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The producers: now playing: the original stars in nature's longest-running hit

Natural History, April, 2002 by Richard Panek

And now it's springtime for galaxies and nebulae!

No, this isn't just one more gratuitous magazine reference to the biggest Broadway hit in decades, a pathetic attempt to sprinkle a little show-biz glitter on a thoroughly unglamorous topic. Well, maybe it is, but at least it's not entirely gratuitous or pathetic. Springtime, in fact, provides one of the best seasons for sky watchers to go exploring outside the Milky Way Galaxy. And what they'll find there--galaxies, globular clusters, and other cosmic exotica--ain't exactly chopped liver.

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As this column explained back in September 2001, our home galaxy is in the shape of a disk. Proportionally speaking, it's flatter than a pancake--if you consider that pancakes really aren't all that flat. They have something inside, only the Milky Way is thick with stars instead of heated batter. Now imagine you're standing on a microscopic crumb inside that pancake, about two-thirds out from the center. What would you see if you looked toward the center, along the disk? If you were inside a galaxy instead of a flapjack (which, fortunately, you are, at least according to the latest astronomical observations), what you'd see is the dense, seemingly impenetrable spill of stars and dust in our night sky that, somewhat confusingly, we also call the Milky Way.

Now imagine you're looking away from the plane of the disk--straight "up" or "down," so to speak. You'd still see stars, the ones that lie between out solar system and the "top" or "bottom" of the galaxy. But they would be nowhere near as numerous as those you saw when you were looking directly along the disk's edge. These stars wouldn't appear densely packed at all. In fact, what you'd mostly see would be the gulfs between the stars, the black expanses that serve as our windows on the rest of the universe.

Not that those gaps are exactly empty. As I said in that same September column, our galaxy seems to be surrounded by something that astrophysicists call dark matter--the unknown material that's detectable only indirectly, through its gravitational effects on "normal" matter (which is the kind we can detect directly). It's the presence of this kind of material that challenges traditional definitions of what galaxies are and how they look and where they extend. Hence the title of that earlier article: "Milky Way Mystery."

Since then, the plot has only thickened--and so has our galaxy. This past January, a team of astronomers announced that they've discovered what they termed a "corona" of hot gas surrounding the Milky Way. This is hot the mysterious dark matter. Nor is it the previously identified "halo" that reaches about 5,000 to 10,000 light-years in every direction from the plane of the galaxy; for years astronomers have speculated that this halo, in part a repository of gas from the Milky Way's exploding stars, operates on the same principle as the water cycle on Earth, seeding future generations of stars.

The corona, however, is wholly other. It serves as an outer atmosphere of sorts for the Milky Way, encapsulating it in a "shell" at least 100,000 light-years thick. Astronomers detected its existence by observing hydrogen clouds (perhaps gas left over from the formation of the galaxy) that are barreling into it at nearly a million miles per hour and, in the process, being heated to between 100,000 [degrees] and 1,000,000 [degrees] F. In other words, the Milky Way might still be growing. As one astronomer observed at the press conference announcing the discovery of the corona, "Any formation models of galaxies will now have to include this."

None of this activity will affect your view out of the galaxy. The galactic corona and halo can be detected only through telescopes that operate at wavelengths that the eye doesn't--ultraviolet and X-ray, for instance--and dark matter isn't observable in any band of the electromagnetic spectrum. So when you look through the "top" or "bottom" of the galaxy--the parts that swing overhead on spring and fall nights--your window on the rest of the universe will be clear.

To experience in a particularly dramatic fashion the feeling of being on the inside of the galaxy looking out, try observing in the first hours after nightfall this month. If you can find a spot where the horizon is unobstructed in every direction, you'll see a whitish wash entirely encircling you. That's the disk of the galaxy, flat as the proverbial pancake, and you're part of it. Now look up--and therefore out. Equipped with a sky chart and binoculars or a telescope, you'll have more galaxies to choose from than you could possibly manage to see in one night.

What's true of our own galaxy applies to all those other galaxies as well. They're seething with unseen activity, they're bigger than they appear at first sight, and they might be constantly replenishing themselves. They are, in a word, producers--and you can see them on any clear night, no lines, no waiting, for free.

The producers: now playing: the original stars in nature's longest-running hit

Natural History, April, 2002 by Richard Panek

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