Before you ask why - we're building a model of a galaxy, and galaxies aren't exactly circular. Now make the ellipses spin, with the little ellipses in the center spinning faster than the big ones at the edge. Imagine stretching them out so the ripples are ellipses rather than circles. They usually look like a bunch of concentric circles, with smaller ones inside larger ones. Well, take the ripples in the pond and freeze them for just a moment. They have ripples, but those ripples are most certainly not spirals." "Wait, wait, wait," you may say, "I'm a person of the world. The best picture astronomers have, so far, is that the spiral arms are actually - get this - density waves. The fact that spiral arms are only slightly more dense - but not crazily so - than the rest of the disk is a clue. This image is a digital combination of a ground-based image from the 0.9-meter telescope at Kitt Peak National Observatory and a space-based image from the Hubble Space Telescope highlighting sharp features normally too red to be seen. At only 30 million light years distant and fully 60 thousand light years across, M51, also known as NGC 5194, is one of the brightest and most picturesque galaxies on the sky. The Whirlpool Galaxy is a classic spiral galaxy. Positively infested! The arms themselves aren't all that much denser than the seemingly empty gaps. But deeper observations reveal what the human eyeball can't: The disks of spiral galaxies are filled with stars. So if spiral arms aren't things, then what the heck are they? From looking at a spiral galaxy, it seems as if all the stars are bound up in the central bulge and in the arms, with vast tracts of wasteland everywhere else. If spiral arms were actually things attached to the center of the galaxy and spinning as fast as all the stars, the arms would've been tightly wound together like pasta on a fork a long time ago. This means that the inner parts of a galaxy spin faster than the outer parts. But galaxies aren't Frisbees they're not solid objects all connected to themselves with plastic and glue. Blow on a galaxy (hard enough), and you just might make the galaxy spin faster. If you just look at a spiral galaxy, it seems as if the arms are massive pinwheels. It must be Julia Child's universe, because that's a lot of perfectly cooked soufflés. Instead, between half and two-thirds of all galaxies feature spiral arms. If spirals were only temporary, once-in-a-galactic-life-time thing, astronomers would hardly see any such galaxies left today. So this means the spirals appear sometime after the galaxy itself assembles, and that whatever makes the spirals keeps on making them over eons. Even though a spiral galaxy may have formed a long time ago, triggering an intense burst of star formation, whatever's happening inside the spirals is an ongoing process, something that doesn't burn up too much or too little of a galaxy's gas reserves. But the spirals themselves are a bright, blazing blue, cluing astronomers in that stars there are young, and the party lives on. Portions of spiral galaxies are red, too: the central bulge and the "halo," or the sprinkling of stars that live above and below and main, flat disk of the galaxy.
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