Stellar Evolution Page 4

Main Sequence Stars

Very much of  the further life cycle of the star depends on how massive the star is. The mass of a star determines the  temperature of its photosphere, and this in turn determines its luminosity, or energy output.

Astronomers use the Hertzsprung-Russell diagram where known stars are plotted according to temperature and luminosity. A diagonal band across the diagram, called the Main Sequence, contains all stars that are fusing hydrogen into helium, as all stars do during most of their lifetime. The young star now has taken up a position in the main sequence, according to its temperature and luminosity.

Hertzsprung-Russell diagram plots stars according to temperature (horizontal)
and luminosity (vertical).

Source http://qbx6.ltu.edu/s_schneider/

Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun. However, it is all relative: the Sun is ten times brighter than a star half its mass. The more massive a main sequence star, the brighter and bluer it is. For example, Sirius (spectral class A1), is more massive than the Sun
(spectral class G2), and is noticeably
bluer. Spica (spectral class B1) is about ten times more massive than the Sun, and has a luminosity 2,300 times that of the Sun. On the other hand, Proxima Centauri (spectral class M5), our nearest neighbour, is less massive than the Sun, and is thus redder and less luminous.

Luminosity is the same as the power (energy per second) of a light bulb that we express in Watt.

Astronomers use the term luminosity and express it in comparison to the luminosity of the Sun (Lsun).
So a luminosity of 100 Lsun  is a power of 100 times that of the Sun.

Read more about luminosity, spectral class and the Hertzsprung-Russell diagram in our Module “Stellar Radiation”.

All stars have a limited supply of hydrogen in their core, and therefore have a limited lifetime as main sequence stars. This lifetime is strongly dependent on luminosity or mass. Massive stars have a very high rate of fusion and emit a very large amount of energy, and therefore have a relatively short life span. More average stars like our Sun fuse their hydrogen slower and stay in the main sequence for several billion years. For a much more massive star, that period is only millions of years (see diagram).

The expected lifetime of our Sun is 10 billion years. The age of our Solar System is 4.5 billion years so we are now about half way of its total life time.

Dwarfs in various colours

The Main Sequence stars with lowest mass are called Red Dwarfs.  The oldest Red Dwarfs must have formed in the early Universe and still have a
long way to go. Therefore the end stage in the life cycle of Red Dwarfs has
never been observed.

Proto stars that never became large enough to sustain a fusion reaction in the core
may end up as a Brown Dwarf. Generally anything between the size of 15 and 75
times the size of Jupiter is considered a Brown Dwarf, sometimes called a “Failed Star”.
They can radiate some energy due to contraction, but are hard to see.
Only nearby Brown Dwarfs are visible (see Image Tour Orion Nebula on the previous page).

White Dwarfs are an end stage of medium sized stars as we will see later in this Module.