Moon and Regulus

Regulus, the bright star that marks the heart of the lion, is a lot older than it looks. Until about a decade ago, astronomers estimated its age at no more than a hundred million years. But the discovery of a dead companion star changed things. Today, the best estimate is a billion years.

In most cases, astronomers determine a star’s age by comparing its mass, its temperature, and its total brightness. Regulus is several times as massive as the Sun, thousands of degrees hotter, and hundreds of times brighter. Crunch the numbers, and that indicates a vigorous young star.

In this case, though, the numbers don’t tell the whole story. That’s because Regulus has a close companion — a tiny stellar corpse known as a white dwarf. It’s so small, faint, and close to Regulus that it’s impossible to see directly. Instead, it was discovered with the help of sensitive instruments.

When Regulus and the companion were born, the companion was the heavier of the two stars. That means it aged more quickly. As it aged, it got much bigger, so it began dumping the gas in its outer layers onto the surface of the star we see as Regulus. That made Regulus much bigger and heavier. In turn, that revved up the nuclear reactions in its core, making the star bluer and brighter. That change disguised the star’s true age: a billion years or older.

Look for Regulus tonight just a whisker away from the Moon — a bright star that’s older than it looks.

Script by Damond Benningfield