Greetings stargazers.
While gazing at stars is fun, gazing at groups of stars is even more fun. The more the merrier. There are two types of star clusters that are popular observation targets with both amateur and professional astronomers. Those are open clusters and globular clusters. Stars inside open clusters are some of the youngest stars in the galaxy, while stars in globular clusters are some of the oldest.
Stars form when a large cloud of interstellar gas and dust collapses because of internal gravitational attraction. Inside the cloud, the bigger clumps collapse a bit faster, forming the hot blue, short-lived stars that are most easily seen in the night sky. The much more numerous, smaller clumps form the cooler, redder stars. The group of stars that remain after the entire cloud has been consumed or scattered away by radiation pressure is called an open cluster.
Stars in a brand-new open cluster show the full range of possible colors and sizes. The age of older clusters can be determined by what stars are missing from the distribution. The bright blue ones burn up their fuel the quickest, so they are the first to disappear. A very old open cluster might only have yellow, orange and red stars remaining.
Globular clusters are at the extreme other end of the age spectrum. These clusters formed at the same time as the Milky Way galaxy and are a bit like miniature galaxies themselves, containing hundreds of thousands of stars. However, they no longer have any active star forming regions and so consist of much older stars that are generally less massive than those found in the rest of the galaxy. Many have evolved toward the end of their lifetimes. These old stars include stellar giants and variable stars, and many star remnants, such as white dwarfs.
Globular clusters are too distant to make out individual stars without a telescope. Even through a telescope, most of them appear as fuzzy puffballs. Many globulars are visible through binoculars, and a few are even visible to the naked eye under very dark skies.
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The winter half of the Milky Way is setting shortly after it gets dark, and the much more obvious summer Milky Way is rising by midnight. All along the Milky Way, hundreds of open clusters can be seen through a small telescope. Scores can be seen with binoculars, and quite a few are visible to the naked eye. The Pleiades is a classic open cluster visible to the unaided eye, but right now it is setting at sunset. The brighter summer Milky Way has even more visible open clusters.
The distribution of globular clusters centers around the core of the Milky Way, which can be seen in Sagittarius. This brightest part of the Milky Way is now peeking over the eastern horizon before midnight, but by the end of the month, it will be rising at late twilight.
This month I have been working with several Fort Lewis College students to get the observatories on the roof of Sitter Hall and at the Old Fort Lewis tuned up and ready for remote data taking and in-person viewing. The Air Force’s Falcon telescope is scheduled for some upgrades, and we should be able to add that to the list of available research scopes. We also are trying to get a set of telescopes ready for visual use once public outreach star parties can be organized. Until then, look for star parties that typically get scheduled over the summer at Mesa Verde, Chimney Rock and the Durango Nature Center.
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.
