Star*Points for July, 2014
Globular Star Clusters

Last month we talked about how stars are regularly found to exist in clusters. We focused on the ?open cluster? type in which groups of stars form, disperse, and then go on their merry way. Now we?ll look at the other major class of star clusters, the ?globular clusters.?

The globular?s name comes from their appearance:  They look like a big glob of stars. By big I mean they are very large in comparison to open clusters.  The greatest globular clusters are estimated to contain several million stars, whereas the most populous open clusters contain merely thousands.

The name has a bearing on their shape as well: The clusters are spherical, like a globe.  They are big round, and nearly symmetrical big balls of stars. They?ve always reminded me of the popcorn balls my grandmothers used to make at Christmastime.  In that model, each kernel of popcorn represents a star in the cluster.  The cluster?s stars are bound by gravity such that, unlike open clusters, their stars do not disperse over time.

Globular clusters contain some of the oldest known stars.  According to the theory of their formation, the globular clusters were some of the first stars to form.  Whereas newer stars in the open clusters are being formed from the dust and metals produced in earlier generations of stars, globular clusters have stars that are ?metal poor,? an indication of greater age. 

Young stars and open clusters exist in the spiral arms of our Milky Way galaxy.  The globulars, on the other hand, are located closer to the galaxy?s central hub, the so-called halo region. There are some 150 known globular clusters in the Milky Way.  Therefore, globular clusters are more commonly viewed in the summer when we are looking toward the center of the Milky Way in the southern night sky.

Typically, the larger the galaxy, the more globular clusters it contains.  The largest galaxies may have globular clusters numbering in the thousands.  

The Milky Way?s largest globular cluster is Omega Centauri, in the southern constellation Centaurus.  At its highest, it just barely peeks above the horizon in Westminster making it difficult to locate.  The best views I?ve enjoyed of Omega Centauri  were from the tropics.  Places like South America, Mexico, and aboard cruise ships in the southern Caribbean Sea in places including such the Windward Islands -- such as Grenada , and the Netherlands Antilles.

For those of us in the temperate zone of the northern hemisphere the foremost globular cluster is Messier 13 in the constellation Hercules.  M13 was discovered in the 17th century by Sir Edmund Halley of Halley?s Comet fame.  It?s easy to locate with binoculars and a star chart once you know how to locate Hercules? four-star ?keystone? asterism.  The globular is located between two of the stars.  In binoculars M13 looks like a fuzzy star.

Another fairly easy to find globular is M22 just off the lid of the teapot asterism in the constellation Sagittarius.  Again, a simple star chart or online guide showing the constellation as well as the globular cluster will help you locate it with binoculars.  

A small telescope is all you need to actually resolve the brighter globular clusters into individual stars.  When I was in high school, the first ?deep sky? object I saw through my telescope, which wasn?t an ordinary star or planet, was the globular cluster M3 in Canes Venatici, the hunting dogs.

One good thing about globular clusters is that their compact size gives them good surface brightness making them relatively easy to locate from moderately light polluted areas.  When I first moved to Maryland I lived in light polluted Montgomery County for two years.  During the summer I used to observe globular clusters from my south facing balcony with a small telescope.