White Dwarfs III

A couple of our stellar neighbors are ancient. They were born more than 11 billion years ago. And for most of that time they’ve been “dead” — they’re corpses known as white dwarfs.

A white dwarf is the final stage for a star like the Sun. When the star can no longer produce nuclear reactions, it ejects its outer layers, and its core collapses to about the size of Earth.

The tiny white dwarf is still quite hot. In fact, astronomers determine the age of a white dwarf by measuring its temperature and mass. They know the rate at which a white dwarf of a given mass cools, so taking its temperature tells them how long it’s been a white dwarf.

The nearby white dwarfs — they’re roughly a hundred light-years away — are much cooler than the Sun. One of them is about two-thirds the mass of the Sun, while the other is about three-quarters the Sun’s mass.

Those numbers tell us that the smaller white dwarf began life at about twice the mass of the Sun, about 12 billion years ago. It became a white dwarf roughly a billion years later. So it’s spent the last 11 billion years as a corpse.

The heavier white dwarf started at three times the Sun’s mass, at about the same time as the other. It shed its outer layers in just a quarter of a billion years, leaving a stellar corpse: a white dwarf.

One of those neighbors lines up in front of the Pleiades, which is high in the sky at nightfall. The other one is below the bowl of the Big Dipper.

Script by Damond Benningfield