The Sciences

A box of jewels

Bad AstronomyBy Phil PlaitApr 18, 2006 1:58 PM


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No man is an island, as they say. But no galaxy is an island Universe either. That's what galaxies are sometimes called, when astronomers try to get poetic. A galaxy is really a collection of stars, gas, and dust, held together by their mutual gravity. Our Galaxy, called the Milky Way, is shaped like a vast, flat disk, like a dinner plate. Now take a scoop of mashed potatoes and plop it in the center. That's the central hub, a bulge of stars. Take a pea and place it halfway from the center of the plate to the edge. That's about where the Sun is, though on this scale the Sun would be submicroscopic: the Galaxy is 100,000 light years across, or about 1,000,000,000,000,000,000 kilometers. That's kind of a long way. That scale is dwarfed by intergalactic distance, too. The nearest big galaxy is about 2.9 million light years away. The most distant objects seen are over 13 billion light years away. But we're not alone in the dark. The Milky Way has several companion galaxies, smaller dwarf galaxies that are bound to us by our gravity. Two of them are easily visible to the unaided eye if you happen to live south of the Equator. They are called the Magellanic Clouds, because they were first noted (by Westerners I should add) by the explorer Magellan.

To the eye they are faint, glowing patches in the sky. With binoculars you start to see that they have shape, and contain bright spots of light. To the telescope, they reveal themselves to be galaxies in their own right: massive collections of stars and gas. They are not like the Milky Way. For one thing, they are actively and aggressively making stars; the rate of star birth in the Clouds is higher than in the Milky Way. Images of the Clouds reveal tremendous regions loaded with dense gas clouds. Stars are born in these giant gas clouds, and it's rare that they're born one at a time. Instead, they're created en masse, churned out thousands at a time. Sometimes, their own gravity keeps these newly born stars together, bound into what astronomers call open clusters. The image at the top of this page -- taken by Hubble, and released to the public just this morning-- is of one such cluster, called NGC 290. It's in the Small Magellanic Cloud, so it's about 200,000 light years away. At 65 light years across, it contains many thousands of stars. The brightest of these are already starting to die, and have turned red as they've expanded and cooled, preparing to explode someday as supernovae. Astronomers like open clusters for a lot of reasons. For one, all the stars are at the same distance, so if two stars appear to be the same brightness, it's because they really are the same brightness. This isn't always the case; when you see a bright star in the sky, it might be an intrinsically faint star, but be very close by, while a fainter-looking star might be a powerhouse located very far away. Another reason is that all the stars in a cluster were born at about the same time, so they are all about the same age. That means when we see difference between two stars, it's not because one is much older than the other. Third, the stars were all born from the same cloud, so they have about the same chemical content-- they have the same amount of hydrogen, helium, iron, calcium, titanium, and so on. This is important too, as small changes in the amounts of some of these elements can really change the way a star lives its life (hmmmm... something like us, as well). Manganese, for example, is very good at absorbing light, so if a star has a little bit extra manganese its heat from the interior gets trapped, making the star hotter. It doesn't take much extra, either, so life is a lot easier for a scientist when she studies a cluster. The stars tend to be better behaved.

The Small Magellanic Cloud

Then, too, scientists can compare clusters to each other, to look for similarities and differences. Looking at our companion galaxies is a good way to do this, because again the clusters are at about the same distance. They may be different ages and have different chemical compositions, but at least we can ignore distance issues. That helps a lot. When I see a cluster like NGC 290, I know we can learn a lot about it; how stars are born, how they live, how they die. But I also look at it and think, "Wow, that is absolutely gorgeous!" The colors, the patterns, the contrast between bright stars and deep space. Another thing, too: -- the images of the clusters above are from Hubble, which observed them in the Small Magellanic Cloud in November 2004. This is the same time I was in Australia, and seeing the Clouds with my own eyes for the very first time. For my PhD I studied a star in the Large Magellanic Cloud which exploded in 1987, so seeing them for myself for the first time was very moving for me. It was an incredible experience. There is beauty in observing the cosmos, and there is beauty in knowing it, too. And there is plenty of room for both, I think.'

NGC 265 and NGC 290 in the Small Magellanic Cloud

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