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Find some of the oldest stars in globular clusters

Greetings, Stargazers.

I need to make myself a set of cheat cards for my favorite objects in the sky so I can answer questions in more detail at the next star party. I have to admit that I rely on readily-available online information sources far too much to remember specific facts about specific objects from one star party to the next.

I do try to restrain myself, but if one of my students asks a very specific question, I tend to respond by saying, “What, you don’t know how to use Google?” Or maybe, “Can someone with a smartphone help us out here – how far away is the Hercules cluster?”

At the recent Mesa Verde star party, the visitors were much more self-organizing than I have seen before. Quiet, orderly, parallel lines of people formed in front of the various telescopes, and as far as I could tell, the lines kept moving until everyone had a view through all the telescopes. At previous public star parties, I have typically focused the telescope on a variety of different objects, but this time I started out on M13, the globular cluster in Hercules, simply because I think it looks cool and didn’t think anyone else was starting with it. I ended up staying there all night until the crowds dissipated.

M13 is short for Messier 13, or the 13th object on Charles Messier’s list of things that weren’t comets. (Not being a comet is the only thing objects on the list have in common.) Questions I got included how far away is it? How many stars are in M13? How close are the stars? In general, I know that all globular clusters are tens of thousands of light years away, contain hundreds of thousands of stars and are dozens of light years in diameter.

I was afraid that information was a bit too general, and I knew that better answers were available somewhere, so for those specific questions, here is my cheat sheet for M13:

M13 is 22,000 light years away and is 11.65 billion years old. It has a total estimated mass of 600,000 suns and a radius of 84 light years. That would make the average density of stars inside M13 roughly 100 times that of the region around the sun, and much higher in the center of the cluster.

Other facts that I could share: Globular clusters are named that because they look like a glob of stars. All are fairly spherical and appear to get denser and denser as you look toward the center. Globulars are generally outside of the plane of the Milky Way and form a spherical halo surrounding its center. They are made up of some of the oldest known stars. These clusters formed while the matter that became the Milky Way was still being pulled together by gravity. Once the stars in these clusters formed, the clusters continued in their elliptical orbits about the center of mass of the Milky Way. There are roughly 180 globular clusters orbiting the Milky Way. This number is only an estimate because the view in some directions is blocked by interstellar gas and dust.

This month

You can find Hercules in the sky by looking on the line between Vega and Arcturus. Those are the two brightest stars in the sky right now, with Vega almost directly overhead and Arcturus near the western horizon. Find them early in the evening, as Arcturus will be setting shortly after 9 p.m. Between those two bright stars are two constellations. Corona Boreales, the Northern Crown, is a bit closer to Arcturus, and Hercules is a bit closer to Vega. Hercules is a large constellation that, I think, requires a bit of imagination to see the mythical hero. However, the torso of Hercules is right on the line between Vega and Arcturus and is a nearly-trapezoidal pattern of four stars called the keystone. M13 is along the western edge of the keystone. On a moonless night, you can see the magnitude 5.8 M13 with unaided, dark-adapted eyes. With binoculars you can see what looks like a fuzzy cotton ball, and through an 8-inch telescope, you can see many individual stars.

Jupiter and Saturn are still great binocular and telescope targets. The planet Uranus is at opposition this month but only visible as a tiny blue dot. It is barely visible to the naked eye in the constellation Aries, but you should use an astronomy app, or a current star chart, to find it since it isn’t near any obvious pointer stars.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.

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