The summer Milky Way is in the sky right after dusk, with the brightest parts toward the constellation Sagittarius. That means this is a good time of year to look for globular clusters.
The name “globular cluster,” coined by William Hershel, is very descriptive of their appearance – a glob of stars. They contain hundreds of thousands of stars in a very tight grouping that is typically only a few score light years across. There are fewer than 200 globular clusters known around the Milky Way, and unless you are familiar with specific ones, they all look very similar.
With binoculars, or low power telescopes, they look like little cotton balls. And with these modest instruments, you can see most of the Milky Way’s globular clusters that happen to be above the horizon. On a clear night, with dark-adapted eyes, you can see M13, the globular cluster in Hercules, with your naked eye. (I am sure that some of you with exceptional eyesight could see more of them if you had a good chart showing you where to look.) The bigger your telescope, the more stars you can resolve toward the center of a cluster.
Globular clusters are ancient, generally comprised of what are called Population II stars (old ones with fewer heavy elements). There are no longer any luminous, blue, main sequence stars, and the remaining distribution of star colors is noticeably redder than that found in open clusters in the spiral arms.
These clusters condensed while rest of the Milky Way was still forming. Rather than being in the disk, like most of the rest of the matter, these clusters reside in a spherical halo. Unlike objects in the disk, which have a mostly orderly rotation about the galactic center, the orbits of globular clusters are random.
It was the three dimensional distribution of globular clusters that Harlow Shapely used to find the center of the Milky Way. His 3-D map showed a spherical distribution, but our solar system was not anywhere near the center. Instead, the center was in the direction of the constellation Sagittarius. That is why this is a good time to look for these clusters.
Globular clusters are not unique to our galaxy. All large galaxies are observed to have globular clusters in a halo.
Look for Sagittarius in the southern sky right after dark. It contains the asterism, or pattern of stars, that looks like a teapot. The Milky Way is the steam coming up out of the teapot. Beside globular clusters in that region of the sky, you also can see numerous open clusters. I will have to talk about those much younger clusters in another article.
The annual Perseid meteor shower will peak between the nights of Aug. 11 and 12, so you might try looking both nights. The meteors will appear to originate from the constellation Perseus, which rises around midnight.
The next Durango Nature Studies new moon event is scheduled for Aug. 6. Join the Four Corners Stargazers to get more information about these events and other local star parties.
firstname.lastname@example.org Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.