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Moon and Regulus

April 16, 2016

Getting close to someone can sometimes lead to a broken heart. For a star, getting close can lead to a shrunken heart.

Consider Regulus, the brightest star of Leo, the lion, which stands just above the Moon as night falls. What we see as Regulus is a big, bright star that spins so rapidly that it looks like a flattened beachball. It has a tiny, invisible companion — the core of a dead star that’s only about half as massive as theory says it should be. The quirks of both stars can be explained by their closeness — they’re only about 30 million miles apart.

When the system was born, the companion was the heavier of the two stars. As a result, it burned through its nuclear fuel faster, so it “aged” more quickly. As it neared the end of its life, it puffed up to giant proportions. It then began to dump some of its gas onto the surface of the star we see as Regulus today. That caused Regulus to spin much faster — so fast that it bulges out at the equator.

Eventually, the dying star lost all of its outer envelope of gas. That should have left a hot, dead core more than half as massive as the Sun. Instead, though, Regulus just kept pulling material away from the core. That left the core with only about a third of the Sun’s mass.

So today, the Regulus system consists of one star that whirls like mad, and another that’s nothing more than a shrunken heart.

We’ll talk about the Moon and another bright light tomorrow.


Script by Damond Benningfield

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