StarDate: July 30, 2009

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A globular cluster is an impressive sight: A tight-packed agglomeration of hundreds of thousands of ancient stars, held close to one another by the force of their own gravity. What may be the closest globular cluster to Earth appears in the south on summer nights. But you need binoculars to see it.

This globular cluster is named M4, because it's the fourth entry in a catalogue compiled centuries ago by French astronomer Charles Messier. Like several other globulars, M4 resides in Scorpius, the scorpion. That's because many globular clusters lie closer to the center of our Milky Way galaxy than the Sun does, and when we look at Scorpius and neighboring Sagittarius, we're looking toward the galaxy's hub.

To professional astronomers, globular clusters are more than just a pretty sight. They preserve clues to the Milky Way's early history. Many globular clusters formed when the galaxy itself did, 13 or 14 billion years ago. In fact, M4 was born so long ago that it acquired few of the heavy elements that make life possible on Earth -- elements like oxygen and iron.

To find M4, look south as darkness falls for the bright red star Antares. Tonight, it's not far to the left of the Moon. Look at Antares through binoculars, then scan for a fuzzy glow just to its right -- M4. Antares is about 600 light-years from Earth, while M4 is more than ten times farther. Even at that great range, though, M4 may well be the closest globular cluster to Earth.

Script by Ken Croswell, Copyright 2009

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