Moon and Spica

Two big stars with quite different futures trail the Moon across the sky tonight. Their light blurs together into a single point — Spica, the brightest star of the constellation Virgo. They’re below the Moon at nightfall.

Spica consists of two stars that are locked in a tight orbit. They’re separated by just a few million miles — far closer than Earth is to the Sun.

One of the stars is at least 10 times as massive as the Sun. That great heft squeezes its core, revving up the rate of nuclear reactions. So it will live a much shorter life than the Sun will — in the tens of millions of years, versus 10 billion years for the Sun.

When that lifetime is over, the core will collapse to form a neutron star — a ball of matter about twice as massive as the Sun, but only as big as a mid-sized city. The star’s outer layers will be blasted into space, forming a supernova. For a few days or weeks, it’ll shine as brightly as billions of normal stars.

Spica’s other star is only about six or seven times the Sun’s mass. It’s still a monster as stars go. But it’s not heavy enough to trigger a supernova explosion. Instead, the star will expel its outer layers in a much gentler process. Its core will become a white dwarf. It’ll be roughly as massive as the Sun, but a little bigger than Earth — many times larger than a neutron star.

Neither star is near its end, though, so Spica should continue to shine brightly for a long time to come.

Script by Damond Benningfield