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M83's nursing arms

Bad Astronomy
By Phil Plait
Sep 2, 2008 6:50 PMNov 5, 2019 7:10 AM


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Who doesn't love spiral galaxies?

That beauty is M83, as seen by the European Southern Observatory's (ESO's) Wide Field Imager on the 2.2 meter telescope in Chile. Whoa. The image is in natural colors, more or less -- the camera used blue, yellow, and red filters to get the image, plus a filter tuned to a narrow range of colors that are emitted specifically by warm, glowing hydrogen gas. That's the ruby red color you see in the galaxy. Can you see how the gas seems to follow the spiral arm? That's because the gas in the galaxy piles up along that pattern as the galaxy rotates (see point #8 in that link specifically). It gets compressed and forms new stars. When these stars turn on, they heat up the gas, exciting the electrons in the hydrogen atoms. The atoms respond by emitting light around 656 nanometers, in the red part of the spectrum. Scientists call this specific wavelength "hydrogen alpha", or just H-alpha for short (or Hα for shorter). When you detect it, it's a sure sign you're seeing hydrogen gas being poked by some process. It may not be a star; we see it emitted by hydrogen in supernova debris, and in the swirling disks of matter near a black hole, and in the regions where gas clouds violently collide. But most of the time, the vermillion glow of Hα is from the swaddling cloth of gas still surrounding the wailing newborn stars formed from it. Because of this, we can see these fledgling stars even from millions of light years away; in the case of galaxy M83, from 15 million light years away: 150 quintillion kilometers (90 quintillion miles) remote! One of my favorite aspects of the Universe is not only that we can see it, but that in many cases it's so easy to see it, and to study it. That image of M83 was composed of four exposures totaling less than two hours, and constructed by one person -- David De Martin from Sky Factory. Certainly, he had help: the telescope was built by dozens of engineers, hundreds of workers, managed by scores of people at the ESO, and is the end product of a long line of work by hundreds of other people. The pedigree of that image is lengthy and arguably stretches back to the first humans who looked up at the sky. But here we are, thousands of years later, and we're still looking up. Our tools are a lot better now, as is our understanding, and I'm very, very glad for people like Davide and others at the ESO, who strive to stretch our knowledge of the Universe -- and our appreciation of its beauty -- even further.

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