July 25, 2011
Our Milky Way galaxy is often described as a flattened disk of hundreds of billions stars. That description leaves out some of our galaxy’s most interesting parts.
Hovering above and below the main disk are the suburbs of our galactic city — 150 or so globular clusters of stars. Along with some stray stars and occasional gas molecules, globular clusters are the main constituents of what is more properly called the “galactic halo.”
The best of the globulars visible from the northern hemisphere is aptly named the “Great Globular” although most TN’s (telescope nuts) refer to it by its catalog number, M13. It can be found fairly easily right now in binoculars in the constellation Hercules, but don’t expect to see much except for a small, round fuzzy patch.
Remember, our planet is located in the main disk of the galaxy. M13 is out there in the suburbs. At 25,000 light years, or 150,000,000,000,000,000 miles away, even a cluster of a million stars doesn’t look like much. However, even a small department-store telescope will begin to resolve M13 into stars around its edges. In an eight or ten-inch diameter amateur telescope, the cluster resolves into stars across its entire surface. In a telescope as large as the one at Perkins Observatory, M13 looks like a diamond dust piled on the sky, a stunningly complex and indescribably beautiful explosion of stars. You should come to one of our Friday-night public programs in August and check it out.
The oddest thing about M13 is that it took humans so long to discover and catalog it. It is first mentioned by Edmund Halley (of Halley’s Comet fame) in 1715, more than 100 years after Galileo used the first astronomical telescope. However, Halley himself notes that M13 is visible to the unaided eye “when the sky is serene and the moon is absent.” To that we must add that you’ll have to be far from city streetlights under a dark, rural, sky. Countless observers must have seen M13 before 1715 without taking any special note of it, but we city dwellers can’t see it nearly so well.
If globular clusters existed in isolation from their parent galaxies, we might even think of them as mini-galaxies. Imagine one million stars in a tightly packed ball, dense at the center and gradually less so at the edge. Of course, by Milky Way standards, globulars don’t amount to much. The disk of our galaxy is about 100,000 light years wide. (One light year is approximately equal to 6 trillion miles.) The part of M13 we can see in small telescopes is perhaps 100 light years wide, but it probably extends to a diameter of 200 light years. A million stars is a lot to pack into such a relatively small space. Near the edge of the cluster, the stars are probably several light years away from each other.
That distance compares favorably with the distance from star to star in Earth’s section of the Milky Way. The nearest star to our sun is about four light years distant.
Near the center of M13, the stars are very tightly packed, at least for stars, at roughly a tenth of a light year apart. Before you conjure up a picture of stars crashing into each other, consider that one tenth of a light year is still 600 billion miles, which gives the stars plenty of room to move around without coming even remotely close to each other.
In his “Celestial Handbook.” Robert Burnham creates a visual model of M13 to give some sense of the emptiness. He reduces each star to the size of a grain of sand. On that scale, the stars would be distributed in a spherical space 300 miles in diameter. Even in the dense center of the cluster, the stars/grains would be separated from each other by almost a mile. Even at the center of globular clusters, among the most crowded places in the universe, the stars still live in lonely but magnificent isolation.
Saturn continues to sink in the west as darkness falls in the evening. Better get to a Friday night program at Perkins Observatory if you still want to see it! Call 740-363-1257 for more information about our Friday-night public sessions. Jupiter is still a morning object. Look low in the east around 4 a.m. for the brightest object in that direction.
Tom Burns is the Director of Ohio Wesleyan University¹s Perkins Observatory. He can be reached at firstname.lastname@example.org