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!

Globular Cluster Photo Series (Part 3): M13

Today we're taking a look at another of the biggest and brightest globular clusters in the sky, Messier 13, also known as the Great Globular Cluster in Hercules. (It's located around the area of his kidney. We can but hope it's not a Herculean kidney stone!) It is nearly the same size as Omega Centauri, about 170 light years across. However, it has only a 3^rd as many stars, about 300,000.

Messier 13, the Great Globular Cluster in Hercules. Click for larger image.

Although it's as large as Omega Centauri, M13 appears 2/3 as large on the sky because it is almost twice as far away; 25,000 light years compared to Omega Centauri's ~16,000. (Its apparent diameter is 20 arcminutes compared to Omega Centauri's 36.3, so M13 is about 2/3 the size of the full Moon on the sky.)

Other than that, I don't have too much to say about M13. It, Omega Centauri, and another cluster called 47 Tucanae are generally considered the best and most beautiful globular clusters. 47 Tucanae is just below the horizon from Hawai‘i, so I won't be showing you any pictures of it. In terms of visual impressiveness it will probably go down from here since most globular clusters are either smaller or further away (or both) than these three, leading them to appear smaller on the sky.

Of course, one reason I started this project was to catalog the differences between the many Milky Way globular clusters. On the face of it they're not particularly interesting, big balls of stars held together by self-gravitation. Just as all stars are big balls of gas held together by self-gravitation. And yet, just as there is almost infinite variation among stars, I'm already starting to see differences just between the three globular clusters I've shown here. Some are larger, some smaller, some very tightly packed, some less so. My goal for this series is to get a sense for that same variability found in globular clusters.