to this orbital point for commonly used systems. For an object orbiting the Sun see perihelion, and for an Earth satellite see perigee.

The point in its orbit where an Earth satellite is closest to Earth. Opposite of apogee.

The point in an elliptical orbit where a celestial object is closest to the Sun. Opposite of aphelion.

The interval of time required to complete one revolution in an orbit or one cycle of a periodic phenomenon, such as a cycle of phases

Period-Luminosity Relation
A relation between the periods and luminosities of certain stars, discovered by Henrietta Leavitt in 1912. Cepheid variables (Cepheids) are stars that obey this relation. The longer the Cepheid's pulsation period, the more luminous the star. By measuring a Cepheid's period, the period-luminosity relation determines the Cepheid's luminosity. Compared to the actual brightness in the sky, astronomers can calculate the distance. If the Cepheid is part of another galaxy, the Cepheid's distance gives the distance to the entire galaxy

Moon phasesPhase
Differences in the appearance of a non-luminous object (e.g. the Moon, or of Mercury and Venus, as seen from Earth) that is illuminated by a star. It is caused by the observer seeing only a part of the body lit by the star. (see example of Lunar phases).

Phase angle
An angle between 0o and 180o, measured at the centre of an illuminated body between the light source and the observer. For the Moon phase as seen from Earth, 0o corresponds to full moon and 180o to new moon.

Phobos (mission)
Two Soviet missions to Mars and its moon Phobos in 1988. Both failed due to technical problems during the mission. More.

Phobus Grunt (mission)
Russian mission to Mars and its moon Phobos, scheduled for launch in 2009. More.

Phoenix Mars Lander
NASA's Phoenix landed successfully on Mars on 25 May 2008. It investigates atmosphere and soil in the North polar region of Mars. More.

Photoelectric effect
An effect explained by Einstein which demonstrates that light seems to be made up of "particles", or photons. Light can excite electrons (called photoelectrons in this context) to be ejected from a metal. Light with a frequency below a certain threshold, at any intensity, will not cause any photoelectrons to be emitted from the metal. Above that frequency, photoelectrons are emitted in proportion to the intensity of incident light.
The reason is that a photon has energy in proportion to its wavelength, and the constant of proportionality is the Planck constant. Below a certain frequency -- and thus below a certain energy -- the incident photons do not have enough energy to knock the photoelectrons out of the metal. Above that threshold energy, called the work function, photons will knock the photoelectrons out of the metal, in proportion to the number of photons (the intensity of the light). At higher frequencies and energies, the photoelectrons ejected obtain a kinetic energy corresponding to the difference between the photon's energy and the work function.

The smallest (quantum) unit of electromagnetic energy. Photons are generally regarded as quanta with zero mass and no electric charge.

The photosphere is the region of a star closest to its centre, from which the light we receive, originates. We see the Sun in visible light as a sphere, but that sphere, the Photosphere, is the region below which the Sun is opaque for visible light.

The conversion of light energy into chemical energy by living organisms. The raw materials are carbon dioxide and water; the energy source is sunlight; and the end-products are oxygen and carbohydrates, for example sucrose, glucose and starch. This process is arguably the most important biochemical pathway, since nearly all life on Earth either directly or indirectly depends on it.

The constant equal to the ratio of the circumference of a circle to its diameter, which is approximately 3.141593.

Pioneer (mission)
Series of early space missions by NASA, from 1958. Most notable are Pioneer 10 and 11 that are on their way out of the Solar system. More.

Pioneer Venus (mission)
NASA Orbiter and multi probe missions to Venus, 1980 - 1992. Four probes have been launched into Venus' atmosphere. More.

Planck constant (h)
The fundamental constant equal to the ratio of the energy of a photon to its frequency. It has the value 6.626196 x 10-34 J s (see scientific notation).

Planck equation
The quantum mechanical equation relating the energy of a photon E to its frequency ν (Greek character nu): E = h x ν

Planck, Max (1858-1947)
Planck was a German physicist. He is famous for his contributions to black body radiation and worked with Einstein on the Theory of Relativity. He is considered to be one of the the founders of quantum theory, and is one of the most important physicists of the twentieth century.


