The Andromeda galaxy is our nearest big galactic neighbor. In fact, it’s bigger than our own galaxy, the Milky Way. Despite its vastness, though, the last time anyone saw a supernova explode there was more than a century ago. But astronomers recently discovered a star in Andromeda that seems destined to explode — in a million years.

The star is a white dwarf — the small, dead core of a once-normal star. A white dwarf explodes if it gets too heavy — in particular, when it gets more than 1.4 times the mass of the Sun.

This white dwarf already becomes a nova at least once a year. A nova is a much less dramatic flare-up than a supernova. It doesn’t get as bright as a supernova, and it doesn’t destroy the star. But the white dwarf is already close to the critical mass of 1.4 Suns. As a companion star dumps more material onto it, the white dwarf is getting heavier. It should exceed the critical weight limit in less than a million years, and this periodic nova will become a supernova.

Actually, if this estimate is right, then the star has already exceeded the weight limit and exploded. We haven’t seen it yet because the Andromeda galaxy is two-and-a-half million light-years away, so its light takes two-and-a-half million years to reach us. So it’s possible that the star blasted itself apart more than a million years ago. Now, we just have to wait for its light to reach Earth, allowing us to see a supernova flare up in the Andromeda galaxy.

Script by Ken Croswell, Copyright 2017