White Dwarfs

Three illustrations showing white dwarfs 

orbiting each other and then colliding.

Our Sun is said to be a main sequence star, meaning that it fuses hydrogen to form helium. What happens when it exhausts its supply of hydrogen fuel? Once hydrogen fusion stops, there is not enough pressure on the star's core to create a fusion reaction with helium. Because there is no longer any outward push from fusion, the star begins to collapse from the crush of gravity. This collapse create more and more pressure in the core until helium fusion begins, while some of the remaining hydrogen burns outside of the core. The products of helium fusion is carbon and oxygen. The star expands and becomes a red giant. Its life as a red giant is brief on an astronomical scale—only a few million years. It will then eject its outer layers as it reaches the end of its burn cycle, leaving behind a collapsed core known as a white dwarf surrounded by a planetary nebula. 
  A white dwarf is the final evolutionary state of about 97% of all the stars contained in the Milky Way. White dwarfs are very dense—they pack a mass comparable to that of our Sun into a volume similar to the size of our Earth. A white dwarf is composed mostly of the fusion products of helium: carbon and oxygen. It is also surrounded by a thin layer of helium, and sometimes hydrogen. Fusion quickly stops so the white dwarf has no source of energy and will cool over many billions of years, eventually to the point where it no longer shines at all.
  Astronomers have recently found two white dwarfs stars orbiting one another once every 39 minutes. Binary white dwarfs are exceedingly rare. Out of the 100 billion stars in our galaxy, only a few dozen have been found. Located about 7,800 light-years away in the constellation Cetus, these two stars orbit each other at a distance less than that from the Earth to the Moon.
  The fate of these stars is already determined. Because they orbit one another so closely, they produces gravitational waves, ripple-like distortions in spacetime that were predicted by Albert Einstein in his theory of general relativity. The gravitational waves carry away orbital energy, which has put the stars in an ever-tightening spiral. In about 40 million years, they will crash into one another and merge to form a new star. If their combined mass were more they would collide and turn supernova, but since they aren't massive enough, they will be reborn and begin to shine once again.