| 1 | It orbits the "white dwarf" core of a star . | |
| 2 | One class of white dwarfs has almost pure helium atmospheres. | |
| 3 | Instead, they should eventually just cool to become white dwarfs and then black dwarves. | |
| 4 | Estimating how long white dwarfs have been cooling can help astronomers learn much about the age of the universe. | |
| 5 | White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel. |