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Moon and Regulus

February 18, 2019

When you look into the night sky, all of those little pinpoints of light look pretty much the same. A few are brighter than the others, and a few show some color. But otherwise, there’s little to tell them apart.

From up close, though, it’s a different story. Consider Regulus, the bright heart of Leo, the lion. It rises below the full Moon this evening and follows the Moon across the sky later on.

To the eye alone, Regulus is simply an especially bright star. But telescopes reveal that Regulus is actually two stars — the big, bright one that we can see, plus a tiny stellar corpse known as a white dwarf. And the bright star is quite different from our own Sun; instead of being nice and round, it’s about a third bigger through the equator than through the poles.

That shape is caused by the star’s rotation: It makes one full turn every 16 hours, compared to almost a month for the smaller Sun. That rapid spin may be the result of interactions with its tiny companion.

The companion probably began life as the bigger and heavier star. Because of its greater mass, it aged more rapidly, puffing up to giant proportions. As it grew, it began dumping gas on the bright star we see as Regulus. That made Regulus heavier, causing it to spin faster. Regulus eventually took most of the other star’s gas, leaving a small white dwarf — and a star system that’s quite different from most of the others that twinkle through the night sky.

Script by Damond Benningfield

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