The stars in the sky may seem ageless and unchanging, but eventually most of them will turn into white dwarfs, the last observable stage of evolution for low- and medium-mass stars. These dim stellar corpses dot the galaxy, leftovers from brightly burning stars.
Main-sequence stars form from clouds of dust and gas drawn together by gravity. How the stars evolve through their lifetime depends on their mass. The most massive stars, with eight times the mass of the sun or more, will never become white dwarfs. Instead, at the end of their lives, they will explode in a violent supernova, leaving behind a neutron star or black hole.
Smaller stars, however, will take a slightly more sedate path. Low- to medium-mass stars, such as the sun, will eventually swell up into red giants, eventually shedding their outer layers into a ring known as a planetary nebula (early observers thought the nebulae resembled planets such as Neptune and Uranus). The core that is left behind will be a white dwarf, a husk of a star in which no hydrogen fusion occurs.
Smaller stars, such as red dwarfs, don't make it to the red giant state. They simply burn through all of the hydrogen within the star, leaving behind the shell that is a white dwarf. However, red dwarfs take trillions of years to consume their fuel, far longer than the 13.8-billion-year-old age of the universe, so no red dwarfs have yet become white dwarfs.
When a star runs out of fuel, it collapses inward on itself. White dwarfs contain approximately the mass of the sun but have roughly the radius of Earth. This makes them incredibly dense, beaten out only by neutron stars and black holes. The gravity on the surface of a white dwarf is 350,000 times that of gravity on Earth.
White dwarfs reach this incredible density because they are so collapsed that their electrons are smashed together, forming what is called "degenerate matter." This means that a more massive white dwarf has a smaller radius than its less massive counterpart. Burning stars balance the inward push of gravity with the outward push from fusion, but in a white dwarf, electrons must squeeze tightly together to create that outward-pressing force. As such, having shed much of its mass during the red giant phase, no white dwarf can exceed 1.4 times the mass of the sun.
One last kick
While many white dwarfs fade away into relative obscurity, eventually radiating away all of their energy and becoming a black dwarf, those that have companions may suffer a different fate.
If the white dwarf is part of a binary system, it may be able to pull material from its companion onto its surface. Increasing the mass can have some interesting results. [VIDEO: Cannibal White Dwarf Feeds on Companion Star]
One possibility is that adding more mass to the white dwarf could cause it to collapse into a much denser neutron star.
A far more explosive result is the Type 1a supernova. As the white dwarf pulls material from a companion star, the temperature increases, eventually triggering a runaway reaction that detonates in a violent supernova that destroys the white dwarf. This process is known as a single-degenerate model of a Type 1a supernova.
If the companion is another white dwarf instead of an active star, the two stellar corpses merge together to kick off the fireworks. This process is known as a double-degenerate model of a Type 1a supernova.
At other times, the white dwarf may pull just enough material from its companion to briefly ignite in a nova, a far smaller explosion. Because the white dwarf remains intact, it can repeat the process several times when it reaches the critical point, briefly breathing life back into the dying star over and over again.