Karl Ernst Ludwig Marx Planck

Planet-C (mission)
Also named Venus Climate Orbiter (VCO), this Japanese mission to Venus is scheduled for launch in 2010. More.

Planetary Nebula
A bubble of gas surrounding a hot, dying star. The star is so hot that it makes the planetary nebula glow, which makes it visible. The star was once the core of a red giant, which ejected its outer atmosphere and created the nebula. A planetary nebula has nothing to do with a planet, but through a small telescope, it looks like a planet's disk, hence the misleading (historical) name.

A completely ionized gas; the so-called fourth state of matter (besides solid, liquid, and gas) in which the temperature is too high for atoms to exist. The plasma consists of free electrons and free atomic nuclei.

Major tectonic platesPlate tectonics
The theory that describes the large scale motion of parts of the Earth's crust (tectonic plates). These plates move with respect to each other, typically with a speed of a few centimetres per year. Along the plate boundaries earthquakes, volcanic activity, mountain-building, oceanic trench formation, and sea floor spreading occur.

Major tectonic plates

The direction in the sky to which the telescope is aiming. Pointing also describes how accurately a telescope can be pointed toward a particular direction in the sky.

Polar Axis
The axis of an equatorially mounted telescope that points towards the Celestial Pole and is therefore parallel to the Earth's axis of rotation.

Polar diameter
Diameter of a celestial body measured from pole to pole.

A special property of light. Polarisation is a condition in which the planes of vibration of the various rays in a light beam are at least partially aligned.

The points on a rotating body where the axis of rotation intersects the surface. One point is defined as the North pole and the opposite point as the South pole.

The antiparticle to the electron. The positron has most of the same characteristics as an electron except it is positively charged.

Potassium (K)
a chemical element with atomic number 19. It is highly reactive with water and occurs naturally in various compounds. It has an important function in the neurological system in animals (including humans).

The force exerted over a surface divided by its area. Atmospheric pressure is the weight force exerted over a unit area onto the surface of a celestial body. The SI unit for pressure is the Pascal (Pa), equal to one Newton per square metre (N m-2). Non-SI measures such as pound per square inch (psi) and Bar are (still) used in parts of the world.

Proto star
Very dense regions of molecular clouds where stars are in the process of forming.

A particle with a positive charge commonly found in the nucleus of atoms.

P-P chainProton-proton chain
A series of thermonuclear reactions by which the Sun and all other main-sequence stars with less than 1.5 Solar masses fuse hydrogen into helium.


Proton - Proton Chain

Ptolemy (83 - 161 AD)
Greek mathematician and philosopher. Ptolemy believed the planets and Sun to orbit the Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations. He is famous for three books on astronomy and geography: Almagest, Geography and Tetrabiblos). The Almagest includes a star catalog containing 48 constellations, using the names we still use today.

Claudius Ptolemaeus

A rapidly rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies as well.

PVO (mission)
Pioneer Venus Orbiter. NASA mission to Venus 1978 - 1992. More...

In Physics a Quantum is an indivisible amount of energy, momentum or other physical quantity, linked to elementary particles. A photon is the quantum of energy and the electron the quantum of electric charge. The notion of quantum is a key aspect of Quantum Mechanics and the discrete nature of matter.

Quantum Mechanics
Quantum mechanics is a mathematical theory that can describe the behaviour of objects at the atomic or sub-atomic scale. Quantum particles move from one point to another as if they are waves. However, at a detector they always appear as discrete lumps of matter. There is no counterpart to this behaviour in the world at the human scale. One cannot rely on every-day experience to form some kind of "intuition" of how these objects move and therefore some consequences of the theory seem to defy logic. Quantum Mechanics replaces the classical Newtonian Mechanics at the micro-scale.
It is a very successful theory in that it accurately describes behaviour of sub-atomic particles, and just about all of modern electronics is based on this theory.

An enormously bright object at the edge of the observable Universe which emits massive amounts of energy. In an optical telescope, they appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar). Current theories hold that quasars are one type of AGN.

