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March 12, 2018

The proper names of the three brightest stars of Leo, the lion, all sound like destinations from a spy thriller: Regulus, Denebola, and Algieba. And the fourth-brightest star sounds pretty exotic, too: Zosma. It’s from a Greek words that means “girdle,” because the star marks the lion’s hip.

Unfortunately, though, the star itself isn’t all that exotic. It’s in the prime of life, so it’s fusing the hydrogen in its core to make helium. It’s the same phase of life the Sun is in, known as the main sequence.

But for Zosma, that phase won’t last nearly as long as it will for the Sun. That’s because Zosma, which is also known as Delta Leonis, is a bit more than twice as heavy as the Sun. And as a star’s mass increases, so does the rate of nuclear reactions in its core. So while the Sun will spend about 10 billion years on the main sequence, Zosma will stay there for only about one billion years.

And it’s getting toward the end of that span already. Astronomers estimate it will end its “prime” lifetime in just a few hundred million years. After that, it’ll puff up to giant proportions, and shine hundreds of times brighter than it is now — making the lion’s “girdle” a bit more exotic.

Leo is in the east as night falls, with Regulus, its leading light, about a third of the way up the sky. Zosma stands well to the lower left of Regulus, and directly above Denebola, the lion’s tail.

Tomorrow: counting galaxies.

Script by Damond Benningfield

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