For amateur astronomers, spring may be galaxy observing season, yet summer provides the opportunity to observe most of the globular clusters that inhabit the outer halo of our galaxy. Globular clusters are to me, the most beautiful objects to observe through a telescope as they are tight swarms of stars- containing well over 100,000 stars (and some many more!) all within an apparent diameter of a few arc-minutes. Many folks describe them as diamonds spilled on black velvet and that description is apt. At left is an image I took of the large globular cluster M 22 in Sagittarius last summer with my Canon T2i through my TEC 140. (It is a single exposure, not multiple exposures integrated)
These clusters are tightly bound by gravity which gives them a general spherical shape and a higher density of stars toward the center. There are upwards of 150 known globular clusters in our Milky Way galaxy. While they contain some of the oldest stars in the Universe, the origin and role of globular clusters in the evolution of galaxies remains unclear and this is an area of active research. Astronomers who study galactic evolution have been able to detect globular clusters in most of the galaxies they have studied, and some galaxies contain extraordinary numbers. For instance, the great Andromeda Galaxy has more than 500, and the giant elliptical galaxy M 87 in Virgo contains approximately 13,000! (See this post for my observation of the jet emanating from the center of M 87).
Last night I was out observing and spent quite a bit of time observing globular clusters in the constellation of Ophiuchus. There are 20 reasonably bright globular clusters in this constellation, most if not all of which should be visible in my 140mm refractor under reasonably dark skies. Among these globulars are 7 Messier objects, from M 107 which is the dimmest of the Messier globular clusters to the majestic clusters M 10 and M 12. After observing these, as well as M 62, M 9 and M 19, I slewed the telescope to M 14 and was surprised at what I saw.
I have observed all of the Messier objects at some point years ago, yet I am not sure that I have returned to this cluster since that time. The cluster, discovered by Messier in 1764, lies in a remote area of the constellation and at first glance the cluster is unimpressive. It is difficult to resolve many stars and appears more nebulous than cluster like...which is probably why it did not stick in my memory as a great visual target. More experienced now than I was those years ago, I began to wonder why this cluster appeared so large and reasonably bright in my telescope yet resolution of the stars was so difficult. A little reference checking and I learned that this globular contains several hundred thousand stars and has an absolute luminosity about 400,000 times that of our own sun! This cluster is about 30,000 light years away, and as it turns out, the light from the cluster is obscured by an extensive amount of dust in our galaxy. Below is my sketch from last night of M 14 at a magnification of 163x.
The cluster has a brighter central region with a fainter outer halo and it is difficult to resolve member stars (the sketch is generous in this regard). The cluster has a grainy appearance and with averted vision it does seem that some faint stars come and go. The cluster is approximately magnitude 7.6 and has an apparent diameter of 11 arc minutes. Again, with a cluster this large and bright one would expect to resolve stars. Often, it is not simply the aesthetics of objects in the eyepiece that make an observation interesting; it is knowing what it is that one is looking at. In this case, one of the most impressive facts about this cluster is that it has a diameter of approximately 110 light years. Our galaxy is approximately 100,000 light years across, so it would take only 1000 of these clusters to span the galaxy. that may not sound impressive until one considers that our galaxy has between 200 and 300 billion stars...