Many summers ago as I stood at the counter at Khan's scopes waiting for Ray to figure out how much my latest purchase would set me back, when I overheard a couple of customers talking about M13 and how spectacular it was. The tone of their voices and Ray's approving nod suggested that it might be worth a look. I had seen globular clusters before and they were interesting to look at, but 'spectacular' did not spring to mind when describing their attributes. That weekend I travelled to Mount Forest and set up my scope in the usual fashion, and scanned the sky for the constellation Hercules. As this was my pre-Telrad, and pre-setting circles phase, I simply eyeballed the approximate location of M13 in the 'keystone' of Hercules. A peek through the eyepiece proved that this object is not hard to find, as this blue shimmering ball of symmetrical stars filled the field of view of my scope's eyepiece. After 5 minutes of being frozen with awe, I decided to drop in my 8.8mm ultra wide angle eyepiece in order to observe the core of this huge globular. It was like staring into the embers of a brilliant blue camp fire, millions of stars seemed to leap out of my eyepiece in a celestial dance of light.
Globular clusters are the oldest known objects in our galaxy and once ruled the Milky Way numbering in the thousands. These stars formed early (some say before) in the formation of our galaxy. Today there are only a few hundred left as over the millennia they have been destroyed by collisions with each other or with our galactic centre. It is believed that there are no young globular clusters around as the primordial conditions that formed them have long since passed in the Milky Way. The conditions do seem right in our neighbouring Large Magellanic Cloud galaxy as its globular cluster NGC 1818 is believed to be only 40 million years old; a baby compared to the 12 billion year olds in our own galaxy.
M13 (NGC 6205) is by and far the largest globular cluster visible to the amateur astronomer. At a magnitude of 5.8, this star cluster is estimated to be 24 billion years old, 150 light years in diameter, and a distance of approximately 22800 light years from earth. M13 is composed of more than a million stars that are packed together at a concentration of 1 star per 100 cubic light years at the outer regions, but may be as high as 3-30 stars per cubic light years at the core. This concentration is 500 times greater than stars found within our galactic disk. If the earth orbited a star in the core of M13, its nearest neighbours would be a few light months away. The sky would be filled with evenly distributed stars so bright that there would be no such thing as night.
M13 was discovered in 1714 by Edmond Halley who noted it as a naked eye object in a moonless sky. It was later included in Charles Messier's catalogue around 1781 where it obtained its current, and most popular name. Messier commented that M13 appeared as a round nebula without any star (as Homer Simpson would say: doh!). In 1915, Harlow Shapley's research on M13 and other globular clusters rocked the astronomical community with findings that suggested that our sun was not at the centre of the Milky Way galaxy as was the belief at the time(first it was Copernicus who dethroned the earth as the centre of the solar system, now it was Shapley who wanted to dethrone the sun form the centre of the galaxy; will these 'troublemakers' ever stop?).
Shapley believed that due to the great distances of globular clusters from earth(based on his calculations of M13's distance), they were not like the open clusters found within the disk of the Milky Way, but must be located just outside our galaxy. He knew that our galaxy had the shape of a flattened disk with a central bulge, and believed that these globular clusters were still bound by its gravitational pull. Since the pull of galactic gravity would be even along its spherical shape, Shapley postulated that globular clusters should be evenly distributed throughout the galactic halo. This theory flew in the face of what could be observed, as globular clusters tended to be more numerous (91%) in the southern horizon (around Sagittarius and Scorpius, now known as the direction of our galaxy's centre) than they are in the northern horizon. Aesthetically, Shapley could not accept the notion of a lopsided galaxy therefor he decided to see what would happen if he theoretically moved the sun away from the centre of the galaxy as in the model below. With the sun moved to one side of the galactic plane(b), both the expected distribution and the observed distribution of globular clusters were in agreement with one another.
In 1974, M13 was chosen as the site for the first radio message intended to reach intelligent exrta-terrestrial lifeforms. Arecibo Observatory sent this message via their large radio telescope which will take 23,000 years to reach M13. It will take that long again for a return message to reach earth (assuming anyone recieved our message, 46000 years is one heck of a lag time).
Since last summer my appreciation for globular clusters has grown. In context of a giant like M13, and subtle beauties like M4, I have come to see how these 'grandfathers' of our galaxy can be seen as spetacular. All in all, I am forced to conclude that astronomy is as much a philosophic endevour as it is a scientific pursuit. It is not only what we see, but what we think about what we see that challanges our imaginations, modifies how we percieve ourselves, and the world around us.
