Gould’s Belt

Logarithmic History has had a lot of geology and biology lately, not so much astronomy. But all is not peaceful in the heavens.

Benjamin Gould is a nineteenth century astronomer who noted that a lot of bright stars in the sky — especially the bright blue stars that we know are very young — seem to fall along a ring tilted at a 20 degree angle to the Milky Way. This ring has come to be called Gould’s Belt. The Belt is an ellipse about 2400 by 1500 light years across where there has been a recent wave of star formation. Our Sun lies within the belt, somewhat off center; the center lies in the direction of the Pleiades.

The Belt began forming maybe thirty million years ago. We’re not sure what happened. A supernova may have set off star formation, but it would have to have been a huge one. Or it may be that a gas cloud or a clump of dark matter passed at an angle through our part of the Milky Way, and started stars forming with its shock wave. In any case, the Belt is one of the really striking features of our part of the Milky Way.

If you’re in the Northern hemisphere and look west after sundown tonight, you’ll see Venus and the Moon low in the sky, and the Milky Way above them. Gould’s Belt intercepts the Milky Way to the north and passes below it, through the bright stars of Perseus, Orion, and Canis Major and then disappears below the southern horizon. So tonight look at the stars, and drink a toast if you want to your ape ancestors who were just on the cusp of splitting off from monkeys thirty million years ago.

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