by Robert J. Vanderbei
Our sun is just one of hundreds of billions of stars in our galaxy, the Milky Way.
A galaxy is an interesting thing. At the center of most large galaxies is a supermassive black hole. Many galaxies, such as our own, have spiral arms, places where the density of the stars is greater than in the surrounding areas.
There is also a great deal of dust, which makes for many interesting nebulae that we can see—and photograph—in our night sky.
In addition, we find so-called globular clusters of stars associated with galaxies. These are swarms of stars that are mutually gravitationally bound to each other, with the entire group loosely bound to our galaxy.
Our galaxy is home to more than a hundred such clusters of stars.
A picture of M13 taken through a four-inch refractor telescope.
Photograph by Robert J. Vanderbei
Over the course of a human lifetime, the stars in a globular cluster appear to remain fixed relative to one another. But if it were possible to observe such a cluster for millions of years, we would see that the stars are swarming around their common center, much like bees around a beehive.
For observers in the Northern Hemisphere, the most spectacular globular cluster is the Great Globular Cluster in the constellation Hercules, also known as the Hercules Cluster, or simply M13.
It consists of about 300,000 stars. Visually, in a small telescope the cluster looks like a faint, diffuse smudge. But in larger apertures under dark skies, the object reveals itself as a wonderful spectacle of stars of varying brightnesses and colors. The individual twinkling of thousand of small points of light in a beautifully symmetrical spherical glob is really something to behold.
M13 is about 145 light-years in diameter and about 25,100 light-years away from us here on Earth. It is just one (the best!) of several globular clusters than can be seen in the summer months. Other examples include M4, M5, M12, M53, M22, and M92.
Currently M13 rises in the northeast around the time the sun sets. It rises higher and higher in the sky as the evening progresses, passing almost directly overhead (for those of us in mid-northern latitudes) at about 2:30 a.m.
To find it, stand facing south (or southeast if it’s early evening) and look high in the sky. The brightest star you will see is Vega. The constellation Hercules will appear to the right (westward) of Vega.
The star chart shown here can help you locate the Hercules Cluster lying along the line connecting the two rightmost stars of the Hercules “keystone” asterism, about two-thirds of the way from the more southern of the two stars to the more northern one.
Robert J. Vanderbei is chair of the Operations Research and Financial Engineering department at Princeton University and co-author of the National Geographic book Sizing Up the Universe. Vanderbei has been an astrophotographer since 1999, and he regularly posts new images on his astro gallery website.