Late summer star clusters


So did you catch the Perseid meteor shower? I must confess that the annual event always draws me toward the darkest rural sky that I can get to. It involves long hours of waiting for a payoff right before morning twilight, but I stare up anyway in infinite fascination all night long. Why?

During the late summer, our galaxy, the silvery band of light called the Milky Way, stretches from horizon to horizon, starting from the south and ending in the northeast. By late August, the sky is good and dark by 9:30 p.m., so you can see the silvery traces of our galaxy without blowing an entire night’s sleep. Unfortunately, as with the Perseids, you’ll still have to find an observing site far from city lights. Hey, what’s a little drive time compared to the splendor of our galaxy?

There’s much to see if you enhance the view with a small telescope or binoculars. The silvery band explodes into uncountable stars. I’ve spent many a night with a large telescope gathering dust beside me as I scanned the Milky Way in binos from horizon to horizon over and over again.

But don’t stop there. The Milky Way is also studded with star clusters and their cosmic progenitors, the gaseous nebulae in which stars are born.

A quick scan across the Milky Way provides a primer on the early life of stars. Start by buying a decent map of the stars. I’ll wait.

Ready? Now drive to a dark, rural sky. Looking high in the southeast just after dark. The brightest star in that direction is brilliantly white Altair in the constellation Aquila, the Eagle.

Follow the string of stars that form the eagle’s body down and to the right, and you’ll see three fainter stars of the constellation Scutum, the Shield. To the right of the bottom-most star in Scutum will be two small hazy patches visible in binoculars. The top fuzzball is called the Eagle Nebula or, in the parlance of astronomers, M16. Below it is the more-visible Swan Nebula, or M17.

The Eagle isn’t much to look at in binoculars. The Swan, however, is a sight to behold. It resembles a checkmark in a small telescope.

They are both gaseous nebulae, enormous clouds of glowing hydrogen gas. In clouds like these, stars are condensing out of the primordial as you read these lines. In fact, in perhaps its most famous image, called “The Pillars of Creation,” the Hubble Space Telescope managed to capture that magical moment when stars begin to emerge from the parts of the cloud that gave them birth. A similar cloud gave birth to our own sun and some of the bright, nearby stars that can be seen with the unaided eye.

We can see these exceedingly thin clouds of hydrogen precisely because they have young stars inside of them. The stars ignite the gas around them, much in the same way that a spark of electricity ignites the gas in a fluorescent light bulb. Since the Swan Nebula is a mind-altering 40 light years across, you are looking at an area of fluorescence 240 trillion miles wide!

These clouds contain enough hydrogen to give birth to thousands of stars. Since the nebulae are relatively small compared to the Milky Way, the stars that are born there are relatively close together to start with. Stars are born in litters, like puppies, and early in their lives they huddle together close to their cosmic mother.

When a nebula gives birth to many stars, it uses up much of its available hydrogen gas. What remains of the gas is often blown outward into space by the powerful explosions of the young stars. As a result, you’d expect to find some parts of the Milky Way that are dense congregations of stars without any surrounding hydrogen gas.

And that’s exactly what you’ll find when you look down and to the left from the top star in Scutum. Binoculars will show a grainy patch of light called M11, or the Wild Duck star cluster. In a small telescope, the area expands into a gorgeous, tightly packed grouping of stars. A fainter cluster, called M26, is visible down and to the left of the middle star in Scutum.

The Wild Duck contains more than 1,000 stars all at a distance of 5,000 light years. The entire cluster is 50 light years across, about the size of the gaseous nebula that gave birth to the cluster. You are seeing young stars, newly freed from their nebulous star mom.

Most of the stars of the rest of the Milky Way are much more distant from each other than the stars found in clusters. As they grow older, they drift apart and find their own places in our galaxy, leaving us with the Milky Way galaxy, a sight of unparalleled splendor. You don’t need a meteor shower as an excuse to go outside and look.
Tom BurnsStargazing

Tom Burns is director of the Perkins Observatory in Delaware.

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