November 14, 2008

In Today's One Minute Astronomer...

M15 - A Dense Swarm of Ancient Stars

We've looked at 47 Tucanae, a jewel of an object accessible only to observers in the southern hemisphere. Today… Messier 15, a dense globular visible from parts of the south but best positioned for those north of the equator. The extreme concentration of stars near the center of this cluster suggests the presence of a black hole formed billions of years ago.

The Basics

• M15 is one of the easiest globular clusters to find. Just follow the last two stars in the nose of Pegasus, the winged horse, by about 4 degrees and there it is.

• You'll see M15 in a finder scope or a pair of binoculars. In a telescope, it looks very bright and dense near the middle with swirls of stars at the outer edges visible in an 8-inch or larger scope. But the high density of stars in the center make it quite different from other globular clusters.

• M15 is one of the oldest globular clusters: its stars are nearly 13.2 billion years old. This cluster lies more than 30,000 light years from Earth.

A Deeper Look

• Observations from Hubble show the core of M15 has collapsed into a central region packed with stars. Some astronomers believe a black hole lies at the center of the cluster, pulling in the constituent stars to a density much higher than other globulars.

• M15 is one of the few globulars with a visible planetary nebula, the 15th magnitude Pease 1. You can see it on some long-exposure photos. I'm not sure if anyone has seen it visually. A 15 to 18-inch telescope would be the minimal requirement to see this ancient dying star.

M15, found just off the nose of Pegasus

Good To Know

Because of the central black hole which helps concentrate the brightness in the center, M15 is a fine object for urban skies. You can see the effects of a black hole from your backyard!

Personal View

A note from last week's observing log: Very bright cluster! M15 is the opposite of more loosely aggregated clusters like M22 or even M13. Streams of stars at the edge, but core completely unresolved in 8-inch SCT. Definitely not round. No sign of the planetary nebula at edge, even with OIII filter.

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