Barred for life

Bad AstronomyBy Phil PlaitApr 4, 2007 3:16 AM


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It's been a while since I posted a pretty picture just to post it. Click it for a bigger version (or go here to get a choice of images, including a monster image at 48 meg!). Of course, I can't just leave it at that! Astronomy is so much more than just pretty pictures. There's a lot to see in this image, the latest release form Hubble. It's NGC 1672, a barred spiral galaxy (NGC stands for New General Catalog, as my last real episode of Q and BA notes). All by themselves, normal spirals are weird enough. We know that stars closer in to the center revolve around the core of the galaxy faster than stars farther out, so that tells you right away that arms are not just bands of stars. If they were, they'd wind up tighter and tighter with time, and they wouldn't last very long (I'm surprised I haven't seen any creationists use that as an argument against an old Universe). So what are they? Turns out, the best hypothesis so far is that they are standing waves, like cosmic traffic jams. If you were in a helicopter over a traffic jam on the freeway, it would look like the jam is a permanent fixture of the traffic. But in reality, cars leave the jam at the same rate as cars entering it. So while the jam itself stays put, the cars making it up always change. So it is with spiral arms: they are places where the matter in the galaxy is compressed, but stars enter the jam and stars leave. The arm looks permanent, but over time its resident stars, gas, and dust change. Weird, eh? But of course, things always get stranger. NGC 1672 is a barred spiral. The big arms don't go all the way to the center; they seem to emanate from the ends of a rectangular structure. The bar here isn't as obvious as in some galaxies (like, for example, our own Milky Way), but if you look at the high res version of the image you'll see it better. Bars are really weird, and are a result of the complicated gravitational forces due to an extended mass like a galaxy. The gravity from individual objects is easy to understand -- the farther you are from them, the weaker you feel their gravity, and so on -- but when you take a few hundred billion masses and spread them out into a disk, things get messy. In such a situation, I'd go to a bar too. Haha. Heh. Perusing the very high res images, you can see just what a mess things are. Gas and dust are strewn everywhere, stars scattered like dandelion seeds in the wind... people who study such things have their work cut out for them. But for you and me, we can simply scan the image and look for interesting things. Foreground stars -- ones in our own Galaxy -- blaze out. Fainter, more distant galaxies can be seen right through NGC 1672 (I love that). Pinkish clouds denote regions where stars are forming; stellar nurseries. These really define the arms of the galaxy; massive stars are born, and outshine all the other stars. These stars are blue, giving the arms their bluish hue. They don't live long enough to pass out of the arm; that takes millions of years; time the stars don't have. They explode before then, becoming supernovae, and scattering their elements back into the galaxy from which they came. It's the cycle of life, but on a somewhat larger scale than here on Earth. I said above that astronomy is much more than pretty pictures, but actually, there's a whole lot you can learn from simply looking over images like these. I've barely scratched the surface here. Still, it is pretty. Sometimes, as I've said many times in the past, that's enough too.

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