Moon and Regulus

Most of the stars that fill the Milky Way galaxy are in the prime of life. They are steadily “fusing” together hydrogen atoms in their cores to make helium, the next-heaviest element. In astronomical jargon, stars in that phase of life are on the main sequence — a description that comes from a simple plot of a star’s brightness and temperature.

When you plot a lot of stars, most of them lie along the same diagonal line — the main sequence. Hot, bright stars are at one corner of the plot, with cool, faint stars at the opposite corner.

One prominent star that’s on the main sequence is Regulus, the brightest star of Leo, the lion. It follows the Moon across the sky after they rise late this evening.

Regulus actually consists of two stars that are locked in a tight orbit around each other. One of the stars is well past the prime of life. But the other — the one that’s visible to the eye alone — is in its prime.

Regulus is on the bright, hot side of the sequence — it’s hundreds of times brighter than the Sun, and thousands of degrees hotter. And that tells astronomers a lot about the star. It’s several times the Sun’s mass, for example. And since heavier stars consume their hydrogen at a rapid rate, Regulus will spend a much shorter time on the main sequence than the Sun will.

So astronomers can learn a lot about what’s going on inside a star, how long it will live, and more — just by plotting the star’s place on the main sequence.

Script by Damond Benningfield