Quasi-stellar source (QS)
Sometimes also called quasi-stellar object (QO); A stellar-appearing object of very large cosmological redshift that is a strong source of radio waves; presumed to be extra galactic and highly luminous.

Refers to the mass of the Sun, and is used as a unit of mass to express the mass of other stars. It thus makes the numbers smaller than when using kg as the unit of mass and makes it easier to compare with the Sun.

Radial velocity
The speed at which an object is moving away or toward an observer in the direction of the line of sight. By observing spectral lines, astronomers can determine how fast objects are moving away from or toward us; however, these spectral lines cannot be used to measure how fast the objects are moving across the sky, which is called Angular velocity.

Radian (rad)
The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. One radian is approximately 57 degrees. Because the circumference of a circle is defined as 2πR, 2π radians is equal to 360 degrees.

Radian (rad)
The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. One radian is approximately 57 degrees. Because the circumference of a circle is defined as 2πR, 2π radians is equal to 360 degrees.

Emitted energy. In astronomy usually referred to as Electro-Magnetic radiation.

Radiation belt
Regions of charged particles in a magnetosphere.

Long wavelength end of the EM-spectrum. Earth's atmosphere is transparent to radio waves with wavelengths from a few millimetres to about twenty meters. Observing radio waves from space, using Radio Telescopes, is an important part of astronomy (Radio Astronomy).

Radioactive dating
A technique used to determine the age of materials, based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products. The rate of decay (half-life) of many naturally occurring radioactive materials is accurately known and forms the time basis for this technique of dating. It is the principal source of information about the absolute age of rocks and other geological features, including the age of Earth itself, and can be used to date a wide range of natural and man-made materials.

Rayleigh Scattering
Selective scattering (i.e., preferential scattering of shorter wavelengths) of light by very small particles suspended in the Earth's atmosphere, or by molecules of the air itself. Blue light (shorter wavelength) is scattered more than the longer wavelengths such as yellow and red. Therefore the sky in daylight appears blue, and the Sun appears reddish at sunset and sunrise.

Red giant
Later and giant stage in the life cycle of a star of about one Solar Mass or less. Most of the hydrogen has run out and the star's core fuses helium. When the Sun reaches that stage in about 5 billion years, it will have a size as large as up to the orbit of Mars.

An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer. See also Doppler effect and Cosmological redshift.

When light hits a polished surface it is reflected such that the angle of reflection is equal to the angle of incidence. This ideal situation is called specular reflection. Less perfect surfaces will cause a slightly diffuse reflection, where the light rays are spread out about the angle of reflection.

At a transparent glass surface, light rays often partially reflect off the surface and partially refract into the surface (right hand image).

Reflection law
For a wave front intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.

Refraction is the bending of light rays as they pass through the atmosphere, due to variations in the air density along the light path. Generally the density increases on the way down, therefore rays bend in such a way that we see celestial objects higher in the sky than they really are.

Image www.mbmg.mtech.edu

When light passes from one material into a dissimilar material, the rays bend and the light slightly changes its velocity. This effect is called refraction. The angle of refraction depends on the ratio of refractive index of the two materials.
As the atmosphere has a varying air density, light we receive from celestial objects is refracted, which makes us see such objects higher in the sky than they really are. This is usually referred to as astronomical refraction (right hand image).

Relativity principle
The principle, employed by Einstein's Relativity theories, that the laws of physics are the same, at least locally, in all coordinate frames. This principle, along with the principle of the constancy of the speed of light, constitutes the foundation of special relativity.

Relativity, theory of
Theories of motion developed by Albert Einstein, for which he is justifiably famous. Relativity describes the motions of bodies in strong gravitational fields or at velocities near the speed of light more accurately than Newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy.

Resolution (spatial)
In astronomy, the ability of a telescope to differentiate between two objects in the sky which are separated by a small angular distance. The closer two objects can be while still being seen in the telescope, the higher the resolution of the telescope.

