Observing Projects 13B Globular Star Clusters

While open clusters are visible year around, M13 and nearly all the globulars have a distinct viewing season. Since they are gathered around the galactic center they are most prominent when the center of the galaxy is in opposition to the Sun. This occurs in late June when the Sun is in Gemini and Sagittarius is at opposition. A majority of the globulars are located in Sagittarius and two adjoining constellations. So globular hunting is a very popular summertime activity for amateur astronomers.

M13 is not only the most beautiful of the northern hemisphere globulars but also one of the oldest structures easily visible to the amateur. It is some 14 billion years old and may contain as many as a million stars. M13 is very easy to find on the line connecting the two stars that make up the west side of the Hercules keystone. Note the color of the stars within the cluster. Nearly all are yellow and orange in color indicating that the stars within are very old main-sequence stars of what are called "Population II" stars. With globular clusters, use all the magnifying power the weather conditions will tolerate and see how close to the center of the cluster you can resolve individual stars. M13 is categorized as a category V globular, is heavily populated and very concentrated towards the center. Test the Shapely-Sawyer system out and see if you agree.

As summer moves towards its later months and the lackluster constellation Aquarius comes to the meridian at midnight, you will have your best chance to observe the tightly packed category II globular cluster M2. This cluster shines at magnitude 6.5 and spans 16 arc minutes across. At a distance of about 37,500 light years, the cluster is huge, spanning some 300 light years. The measure of how tightly packed this cluster is would be through the measure of half-mass radius. This measure for M2 is only 0.93 arc minutes. This means that in a globular cluster that spans 16 arc minutes (a radius of 8 minutes) half the mass of the cluster is compacted within less than one arc minute of the center. This in turn means that the cluster's tidal radius is enormous, about 233 light years. The cluster would likely capture any object passing within 233 light years of that center. You can see very easily that the structure of this cluster is very different from M13. Finding the cluster is very easy. Find Alpha Aquarii with an equatorially mounted telescope and scan west to find M2. It is 5 degrees due north of Beta Aquarii.

Nearly all the globulars in the heavens are in the half of the sky centered on the core of the Milky Way. One of the few that is not is M79. This globular in Lepus is also therefore one of the few that is best viewed in winter. The cluster is located some 40,000 light years from Earth, but 60,000 light years from the galactic center. The cluster is magnitude 7.7 and spans just a bit less than 10 arc minutes. This makes for a linear extension of 118 light years. The cluster is rated as a category V globular, the same as M13. We know that M79 is not a native to our galaxy but rather was a member of the Canis Major dwarf galaxy, a small irregular galaxy that was once a satellite of the Milky Way and is being gravitationally destroyed and consolidated by our galaxy.

Most visitors to Lyra go seeking the double-double Epsilon Lyrae or the famous Ring Nebula M57. But Lyra is also host to a faint, but interesting globular listed as M56. This little globular is graded as category X on the Shapely-Sawyer system. The cluster is barely three minutes across in most amateur telescopes and shines at magnitude +8.3. In larger instruments, the total extent of this cluster is about 9 arc minutes. The cluster is a long way from Earth at more than 34,000 light years.

These four Messier globulars show you the full range of qualities of globular clusters from large to small and from dense and compact to loose and sparse. All of these clusters are home to stars of extreme age, including many RR Lyrae type variables. Globular clusters teach us what the oldest stars in the universe are about.

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