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Moon and Spica

December 22, 2016

Many of the stars in our galaxy have stellar companions — they’re gravitationally bound to one or more other stars. But just how many stars have companions is poorly understood. One study says about a third of the stars in the Milky Way are members of systems with at least two stars, while other studies say it could be half or more.

Something else that’s uncertain about these systems is how they form. There are a couple of leading ideas, and it’s possible that both of them are right.

All stars are born from the collapse of a giant cloud of gas and dust. As the cloud contracts, a dense clump forms in the middle, with a spinning disk around it. The clump forms the star, while the disk may give birth to planets.

But as the cloud collapses, turbulence may cause it to form more than one clump. Each clump then coalesces to form a star. That may be the best idea for systems where there’s some space between the stars.

A second idea says that as a star is taking shape, a second disk of material forms within the original disk. The second disk then collapses to create a second star that’s quite close to the first one.

One binary where the two stars are especially close is Spica, the leading light of Virgo. It stands to the right of the Moon at first light tomorrow. The bright planet Jupiter is above Spica. The Spica system consists of two massive, brilliant stars in a very tight orbit — stellar companions whirling through the Milky Way.


Script by Damond Benningfield


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