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Why Squiggly Lines Rule: Astronomical Spectra and the Elemental Fingerprints Adam Frank for McGraw-Hill When I was a kid I knew I wanted to be an astronomer. By the age of six I already had caught the "astro" bug and I took every opportunity to nag my parents for yet another trip to the planetarium or a new book about space. Given this background, you can imagine how psyched I was when my parents told me it was time I had my own telescope. Finally, I was gonna get my chance to see the distant stars up close. For weeks as I waited for the telescope to arrive I had visions of watching giant flares erupt off the surface of Vega and boiling torrents bursting from white dwarfs like Sirius B. With so much build-up, my horror and surprise were only magnified when I finally set the telescope up in backyard, trained its eye on a star and found it looked just like … a star. Here I had this beautiful new $100 telescope and still the star looked like a pinprick of light. What a bummer! It took me a while to understand what was going on. In spite of being so big, stars are so very, very far away that even powerful telescopes cannot resolve them into anything more than a point of light. Only later, as my disappointment faded, did I begin wondering how astronomers got past this sad fact and managed to figure anything out about the stars they could not see in detail. That was when I began to learn about the joys of spectra. The architecture of physical reality is astonishing in its simplicity. Consider for a second the scaffolding on which all matter is built. Every atom is made up of a dense nucleus (protons and neutrons) surrounded by a cloud of orbiting electrons. You have heard this before, of course, but have you ever thought about how every element is built by adding one proton and one electron to the element below it? Hydrogen, the simplest element, has one proton and one electron. Helium, the next element up on the periodic table, has two protons and two electrons. Lithium has three protons and three electrons. And so on up the line. Iron has 26 protons and 26 electrons. Nickel has 28 protons and electrons. (The rule for neutrons is not so simple but we don't need to bother with that right now.) The order built into the elements determines all their properties, their color, hardness, texture. Most important for astronomers, the number of electrons determines how an element absorbs and emits light. That simple fact has become the key to our understanding of almost everything astronomical. For the most part it's the electrons in an atom that are responsible for interacting with light. As electrons orbit the nucleus they absorb light energy or spit it out into space. Since each element has a very specific pattern of orbiting electrons, the way the element absorbs or emits light also arranges itself into a very specific pattern. The pattern of light emitted by a star, (which means how much energy goes into each different color or wavelength) is what people mean when they talk about spectra. Since each spectral pattern is unique they form a kind of fundamental fingerprint that astronomers can use to understand what a star, or any distant astronomical object is made of. Along with composition, astronomers also can use spectral patterns to determine speed, temperature density and other properties of the objects they observe. Now you can see why the analysis of spectra is so important. There is a mess of information hiding in astronomical spectra that can reveal critical details of an object that appears to us as nothing more than a pinprick on the night sky. Of course looking at spectra is not exactly as thrilling as looking at a star. Spectra are basically graphs showing brightness vs. wavelength and that is an overblown way of saying they look like squiggly lines. I attended a conference recently about the central regions of galaxies millions of light years away. The speakers told us about monster blackholes billions of times more massive than our Sun that live at the center of these galaxies. Then they showed us spectra after spectra. Each squiggly line had been used to decipher the properties of the blackhole and its environment. It was like being that kid with the telescope in my backyard all over again. I wanted a picture of the blackhole swallowing star systems whole. All I had to content myself with was a bunch of squiggly lines. This time though the disappointment passed pretty quickly as I recognized that every dip and peak in those spectra represented a message that had crossed a good fraction of the Cosmos to get to us. The spectra may not have been much to look at, but once you learned how to read them there was a Universe of stories there waiting to be told. Check Out These Spectra Webpages Spectra of Gas Discharges History of Astronomical Spectroscopy Atomic Spectra Questions to Ponder A doctor will often try and diagnose a problem by asking questions about an ailment rather than cutting a patient open first (at least that is what you hope!). In what ways could the analysis of astronomical spectra be considered a means of diagnosing conditions in distant objects? How can spectra be used to decipher the velocity of a distant object? How can spectra be used to decipher the temperature of a distant object? 
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