The second type of object to have the unfortunate name of ‘degenerate star’ discussed in this three part series is the white dwarf. The white dwarf is of great interest as it will be the end state of our own sun.
The Sun shines because it is constantly converting hydrogen into helium in its center. This thermonuclear reaction takes in hydrogen as ‘fuel’ and releases helium + light as the products. For the past 4.6 billion years the Sun has rather dutifully operated in this way. In the far future the Sun will run out of hydrogen fuel and just stop functioning, analogous to what happens to a car when it runs out of gas.
At this point a star like the Sun will ‘panic’ and flail about for a hundred million years trying to re-establish again a source of power in its center. It will find that by increasing the pressure of the gas near the center it will be able to start using helium (the ‘waste’ of hydrogen fusion) as a new fuel to produce carbon as the product. This type fusion factory will operate for a while, until the amount of useable helium fuel is gone.
Following this chapter in the Sun’s life, the sun will panic again, but this time to no avail. It will lose all its outer layers to outer space, revealing its bare core. This core of nuclear ‘waste’ is very hot, about 10,000 degrees or more, and will slowly cool down over time.
It is this hot core that is referred to as the white dwarf. A white dwarf is no longer a proper star undergoing nuclear fusion but instead is more like a giant white hot coal in space.
Like a coal, it will mostly be made of carbon, but it will be a coal the size of the Earth! Once a coal is this big it will be hard to sustain itself under its own weight. It will be so heavy it will start collapsing, until the powerful force of the electron repulsion presses outward to combat the inward pull of gravity.
This outward force, called ‘electron degeneracy pressure,’ will hold the white dwarf up. It is the fact that this object sustains itself by degeneracy pressure that also explains how it got the name of the ‘degenerate’ star (and not for any morally reprehensible activity on its part)!