Of all the Deep Sky Objects visible in large amateur telescopes 8" and up, globular star clusters are among the most rewarding.
There is nothing quite so spectacular as a globular star cluster in which the brightest stars can be seen individually. We call that resolving the star cluster. The larger the telescope aperture the fainter the stars that can be seen.
For instance the greatest globular star cluster in the north is M13, otherwise known as the Great Hercules Star Cluster because it's located in the constellation of the Greek hero. It appears as a single dim star to a sharp eyed observer. In binoculars it appears as a tiny fuzzy spot. In telescopes up to 6 inch diameter it appears as an increasing large and fuzzy blob. Starting with an 8 inch diameter telescope more and more stars appear superimposed on the haze of the rest of the star cluster. It makes quite a nice 3-D illusion because the bright stars appear in front of the ball of haze. The larger the telescope the more stars appear.
We call these clusters globular because there is another type of cluster. Globular clusters are, lets face it, big globs of stars. There are tens of thousands, up to a million stars in these clusters. The other kind of star clusters are open or galactic star clusters: Open, because the stars in then are rather widely spaced, compared to globulars, with hundreds of stars maximum. Galactic, because they appear mostly near the hazy band of the Milky Way galaxy.
Actually both types are members of the Milky Way. The open clusters lie along the galactic disk, while globulars are arrayed in a spherical cloud centered in the galaxy's nucleus. Two kinds of star clusters, and two kinds of distributions. Why?
In a word: age.
Young whipper-snappers, open clusters lie along the galactic disk, where star formation is occurring now. As a rule of thumb, the farther an open star cluster is from the center of the galactic plane the older it generally is. I'll get to how we can tell how old star clusters are at another time. Cluster M67 in Cancer is an example of an old open cluster which lies 1,500 light years off the galactic plane, and is perhaps 10 billion years old, comparable to the ages of globulars. While open clusters can be a few million years old, or possibly less. There are no young globulars. They just aren't making any anymore.
So what does that tell us? Globulars clusters are an artifact of the early days of the life of the galaxy, when it was a spherical cloud of hydrogen. As the cloud collapsed the globulars were born, and left out of the rest of the evolution of the galaxy. The collapsing protogalaxy found spin axis and formed a flat disk. The rest of the star formation now was confined to the disk, trapping the gas to create stars, the largest exploding as supernovae, spewing their cooked up heavy elements out to be recycled as more new stars, and now heavy solid bodies called planets.
In Genesis 3:19 it states: "for dust thou art and unto dust shalt return". While humbling, we can also take heart that the dust we are made of is star dust. But I digress.
The bright stars that are part of globular clusters appear to be very old. The are red giant stars, operating on helium as a fuel They also have very little of the elements heavier than helium in their atmospheres, which suggests that they were born out of hydrogen gas clouds not yet heavily contaminated by the remains of earlier supernovae.
The bright stars in most open clusters are blue-white or white, massive main sequence stars using hydrogen for fuel. The more massive a star the brighter it is and the shorter the lifetime on the main sequence.
The chart above shows the globular clusters in the Messier catalog. Note how the general trend of the clusters is to cluster around of the center of the Milky Way, which is located in Sagittarius.
For the telescope cluster M22 may be the easiest to resolve. And located off the lid of the Teapot asterism of Sagittarius, it is easy to find. A 6 inch diameter telescope can begin to resolve it. Ultimately though, M13 is the more spectacular of the two, because M13 appears to be more dense in the center.
Whether you can resolve them or not, the globular clusters of summer are a treat to find and study.
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