Resolution (spectral or frequency)
Spectral resolution is the ability of the telescope to differentiate two light signals which differ in frequency by a small amount. The closer the two signals are in frequency while still being seen in the telescope as two distinct components, the higher the spectral resolution of the telescope.

In celestial mechanics, a relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
See also orbital resonance.

The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic; moving in the opposite direction as the great majority of solar system bodies do. Several moons of the Gas planets have retrograde orbits. For objects in the sky it means a motion from West to East, opposite to the motion of the Sun and stars. Planets occasionally have a retrograde motion in the sky.

The complete movement of one celestial body around another. It is often measured as the "orbital period."

Right Ascension
A coordinate which, along with declination, may be used to locate any position in the sky. Right ascension is analogous to longitude for locating positions on Earth.

Ritter, Johann Wilhelm (1776 - 1810)
Ritter is known for discovering and investigating the ultraviolet region of the electromagnetic spectrum.

Roche limit
The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At less than this distance the tidal forces of the larger object would break up the smaller object.

Rocky Planets
The four planets in our Solar System that are closest to the Sun: Mercury, Venus, Earth and Mars. They are also called the "Terrestrial" planets which means Earth-like. They have a solid crust as opposed to the Gas Planets. This solid crust is largely made up of silicates and gives the Rocky Planets a clearly defined surface.

German, UK, US Röntgen satellite mission (1990-1999). More...

The spin of a celestial body on its own axis. All celestial bodies have rotation. In high energy astronomy, this is often measured as the "spin period."

Russell, Henry (1877-1957)
An American astronomer who, along with Ejnar Hertzsprung, developed the Hertzsprung-Russell diagram in 1910.


Henry Norris Russell

Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays, also known as Röntgen rays, in 1895.

A body in orbit around another body is generally referred to as a satellite, expressing a subordinate status in comparison to the central body. Most often it refers to a Moon (or natural satellite) orbiting a planet. Man-made satellites, called artificial satellites, are orbiting Earth since the first Sputnik in 1957.

The process whereby light is absorbed and re-emitted in all directions, with essentially no change in frequency. Scattering by free electrons was the dominant source of opacity in the early Universe. (See also Rayleigh scattering).

Schwarzschild black hole
Theoretical black hole described by solutions to Einstein's equations of general relativity worked out by Karl Schwarzschild in 1916. The solutions assume the black hole is not rotating, and that the size of its event horizon is determined solely by its mass.

Schwarzschild radius
The radius r of the event horizon for a Schwarzschild black hole.

Scientific notation
A compact format for writing very large or very small numbers, most often used in science. The notation separates a number into two parts: a decimal fraction, usually between 1 and 10, multiplied by a power of ten. Thus
1.23 x 104 means 1.23 times 10 to the fourth power or 12,300;
5.67 x 10-8 means 5.67 divided by 10 to the eighth power or 0.0000000567.

Decimal notation Scientific notation
4,000.123 4.000123x103
-0.0000001234 -1.234x10-7
1,234,000,000 1.234x109

Scientific notation
Scientific notation is a way of writing numbers that accommodates values too large or too small to be conveniently written in standard decimal notation. It is written in the form ax10exp, where a is a decimal number with one digit to the left of the decimal point and exp is a positive or negative integer that expresses the power of ten and indicates how many positions the decimal point must be moved (to the right if positive and to the left if negative) to obtain the normal decimal notation.
Motivation: imagine that you have to write Planck's constant (6.626068x10-34) in normal decimal notation!

Second (s)
The fundamental SI unit of time, defined as the period of time equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. This definition brings the second very close to the length of a Solar day, divided by (24 x 60 x 60).

Semi major axis
The semi major axis of an ellipse (e.g. a planetary orbit) is half the length of the major axis, which is the line segment passing through the foci of the ellipse with endpoints on the ellipse itself. The semi major axis of a planetary orbit is also the average distance from the planet to its primary.

A measure of how bright objects need to be in order for a telescope to detect these objects. A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.

Seyfert galaxy
A spiral galaxy whose nucleus shows bright emission lines; one of a class of galaxies first described by C. Seyfert.

