In 1924, Mary Proctor viewed a globular cluster through the 40-inch refractor at Yerkes Observatory. She described it as "Myriads of glistening points shimmering over a soft background of starry mist, illumined as though by moonlight, formed a striking contrast to the darkness of the night sky. For a few blissful moments, during which the watcher gazed on this scene, it suggested a veritable glimpse of the heavens beyond...."
Her words describe what we have all seen before. But once we have done so, we think that there is not much more to view. They all seem the same, at least to the untrained eye. But there is much more to look at; they are not all the same. But first a little science.

THE SCIENCE OF GLOBULAR CLUSTERS

Globular clusters in our galaxy are almost as old as the galaxy. They were formed over 10 billion years ago--not long after the creation of our galaxy or the universe itself. They hold their shape because their stars are gravitationally attracted to each other.
The ones in our galaxy hold from 100, 000 to well over a million stars. These clusters generally fall within a sphere around the galaxy's core. The farthest ones from us are about 60,000 light years away, making them about 30,000 light years from the center. Their diameters range up to 100 light years across.
Almost 130 globular clusters have been identified in our galaxy. One could reasonably assume that another 100 exist, hidden by the dust and other material at the galactic center. Of the 130 known, 29 are Messier objects. Since these clusters are relatively near the center, it is not suprising that nearly 40 percent of the known ones, forty-nine, are located in either Ophiucus, Sagittarius or Scorpius.
What is known about globular clusters in our galaxy--particularly brightness, distance and variable stars within them--can be applied to globular clusters actually viewed in nearby galaxies. Astronomers use this information as a steppingstone in determining distances of galaxies and other objects further out in our universe. But for most amateurs, their greatest characteristic are their beauty. Here are a few things that amateur viewers can use to enhance their beauty and to show that they are not all the same.

DENSITY

Globular clusters vary greatly in the density of the stars within them. This factor and the visual magnitude of them determines how easily the individual stars within them are resolvable. Harlow Shapley devised a classification of these clusters using Roman Numerals I through XII. The clusters with the highest concentration of stars are classified as I while those with a very loose density with almost no central concentration are classified as XII at the other end.

M75 in Sagittarius has a I classification. It has a very rich central area. M2 in Aquarius and M80 in Scorpius are also very compact with a classification of II. . It takes as much power as your telescope allows to resolve any stars anywhere near their centers.

On the other end, M55, a class XI in Sagittarius, and two class Xs, M107 in Ophiucus and M56 in Lyra, are very loose and easily resolvable.

M13, the great Hercules Cluster, has an average density with a V classification. It is considered the best globular cluster to view at our latitude in the Northern Hemisphere. It has over a millions stars. Many are resolvable. The longer you look at it, the more stars you can see. It quickly takes on almost a three-dimensional view.

BRIGHTNESS, SIZE AND DISTANCE

How easy it is to resolve the stars in a globular cluster depends upon the cluster's visual magnitude. Since most to do not vary greatly in actual size, a cluster's brightness is proportionate to its apparent size and actual distance from us. Essentially, the closer a globular cluster is to us, the larger its apparent size and the greater its visual magnitude. And vice versa. However, not every such cluster follows this general rule.
The four furthest Messier globulars with related information are listed in Table 1.

These clusters are hard, if not impossible to resolve. Whenever trying to resolve a globular cluster, use as high a power eyepiece as your telescope and seeing conditions will allow. If you still can't resolve it into at least some individual stars, do not feel bad. Not all can be resolved with amateur scopes.
On the other hand, Table 2 lists the two closest Messier and the two brightest non-Messier globular clusters in the sky.

M4 near Antares is the easiest to resolve from our latitude. Anyone should be able to see huge numbers of individual stars. M22 would be the greatest such cluster for us to view from here were in not for the fact it is so low in the sky and it has all the foreground stars and dust to compete with near the Milky Way's galactic center. Even then it is quite a sight.
The two most impressive globular clusters, Omega Centari and 47 Tucanae, are easily seen with the naked-eye. Unfortunately, they can not be seen at our latitude, although Omega Centari can be seen in the southern states if you get a chance to be there during the spring. M13, M5 and M3 are considered to be the first, third and fourth best clusters to view in Central Illinois, even though they fall in the mid-range of distances from us. The more you continue to stare at each of these, the more individual sparkling gems you can see. They differ from the general rule mentioned earlier because of their actual size and the fact they are high above the horizon.

OTHER THINGS TO VIEW

Areas or lanes of dust have been reported in some globular clusters. Look carefully and see if you can see any. It usually takes high power and larger telescopes. I did note the often-described Y-shaped dust lanes in the east side of M13 in the 40 inch refractor at Yerkes Observatory, but have yet to find it in any other telescope. Such dust lanes are supposed to be visible in smaller scopes. Look closely away from the central core and see if there are small areas devoid of stars. Scientists are still not sure if the dust is within the clusters or in the foreground
Finally, look for strings of stars in many globulars. Sometimes you have to concentrate hard on the clusters until the line patterns appear to stand out. Other times the patterns are easily discernible. In M30 in Capricornus, two such strings are obvious which gives the cluster its unique look.
So the next time you look at globular clusters, look for the things that make them different rather than quickly looking at them and assuming they look the same. Then spend a little extra time to relish each cluster's unique beauty.

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