DSO of the Month
Messier 13
AKA: Great Hercules Globular Cluster; NGC 6205
Due south at 00.04 (BST) on 16 June
Position: 16 hr 41.7 min 36 deg 27 min 37 sec
Image of M13 by HAS Member Martin Gill. Used with permission.
For the most part, in this first year of Deep Sky Objects of the Month, I have been discussing bright and easy to find objects. I promise to make them harder and more esoteric next year! Few objects are brighter or easier to find than the Great Globular Cluster in Hercules, but as the sky hardly gets dark at this time of year, that is just as well. If Spring is the season of galaxies, Summer can be considered to be the season for globular clusters. Globular clusters are relatively rare, they are more common than planetary nebulae but not as numerous as galaxies or star clusters. If one looks at a list of the 25 brightest globular clusters in the northern sky, all but two of them are Messier objects, but five of them have a declination of -30 degrees or below making them very challenging indeed. Eight are in Sagittarius, six are in Ophiuchus, three in Scorpius, two in Hercules and one each in Serpens Caput and Capricorn. Three is visible in the summer, although not technically in a summer constellation, namely M3 in Canes Venatici, M 15 in Pegasus, and M 2 in Aquarius. This makes a grand total of 24 summer globular clusters out of 25. The missing one is M68 in Hydra and the 26th brightest one, M107, is also visible in the summer.
Globular clusters are dense clusters of stars which are located in the Milky Way’s halo—rather than the main disk—and are among the oldest stars in the galaxy. Their formation is still rather mysterious, but their stars are usually (if not invariably) the same age which suggest that they were formed at the same time. In a way, they can be considered to be mini-galaxies. If they are mini-galaxies, one would expect they would contain intermediate mass black holes and there is some evidence for them (e.g. in M15), but the issue is not yet settled as they are difficult to detect. As they are made up of very old stars, they do not contain significant amounts of elements other than primordial hydrogen and helium. Their nature as tight clusters of individual stars was not recognised until the invention of the telescope as they cannot be resolved with the naked eye. The first globular cluster to be identified was M22 in Sagittarius by the German amateur astronomer Abraham Ihle while observing Saturn in1665. However the individual stars in a cluster were not resolved until Charles Messier observed M4 almost a century later in 1764.
The Hercules globular cluster was first observed by Edmond Halley (of comet fame) in 1714, having already discovered the brightest globular cluster Omega Centauri from the island of St Helena in 1677. He reported that “This is but a little Patch, but it shews it self to the naked Eye, when the Sky is serene and the Moon absent.” Obviously the sky in early eighteenth century Oxford was darker than in early twenty-first century Havering. Messier added the cluster to his list in 1764, but it was not resolved into individual stars until 1779. Like most globular clusters it is very distant, being 25,000 light years away. It lies between the very bright stars Vega and Arcturus, being somewhat closer to Vega. It is on the western edge of the famous Keystone asterism, and is closer to Eta Herculis than Zeta Herculis. Surprisingly, it is only the fourth brightest cluster in the northern sky with a magnitude of 5.8; it benefits from its position high in the sky—M22 and M4 are very low and M5 is only slightly brighter.