Shock wave
A strong compression wave where there is a sudden change in gas velocity, density, pressure and temperature.

SI, Système Internationale d'Unités); International System of Units
The international and rationalized system of measurement units, in common use in physics today. The SI unit of length is the metre, of time is the second, and of mass is the kilogram.

Sidereal day
The length of time which elapses between any star reaching its highest point in the sky between two consecutive nights.

Sidereal period
The sidereal period is the time that it takes an object in the Solar system to make one full orbit around the Sun, relative to the stars. This is considered to be an object's true orbital period.

Sidereal rotation
The rotation of a celestial body with respect to the distant stars.

a compound based on the Silicon element (Si), often in combination with Oxygen. Silicates are found in nature as the mineral quartz in its various forms. In geology and astronomy, the term silicate is used to denote types of rock that consist predominantly of silicate minerals. Most of Earth's mantle and crust are made up of silicate rocks. The same is true for the Moon and the other rocky planets.

Silicon (Si)
A chemical element with atomic number 14. Silicon very rarely occurs as the pure free element in nature, but is more widely distributed in dusts, planetoids and planets in various forms of silicon dioxide (SiO2) or silicates. On Earth, silicon is the second most abundant element (after oxygen) in the crust, making up 25.7% of the crust by mass.

In astronomy, a term often used to refer to the centre of a black hole, where the curvature of space time is infinite. In mathematics, a singularity is a condition when equations do not have a solution. This can sometimes be avoided by using a different coordinate system.

Sodium (Na)
a chemical element with atomic number 11. It is highly reactive and similar to Potassium. Sodium occurs abundantly on Earth, e.g. in the form of ordinary salt (Sodium chloride), particularly in seawater. It has many important biochemical functions in animals (including humans), who maintain high concentrations of Sodium in their blood and extra cellular fluids.

Soft x-ray
Low energy x-rays, often from about 0.1 keV to 10 keV. The dividing line between soft and hard x-rays is not well defined and can depend on the context.

Solar and SiderealSolar day
The length of time which elapses between the Sun reaching its highest point in the sky two consecutive days. The solar day is about 4 minutes longer than the sidereal day, because Earth orbits the Sun and during one full rotation of Earth itself, it has moved its position with respect to the Sun.

Difference between
Sidereal and Solar day

Solar flares
Violent and sudden release of magnetic energy from the Sun's surface.

Solar mass
A unit of mass equivalent to the mass of the Sun. 1 solar mass = 1 Msun = 2 x 1030 kg.

Solar System
The system formed by the Sun and all objects that orbit around it, because they are gravitationally bound to it. These objects can be classified as Planets, Moons, small Solar System bodies, and the Inter-Planetary medium. The size of the Solar System can be defined as that of the Heliosphere, which is about 150 AU in average, although the Oort Cloud extends much further to at least 1 light year, or more than 63,000 AU.

Occurs twice a year, when the tilt of Earth's axis is oriented directly towards or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes. The name is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the apparent movement in north or south direction comes to a standstill. It happens around 22 June and 22 December, mid-summer or mid-winter, opposite on the northern and southern hemispheres.

Special relativity
The physics theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference, moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as consequences the relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence. See also General relativity.

Spectral Class
A classification of stars according to their spectrum type and organised in order of temperature of the photosphere. Classes are indicated by letters O, B, A, F, G, K and M, with subdivisions by a number in the range 0-9 for each class. A further subdivision using Roman numerals I, II, III, IV and V indicates the star's luminosity. More on stellar classification in our Module "Stellar Radiation".

Spectral line
Light given off at a specific frequency by an atom or molecule when it is in an excited state by incoming radiation. Each element or compound gives off spectral lines at their own unique set of frequencies. This enables astronomers to determine the chemical composition of the surface of stars, no matter how far away they are. See also absorption line spectrum or emission line spectrum.

Also called spectrograph, spectroscope. The instrument connected to a telescope that separates the incoming light into different frequencies, producing a spectrum. This enables astronomers to distinguish spectral lines.

