Photograph courtesy NASA/Tod Strohmayer (GSFC)/Dana Berry (Chandra X-Ray Observatory)
When they reach the end of their long evolutions, smaller stars—those up to eight times as massive as our own sun—typically become white dwarfs.
These ancient stars are incredibly dense. A teaspoonful of their matter would weigh as much on Earth as an elephant—5.5 tons. White dwarfs typically have a radius just .01 times that of our own sun, but their mass is about the same.
Stars like our sun fuse hydrogen in their cores into helium. White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel.
Fusion in a star's core produces heat and outward pressure, but this pressure is kept in balance by the inward push of gravity generated by a star's mass. When the hydrogen used as fuel vanishes, and fusion slows, gravity causes the star to collapse in on itself.
As the star condenses and compacts it heats up even further, burning the last of its hydrogen and causing the star's outer layers to expand outward. At this stage, the star becomes a large red giant.
Because a red giant is so large, its heat spreads out and the surface temperatures are predominantly cool, but its core remains red-hot. Red giants exist for only a short time—perhaps just a billion years compared with the ten billion the same star may already have spent burning hydrogen like our own sun.
Red giants are hot enough to turn the helium at their core, which was made by fusing hydrogen, into heavy elements like carbon. But most stars are not massive enough to create the pressures and heat necessary to burn heavy elements, so fusion and heat production stop.
Such stars eventually blow off the material of their outer layers, which creates an expanding shell of gas called a planetary nebula. Within this nebula, the hot core of the star remains—crushed to high density by gravity—as a white dwarf with temperatures over 180,000 degrees Fahrenheit (100,000 degrees Celsius).
Eventually—over tens or even hundreds of billions of years—a white dwarf cools until it becomes a black dwarf, which emits no energy. Because the universe's oldest stars are only 10 billion to 20 billion years old there are no known black dwarfs—yet.
Estimating how long white dwarfs have been cooling can help astronomers learn much about the age of the universe.
But not all white dwarfs will spend many millennia cooling their heels. Those in a binary star system may have a strong enough gravitational pull to gather in material from a neighboring star. When a white dwarf takes on enough mass in this manner it reaches a level called the chandrasekhar limit. At this point the pressure at its center will become so great that runaway fusion occurs and the star will detonate in a thermonuclear supernova.
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