Main Sequence

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.


lightbulbLuminosity 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 EBook “Stellar Radiation”.


hrgenericsmlHertzsprung-Russell diagram plots stars according to temperature and spectral class (horizontal) and luminosity (vertical).
Astronomers use the Hertzsprung-Russell diagram in which stars are plotted according to temperature (horizontal and increasing towards the left) and luminosity or absolute magnitude (vertical). 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. It is the “adulthood” of the star.

Temperature is directly related to Spectral Class which has been explained in our EBook “Stellar Radiation”. We already introduced the HR-diagram there. The main classes of stars are from most luminous to faint, O,B,A,F,G,K,M.

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.

The O and B-type stars are the most massive and therefore the highest up (and furthest to the left) in the main sequence. They fuse hydrogen much faster than lower mass stars and are therefore on the main sequence for a much shorter time. O-type stars live only a few million years whereas M-type stars live up to 100 billion years. The bigger stars have a lot more hydrogen to fuse, but they do so much faster (much higher core temperature) and thus live much shorter.


The great thing about the HR-diagram is that once we have located a star in the main sequence, we know pretty much every other basic property of that star, such as mass, luminosity, size, temperature, colour and lifetime.


Please read our EBook “Stellar Radiation”, especially from page 13.
You can also watch this lecture on Main Sequence stars and the HR-diagram (first 15 minutes)


Further below in this EBook we will learn to use the HR-diagram to follow the entire life cycle of a star, according to its classification and we will go off the main sequence. This is another great advantage of the HR-diagram. But first we will explore the all important different end stages in the evolution of stars.


  Dwarfs in colours
Red Dwarf

The Main Sequence stars with lowest mass are called Red Dwarfs. They are on the Main Sequence because they are fusing hydrogen into helium, although much slower than any other type of Main Sequence star. 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. The Universe simply isn’t old enough for that.

Brown Dwarf 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. We can only hope to find Brown Dwarfs when they are close enough.
White Dwarf White Dwarfs are an end stage of medium sized stars as we will see later in this EBook.
Black Dwarf A Black Dwarf is the ultimate fate of a White Dwarf, when it has cooled off after trillions of years of inactivity, and it fades into oblivion.