White Dwarfs

Where do White Dwarfs Come From?

Where a star ends up at the end of its life depends on the mass it was born with. Stars that have a lot of mass may end their lives as black holes or neutron stars. A low or medium mass star (with mass less than about 8 times the mass of our Sun) will become a white dwarf. A typical white dwarf is about as massive as the Sun, yet only slightly bigger than the Earth. This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars and black holes.

Black Hole	Neutron Star	White Dwarf

Medium mass stars, like our Sun, live by fusing the hydrogen within their cores into helium. This is what our Sun is doing now. The heat the Sun generates by its nuclear fusion of hydrogen into helium creates an outward pressure. In another 5 billion years, the Sun will have used up all the hydrogen in its core.
This situation in a star is similar to a pressure cooker. Heating something in a sealed container causes a build up in pressure. The same thing happens in the Sun. Although the Sun may not strictly be a sealed container, gravity causes it to act like one, pulling the star inward, while the pressure created by the hot gas in the core pushes to get out. The balance between pressure and gravity is very delicate.
When the Sun runs out of hydrogen to fuse, the balance tips in the favor of gravity, and the star starts to collapse. But compacting a star causes it to heat up again and it is able fuse what little hydrogen remains in a shell wrapped around its core.

(Betelgeuse) January 15, 1996, Hubble Space Telescope Captures First Direct Image of a Star, A. Dupree (CfA) and NASA.

This burning shell of hydrogen expands the outer layers of the star. When this happens, our Sun will become a red giant; it will be so big that Mercury will be completely swallowed!
When a star gets bigger, its heat spreads out, making its overall temperature cooler. But the core temperature of our red giant Sun increases until it's finally hot enough to fuse the helium created from hydrogen fusion. Eventually, it will transform the helium into carbon and other heavier elements. The Sun will only spend one billion years as a red giant, as opposed to the nearly 10 billion it spent busily burning hydrogen.
We already know that medium mass stars, like our Sun, become red giants. But what happens after that? Our red giant Sun will still be eating up helium and cranking out carbon. But when it's finished its helium, it isn't quite hot enough to be able to burn the carbon it created. What now?
Since our Sun won't be hot enough to ignite the carbon it its core, it will succumb to gravity again. When the core of the star contracts, it will cause a release of energy that makes the envelope of the star expand. Now the star has become an even bigger giant than before! Our Sun's radius will become larger than Earth's orbit!
The Sun will not be very stable at this point and will lose mass. This continues until the star finally blows its outer layers off. The core of the star, however, remains intact, and becomes a white dwarf. The white dwarf will be surrounded by an expanding shell of gas in an object known as a planetary nebula. They are called this because early observers thought they looked like the planets Uranus and Neptune. There are some planetary nebulae that can be viewed through a backyard telescope. In about half of them, the central white dwarf can be seen using a moderate sized telescope.