June 15, 2012
The Milky Way, our galaxy, is one of hundreds of billions (or trillions — who knows?) of tiny islands of stars sprinkled throughout the vast cosmic ocean of space.
Seen from the top, galaxies are often shaped like flat spirals — children’s pinwheels of uncountable stars. Seen from the side, galaxies look much like lenses bulging at the center and tapering to points at the edges. Most of a galaxy’s stars are spread throughout the lens-like structure, the galaxy proper.
Strictly speaking, galaxies are really more or less spherical. They are shaped more like balls than lenses.
Above and below the lens, filling in the sphere, is a sparse sprinkling of star balls called globular clusters. In artist’s renderings of the Milky Way, they look at bit like tiny bees swarming around the galactic hive.
The best of the globulars is visible right now in binoculars or a telescope. Look for M13 pinned to the chest of the mighty constellation Hercules.
First, you’ll have to find Hercules, of course. Look east, about half way up to the top of the sky. You’ll see a fairly faint four-star box, wider on the left than on the right, called the Keystone. Locate the top two stars of the Keystone.
In binoculars, sweep about 1/3 of the way from the left star toward the right. The little, fuzzy lint ball is the globular cluster M13.
That’s about what it looked like to Edmund Halley (you know, the comet guy), when he discovered it in 1714. Halley’s telescope was so small that all he saw was a little fuzz ball.
Later that century, William Hershel, British astronomer and large-telescope builder, used one of his behemoths to estimate the number of stars in M13 at 14,000. People thought it was a misprint. Surely, he had meant to write 4,000.
More than a century later, Harold Shapely counted 30,000 stars. As the telescopes got bigger, the estimates got larger.
The bottom line: You are looking at over one million stars although you can’t possibly see them all. A small telescope will begin to resolve the outer stars of the ball. A larger one will dazzle your eyeballs with countless points of light resolved to the core of the cluster. (And that, by the way, is yet another reason to attend one of our Friday-night programs at Perkins.)
Globulars tend to be denser with stars at their center. As we move toward the edge, the stars are less tightly packed together. The reason a small telescope can’t resolve the stars at the core of a globular is that they are too close to each other. From our vantage, you can’t see the stars on the side of the ball facing away from us. The stars look like they are on top of one other.
That feature is, of course, an illusion caused by our distance from M13. The cluster is perhaps 26,000 light years distant — out at the fringes of our galaxy. (One light year is equal to about six trillion miles.) That’s a heck of a lot of zeros, Earthlings.
M13 is about 35 light years wide, small by comparison to the Milky Way galaxy, which is 100,000 light years wide from one end of the pinwheel to the other.
It is huge by earthly standards, however. Our planet is about 8.5 light minutes from the sun, making M13 over two million times wider than the earth/ sun distance.
As a result, M13 is not all that crowded, even with a million stars to work with. On average, each cubic light year of space is occupied by only one star. Even in the most closely packed places in the universe, the stars are still like dust.
Saturn is still high in the south just after dark. The star above it is Gamma in the constellation Virgo.
The great planetary conjunction of 2011 continues and yet the world has not ended in a fiery cataclysm (yet). Go figure. During predawn twilight, look very low in the east. Jupiter is highest and farthest to the right. Dim Mars is down and to the left. Much brighter Venus is farthest left.
Tom Burns is the Director of Ohio Wesleyan University’s Perkins Observatory. Readers can email him at firstname.lastname@example.org.