A Dispersive Spectrometer is like a prism. It directs light of different energies at different angles.
A Non-Dispersive Spectrometer measures the energy at different frequencies directly.


The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the chemical composition of stars and interstellar clouds. See also spectrometer.

Spectrum (plural: spectra)
A diagram or graph of the intensity of light at different frequencies.

Spectrum of visible light

Speed of light (in vacuum)
The constant speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s. Einstein's Theory of Relativity implies that no physical object can go faster than the speed of light.

Sphere (spherical)
A mathematical object on which every point has the same distance to a central point: the centre. In astronomy celestial bodies are called spherical when they resemble a spherical shape, but they never accurately are, due to e.g. topography and the effect of rotation that causes the polar diameter to be usually shorter than the equatorial diameter. Such objects are called spherical to contrast them to bodies that have an irregular shape, such as most asteroids and many moons.

Spitzer telescope
Formerly known as the Space Infrared Telescope Facility (SIRTF), it is an infrared space telescope, launched by NASA in 2003. The mission life depends on when the onboard liquid helium supply, for cooling the telescope, will be exhausted. As of late 2007, it is expected that this will occur in April 2009. More.

A massive ball of plasma in space that is held together by gravity and that creates and emits its own radiation.

Star cluster
A group of stars (ranging in number from a few to millions) which are bound to each other by their mutual gravitational attraction.

Stellar classification
Stars are given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated as a G2 star.

Stellar wind (Solar wind)
The ejection of plasma off the surface of a star. Solar wind is dominant throughout the Solar system. Stellar wind is strongest near the end of a star's life when it has consumed most of its fuel.

Steradian (sr)
The supplementary SI unit of solid angle defined as the solid central angle of a sphere that encloses a surface on the sphere equal to the square of the sphere's radius.

Sulphur (S)
a chemical element with atomic number 16. In pure form Sulphur occurs abundantly in volcanic regions and thermal pools. Otherwise the element is found as compounds in the form of sulphate and sulphide minerals. Sulphur is an essential component for all living cells.

Cooler (and thus darker) regions on the sun where the magnetic field loops out of the solar surface. They often come in pairs of opposite magnetic polarity.

Super Giant
Super Giants are the most massive stars. They can be found in the top region of the HR-diagram. They are 10 to 70 times as massive as the Sun and up to hundreds of thousands times more luminous. Because of their extreme masses they have short life spans of 30 million years down to a few hundred thousand years and are therefore always young stars.

Supernova (plural: supernovae)
(a)The explosion of a massive star at the end of its life cycle when the core collapses. This results in a sharp increase in brightness followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (SNR).

(b) The explosion of a white dwarf which has accumulated enough material from a companion (binary) star to achieve a mass equal to the Chandrasekhar limit. This type of supernovae (called Type Ia) always has approximately the same luminosity. This can be used to determine distances.

Surface gravity
The acceleration of gravity experienced on the surface of a celestial body. Average surface gravity on Earth is 9.8 m s-2.

Swift (mission)
Swift is an international mission whose primary goal is to study gamma-ray bursts (GRB's) and address the mysteries surrounding their nature, origin, and causes. Swift was launched November 20, 2004. More...

Synchronous rotation
Said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward.

Synchrotron radiation
Electromagnetic radiation given off when very high energy electrons encounter magnetic fields.

Synodic period
The synodic period is the time that it takes an object in the Solar system to reappear at the same point in the sky, relative to the Sun, as observed from Earth. The synodic period differs from the sidereal period since Earth itself revolves around the Sun.

Siderial and Synodic month

Tenma (mission)
The second Japanese X-ray mission, also known as Astro-B (1993 - 1995). More...

Thomson, William (1824 - 1907)
Also known as Lord Kelvin, the Irish physicist who is well known for the Kelvin temperature scale.

Tidal force
A force acting on a celestial body as a result of the difference in the effect of gravity that is exerted by another body. When a moon orbits a planet, the far side of the moon experiences less gravitational pull from the planet than the near side. This causes tidal forces inside the moon. The same happens in the planet. On Earth this causes the tides in the oceans, but also the solid Earth tides, that makes the Earth's solid surface rise and fall with the tides by about 30 cm. If a small object, e.g. a comet, comes too close to a planet or the Sun, it can break up when the material of that object is not flexible enough to withstand the tidal forces. Tidal forces can generate heat inside celestial bodies due to tidal friction.

