Tuesday, October 25, 2011

Globular Cluster Photo Series (Part 13): M92

Today I have an image of the globular cluster Messier 92 for your viewing pleasure. M92 is one of the more spectacular globulars in the sky, but is unfortunately outshone by the slightly more spectacular M13 with which it shares the constellation Hercules. M92 is smaller than M13 at 109 light-years across compared to its 170, but is at roughly the same distance, about 26,700 light years away (M13 is about 25,000). These two factors combine to give it an apparent diameter on the sky of 14.0 arcminutes, a bit smaller than M13's 20.

Messier 92 in Hercules.
M92 is a nice looking globular cluster, still large enough on the sky to look interesting, and fairly concentrated. M92 boasts one of the few eclipsing binary systems known in globular clusters. It has another, more interesting claim to fame, however. The Earth's spin axis slowly precesses over time, taking about 26,000 years to describe a large circle on the sky. (Think of a top slowly wobbling in a circle as it spins. It's the same physical principle.) Precession is the reason that Thuban (a star in Draco) was the North Star for the ancient Egyptians rather than Polaris like it is today. Anyway, in about 14,000 years precession would point the Earth's axis less than a degree away from M92, leading to M92 being a sort of North Cluster. For comparison, M92 is about a fourth of a degree across, so you can see just how close that would be. Polaris itself is about a degree from the North Celestial Pole, which is small enough that it doesn't matter for everyday navigation aiding.

Pretty interesting, no? A hui hou!

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