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September 17, 2011

Clues to the birth of our home galaxy, the Milky Way, are found in its oldest stars — stars that formed when the galaxy itself did. More than a hundred thousand such stars congregate in a globular cluster in the faint constellation Aquarius. Its stars have much less iron and other heavy elements than the Sun has, indicating that they formed quite early in the galaxy’s history — before many other stars had enriched the galaxy with these elements.

M2 is 38,000 light-years from Earth, but it’s bright enough to see through binoculars. It’s in the southeast at nightfall, and wheels high across the south later on. It looks like a fuzzy patch of light in the northwestern corner of Aquarius.

Because of the cluster’s great age, all of its bright, heavy stars have long since expired. So the brightest stars in M2 today are yellow and orange giants — stars that were once like the Sun, but that expanded and brightened as they consumed the nuclear fuel in their cores.

Unlike most stars in the galaxy, those in M2 don’t belong to the Milky Way’s flat disk. In fact, the disk didn’t even exist when M2 was born. Instead, M2 is part of the galaxy’s halo — a shell of stars that surrounds the disk. M2 itself lies an incredible 22,000 light-years south of the galactic plane.

That’s why the cluster appears in Aquarius, which is far from the band of light we call the Milky Way. It’s a reminder of what our galaxy was like at its birth — 13 billion years ago.


Script by Ken Croswell, Copyright 2011


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