Tidal locking
Tidal locking is a gravitational effect in which a larger astronomical body forces a smaller satellite to always face the main body with the same side. An example is the Moon which is tidally locked to Earth, always facing Earth with its same side. This effect originates from the deformation (tidal bulging) the smaller body undergoes under influence of the gravity of the larger body. This produces a torque (turning force) that continuously restores the equilibrium. When the two bodies are of similar size and they are sufficiently close, they may both be tidally locked to each other, meaning that they show the same face to each other. A typical example of double tidal locking is the double dwarf planet system Pluto – Charon.

Two bodies orbiting the one in the centre.
The closer body is tidally locked, while the further is not.
Credit: Wikipedia.

Time dilation
The increase in the time elapsed between two events as measured by an observer who is outside of the reference frame in which the events take place. The effect occurs in both special and general relativity, and is quite pronounced for speeds approaching the speed of light, and in regions of strong gravity. Time dilation would make it possible for passengers in a fast-moving spaceship to travel further into the future while aging very little, in that the ship's clock (and the aging of any human traveling in it) shows less elapsed time than the clocks of observers on Earth.

The passage of a smaller, nearer astronomical object across the face of a larger object in the background, as in a transit of Mercury or Venus across the sun as seen from Earth.

Tritium (3H)
a radio-active isotope of hydrogen, in which the nucleus contains one proton and two neutrons

Tropic of Cancer
The Tropic of Cancer is a parallel (circle of equal latitude) which is the northernmost latitude where the Sun can appear directly overhead at noon. This happens at the summer solstice around 22 June when the Earth’s tilt towards the Sun is maximum at the Northern Hemisphere.

Tropic of Capricorn
The Tropic of Capricorn is a parallel (circle of equal latitude) which is the southernmost latitude where the Sun can appear directly overhead at noon. This happens at the winter solstice around 22 December when the Earth’s tilt towards the Sun is maximum at the Southern Hemisphere.

Turbulence is caused by fluctuations in the wind velocities in the upper atmosphere that mix layers of different temperature, density and water vapour content. This degrades the spatial resolution of astronomical telescopes. Irregular movement of air closer to the ground, and even inside astronomical domes, can also contribute to this problem. Turbulence is a meteorological phenomenon and can vary greatly between nights.

Venus in a turbulent sky. Image www.eso.org

Ultraviolet (UV)
Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.

Ulysses (mission)
Launched in 1990, this NASA solar mission orbits the Sun in a polar orbit. It required a unique flyby along Jupiter to lift its orbit out of the ecliptic. More.


Unit prefix
Unit prefixes are used together with the SI-units to express powers of ten. This avoids the use of large numbers. The table shows the twenty unit prefixes adopted for the International System of Units and Measures. It can also be used for non-SI units.


  • the wavelength of the He Ne laser is 6.33x10-7 m or 633 nm (nanometre)
  • the average distance to the Sun is 150 million km or 150 Gm (Gigametre)
  • a star at a distance of 50,000 Parsec is at 50 kParsec (kiloParsec)
  • light travels the distance of one metre in vacuum in about 3.3x10-9 s or 3.3 ns (nanosecond)
  • 1 eV equals about 1.6x10-19 J or 0.16 aJ (attoJoule).

Universal constant of gravitation (G)
The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role in Einstein's general relativity.
It is equal to (6.67428 ± 0.00067) x 10-11 m3 kg-1 sec-2 (see scientific notation).

Everything that physically exists, the entirety of space and time, all forms of matter, energy and momentum; the entire cosmos. Some speculate that this universe is but one of a set of disconnected Universes, collectively denoted as the Multiverse.

