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A famous before-and-after star system shines at its best on winter nights. Tonight, it’s in the east as night falls, high in the south around midnight, and sets due west a little before dawn.

Procyon is a binary system — it consists of two stars. One of them is big and bright and is nearing the end of its “normal” lifetime. The other is tiny, faint, and dead — the same fate that awaits its brighter companion.

During their normal lifetimes, stars like those of Procyon “fuse” the hydrogen in their cores to make helium. When all the hydrogen is gone, they begin to fuse the helium to make heavier elements. Eventually, the star can’t continue that process. It expels its outer layers, leaving its hot, dense core — a white dwarf.

That’s what’s happened to the smaller star in the system, Procyon B. When it was born, almost two billion years ago, it was about two-and-a-half times the mass of the Sun. At that weight, it aged in a hurry. It became a white dwarf more than a billion years ago.

The star we see — Procyon A — was born with less mass. So it is spending a much longer span in the prime of life. But it appears to be starting to make the transition from fusing hydrogen to fusing helium. Over tens of millions of years, it’ll puff up to form a red giant. And after spending perhaps hundreds of millions of years in that phase, Procyon A also will become a white dwarf — the burned-out remnant of a once-brilliant star.

Script by Damond Benningfield

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