Moon and Spica

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Moon and Spica

A bright star that follows the Moon this evening shows how tough it is to know what’s happening inside a star.

Spica is the leading light of Virgo. It’s to the lower left of the Moon as night falls. It’s one of the brighter stars in the night sky, so astronomers have paid a lot of attention to it over the centuries. And they know a lot about it.

For example, they know that it’s a binary — two stars locked in a mutual orbit around each other. Astronomers can plot the orbit, which tells them how far apart the stars are. It also gives them the masses of the two stars. The bigger star — Spica A — is about 10 times the mass of the Sun. The other star isn’t quite as big, but it’s still impressive.

Studying the individual wavelengths of light from the stars reveals their composition and temperature. Combined with their masses, that tells us about each star’s phase of life.

Spica A is going through a transition. It’s completing its primary phase of life, the main sequence. That’s caused its core to shrink and get hotter, which has made the star bigger and brighter.

But astronomers aren’t sure exactly where it is in the transition. It could be just finishing it up. On the other hand, it could have finished the transition fairly recently, and is now moving full-on into the next phase of life — puffing up to become a giant. Continued studies might help resolve the question of what’s happening inside this big, bright star.

Script by Damond Benningfield

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