Introductory Astronomy: Main Sequence Stars

Stars along the main sequence (MS) all have one thing in common: they are all burning hydrogen into helium in their cores. This process is called fusion. Fusion creates heavier elements from lighter elements (for example, helium from hydrogen). Fusion also releases energy (photons) which travel outward from the core of the star (where the fusion is taking place) and eventually escape from the surface of the star (this is the light we see from the star). These photons exert a pressure on the stellar material as they travel outward from the core; this pressure is called Radiation Pressure.

In the Main Sequence Phase of a star's evolution, radiation pressure pushing outward exactly balances the gravitational pressure pulling inward (this balance is called Hydrostatic Equilibrium). Because these two forces are exactly in balance, the star is stable (this means it neither shrinks nor expands). A star will spend almost 90% of its lifetime on the MS. The lifetime of a star on the MS is determined by its mass. It turns out that the lifetime of stars on the MS is proportional to their mass to the -2.5 power. Therefore, more massive stars spend less time on the MS. For example, a 40 solar mass star will spend about 1 million years on the MS while a 0.5 solar mass star will spend 56 billion years on the MS. A star like our Sun will spend about 10 billion years as a MS star (and since it is already about 5 billion years old, the Sun is half-way through its MS life).

Stars along the MS also obey a mass - luminosity relation. This relation says that the more mass a star has, the greater the pressure needed to hold that mass up, and therefore the faster the fusion reactions need to occur. Therefore, higher mass stars on the MS fuse their hydrogen into helium more quickly than lower mass stars and have hotter cores that release more energy, which makes these stars more luminous. (This also explains why massive stars live shorter lives; they burn through their fuel more quickly.) So, as you move up the MS, you find stars of higher temperature and greater luminosity. The luminosity - radius - temperature relation also tells us that the stars higher up on the MS will have larger radii.

The more massive a star is, the stronger its gravity. The stronger the gravity of a star, the more compressed its core becomes and therefore the higher the temperature rises in the star's core. This means that it is easier for more massive stars to reach temperatures at which fusion can begin. (For hydrogen fusion to occur, core temperatures of 10 million K are needed!) This also means that if the mass of a protostar is below some limit, gravity will never raise core temperatures high enough for fusion to occur. In fact, stars with masses less than 0.08 solar masses cannot raise their core temperatures high enough to ignite hydrogen fusion. These stars are known as Brown Dwarfs. Brown Dwarfs are not in balance (hydrostatic equilibrium) because they have no radiation pressure to counter their own gravity. These stars contract slowly and never become Main Sequence stars.