White Dwarfs

So, we've discussed what happens to stars that have masses up to 8 solar masses. They shed their outer atmosphere and become planetary nebulae. However, what happens to the core? Well, their core turns into an object known as a white dwarf. White dwarfs are the remaining cores of stars, composed of electron-degenerate matter and have masses comparable to their original stars. Electron-degenerate matter is essentially electrons that create their own pressure to counter the pressure of self-gravity. White dwarfs are mainly made up of elements that aren't heavily consumed during hydrogen burning in stars(like carbon, oxygen). White dwarfs with far higher masses may contain more elements down the periodic table like neon and magnesium along with the two mentioned previously. Once a star becomes a white dwarf, it no longer has a source of energy because it can't undergo nuclear fusion. Thus, electron-degenerate pressure is required so that a white dwarf doesn't simply collapse in on itself.

However, electron degeneracy can only do so much. If the star during its lifetime had a mass above 1.4 solar masses, known as the Chandrasekhar Limit, then the dwarf's electron degeneracy still won't be able to hold its self-gravity and the white dwarf will collapse, becoming either a neutron star or even a black hole. However, stars with masses under the Chandrasekhar Limit will stay as white dwarfs much longer, but not technically forever as seen later in the next paragraph.

For stars below the Chandrasekhar Limit, they don't stay as white dwarfs. Remember, white dwarfs don't have an energy source as they can no longer undergo core nuclear fusion. All of their energy comes from the kinetic energy(motion) of atomic nuclei within the core. This atomic motion also gives the white dwarf its luminosity and temperature on the surface. However, the nuclei within the core don't fuse so eventually, the white dwarf radiates out all the heat(and thus, light) from its surface making it cool and dim out gradually. Over time, the excess carbon atoms in the star cool down to form a diamond lattice, making it a diamond star essentially. But, the endpoint of a white dwarf is when it turns(theoretically) into a black dwarf. A black dwarf is basically a white dwarf except it no longer emits sufficient light or heat and is composed primarily of elements like carbon, oxygen, and nitrogen. However, black dwarfs are purely theoretical as of now because the time it is estimated for them to form far exceeds the current age of the universe.