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Beta Boötis

July 30, 2022

They may not look it, but the stars at opposite ends of the stick-figure outline of Boötes, the herdsman, are a lot alike.

The star at the bottom is yellow-orange Arcturus, one of the brighter stars in the night sky. The star at the top, Nekkar, looks only a few percent as bright. But that’s only because it’s more than six times farther away — about 235 light-years.

Boötes is in the west as night falls. Its outline resembles a kite or an ice cream cone, with Arcturus at the bottom of the cone, half-the way up the sky. Nekkar — also known as Beta Boötis — is far to its upper right.

Both stars are in a final phase of life. They’ve “burned” all the original hydrogen in their cores, causing their outer layers to puff up. That makes them giants. Arcturus is a little bigger, but Beta Boötis is a little hotter, so their true brightness is about the same.

Two things that aren’t the same are their mass and age. Beta Boötis is only about 300 million years old, versus seven billion years for Arcturus. But Beta is three times heavier than Arcturus. That extra mass made the star consume its hydrogen much more quickly. As a result, it lived a much shorter “normal” lifetime than Arcturus.

The stars face the same fate. Eventually, they’ll blow their outer layers into space. That will leave only their hot but dead cores — white dwarfs — the same fate that awaits the Sun in several billion years.

Script by Damond Benningfield

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