The Final Stage – all about mass

Stars leave the Main Sequence when they stop fusing hydrogen in their core. As we saw, the lifetime on the Main Sequence is strongly dependent on the mass of the star. 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 of only millions of years. More average stars like our Sun fuse their hydrogen slower and stay in the main sequence for several billion years. Still smaller stars like Red Dwarfs can stay on the main sequence for trillions of years.

Source: P. Armitage

But what happens when the hydrogen runs out and they can no longer fuse hydrogen into helium to maintain the equilibrium between radiation pressure and contraction due to gravity? This too is determined by the mass of the star.

Brown Dwarfs less than 0.08 M_sun.
Although some of these “failed stars” may be big enough to fuse deuterium (a heavier isotope of hydrogen) into helium in some stage of their life, that will stop and they will cool gradually over trillions of years.

Red Dwarfs less than 0.1 M_sun
They fuse hydrogen into helium while they are on the Main Sequence for trillions of years, but at some point their hydrogen will run out. But they are not big enough to fuse the helium into any other elements and they will end up almost entirely consisting of helium. Then they will slowly cool off. Because Red Dwarfs live so long, their decay has never been observed.

Low mass stars less than 0.8 M_sun
They will swell up to a Red Giant star when the hydrogen outside the core starts to fuse (see below) but they are not hot enough to fuse helium. After the Red Giant stage they will collapse into a White Dwarf that is rich in helium.

Low mass stars less than 8 M_sun
These stars (including our Sun) will go through a full Red Giant stage producing elements up to carbon and oxygen. They cannot fuse carbon and once the fusion stops they produce a planetary nebula while the core collapses to a White Dwarf that is rich in carbon and oxygen.

If the star has a mass of about more than 4 M_sun the core temperature will be high enough for the fusion process to continue to produce Oxygen, Neon and Magnesium. The White Dwarf that then forms will be rich in these elements.

Massive stars more than 8 M_sun
They will swell up to a Super Red Giant star and the fusion process can successively produce higher elements such as oxygen, sulphur and silicon. The more massive stars will fuse elements all the way up to Iron. Beyond this element, fusion cannot sustain itself as it will require additional energy. Once fusion thus ends the star will collapse to a supernova and the remaining core will either become a neutron star or a black hole.

The boundaries between these categories are not sharp and astronomers find many stars that have complex properties across some of these categories.

Below we will discuss the final stage of two main stages of post-main sequence stars in more detail.