Uranium (U)
a chemical element with atomic number 92. Discovered in 1789 by Martin Klaproth, eight years after the discovery of Uranus, named after that planet. Uranium has the highest atomic weight of the naturally occurring elements. Along with all elements having atomic weights higher than that of iron, it is only naturally created in supernova explosions and similar cosmic events.

Vega (mission)
The Vega mission combined a rendezvous with comet Halley and an exploration of the atmosphere of Venus. The mission consisted of two spacecraft, Vega 1 and Vega 2, launched in 1984. After successfully delivering balloons into Venus' atmosphere, the two spacecraft encountered comet Halley in 1986. More.

Venera (mission)
A total of 16 Soviet missions to planet Venus, with several attempts of robotic landings through the dense and hot atmosphere 1961 - 1983 Venera 7 was the first successful landing in 1970. The Lander survived for 35 minutes. More.

Venus Express (mission)
ESA's first mission to Venus, launched in 2005, primarily to study Venus' dynamic atmosphere. Mission is expected to continue into 2009. More.

Venus In-Situ Explorer (mission)
Conceptual NASA mission to Venus, to study surface properties of the planet. More.

Viking (mission)
NASA Orbiter and Lander missions to Mars in 1975 - 1976. The Landers obtained high resolution images from the Martian surface. More.

Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of "fluid friction". Water has a low viscosity, while e.g. vegetable oil has a higher viscosity.

Visible light
Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colours ranging from red (longer wavelengths; ~ 700 nm) to violet (shorter wavelengths; ~400 nm.).

Voyager (mission)
Twin spacecraft Voyager 1 and 2 were launched in 1977 by NASA, for a fly by of the Gas Planets. They are both on their way out into interstellar space. More.

The SI unit of power. 1 W = 1 Joule s-1. The quantity power expresses how much energy is released or consumed over a period of time.

Wave-particle duality
The principle of quantum mechanics which implies that all matter exhibits both wave-like and particle-like properties. It addresses the inadequacy of classical concepts like "particle" and "wave" in fully describing the behaviour of objects. EM-radiation behaves wave-like when it travels and particle-like when it interferes with matter.

The distance between adjacent peaks in a series of periodic waves. Within the visible spectrum of light, short wavelengths are towards blue and long wavelengths towards red.

White dwarf
A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a size of about Earth, but it has a mass roughly equal to the Sun's. This gives a white dwarf a density about 1 million times that of water!

WMAP (mission) (Wilkinson Microwave Anisotropy Probe)
A NASA satellite designed to detect fluctuations in the cosmic microwave background. This is radiation that originates from the early Universe, that has been red shifted due to the expansion of the Universe. WMAP has provided much higher accuracy measurements of many cosmological parameters than had been available from previous instruments. The mission that was launched in 2001 has seen several mission extensions. More...

Electromagnetic radiation of very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays. Within X-ray wavelengths, a distinction is made between hard X-ray (shorter wavelengths) and soft X-ray, primarily because of difference in observation techniques.

Z (redshift)
The ratio of the observed change in wavelength of light emitted by a moving object to the rest wavelength of the emitted light. See cosmological redshift.
This ratio is related to the velocity v of the object and the speed of light c. If the velocity of the object is small compared to the speed of light, then z = v / c.

When the object would have a velocity of v = c, this ratio would become 1. However for very distant objects in the Universe, values of z up to 5 or more are observed. This shows that for very high speeds relativistic effects must be accounted for. Objects have been observed with z = 5.8 which relates to a speed of 98% of the speed of light.

Zenith is the point in the sky (or Celestial Sphere) that is directly (vertically) above an observer.

Local co-ordinates.
Image www.srrb.noaa.gov

The Zodiac is a region on the Celestial Sphere along the ecliptic. It is divided in twelve roughly equal parts by the 12 Zodiac constellations that play an important role in Astrology.

12 constellations of the Zodiac
Image www.psylentharmony.net

Ångström, Anders J. (1814-1874)
A Swedish scientist famous for his work on spectroscopy. He worked on the Solar spectrum and recorded his measurements in units of 10-10 metre. This unit was named Ångström in his honour. Astronomers still frequently use this non-SI unit for wavelength.


Anders Jonas Ångström