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A bit of science magic you can do with nothing more than your eyes When a color is more than a color By Wil Milan for explorezone.com Spectrograms are the bar codes of the cosmos, and you can read them.The job of a modern astronomer is not what most people think it is. The popular notion of a professional astronomer is that he or she spends much of his or her time gazing through a telescope, at times taking pictures of objects in the telescope for later analysis. In fact, few astronomers do anything like that. Very few of them ever look through telescopes at all. Even if they wanted to they couldn't: Most astronomy work today is done at computers, not telescopes, and large modern telescopes are not even equipped with eyepieces so that anyone could look through them.

Pink-red color of the Orion Nebula indicates ionized hydrogen. Blue in NGC 1973-75-77 indicates gas and dust that is not ionized, but which reflects light. Image copyright © Wil Milan.

The reason you cannot look through such telescopes is that they are not telescopes as most people think of them, but really giant cameras. A photograph can capture objects much fainter than the human eye can see, and a photograph is a much more accurate record than the human mind or even a human drawing (which is how astronomers used to record their observations). For those reasons all major telescopes built since the early part of this century have been designed as huge cameras, not visual telescopes.

Celestial bar codes

Not only are most modern telescopes not used for visual observing, but most of the photographs taken with such large telescopes are not what most people imagine a picture to be. Instead of an image of a beautiful nebula or galaxy, most of the photos look like celestial bar codes: Thousands of thin, parallel vertical lines aligned in neat rows. It's as if those huge telescopes were no more than giant bar code scanners ­ which in a way is exactly what they are. Those "bar codes" are spectrograms, and spectrograms are the basis of modern astronomy. Here's how they work:
If you take a prism and hold it up to a beam of sunlight, at a certain angle of the prism the white light of the sun will be separated into a rainbow of colors. If you were to carefully control the incoming beam and analyze the "rainbow" very carefully, you would note that it contains hundreds of very thin black lines, each one corresponding to a specific chemical element in the Sun. Those thin black lines (known as Fraunhofer lines for the German optician who first catalogued them) are visible in the spectrum of everything that glows in the universe. That gives astronomers a power that is almost magical: It allows them to do very detailed chemical analysis of distant objects without ever coming near them. That is how we can look at a star millions of light years away and know what is in it ­ quite an amazing thing.

Speed machines

And spectrograms can do even more: If the pattern of lines in the spectrum are shifted a bit from their normal positions, it tells us that the object is moving and how fast it is moving. If the lines in the spectrum appear shifted toward the blue end of the spectrum, the object is coming toward us. If the lines are shifted toward the red, the object is moving away from us. In either case, the amount of shift (which can be very precisely measured) indicates the speed of the object.
Some of the speeds detected by use of spectrographs are downright astonishing: Some galaxies ­ whole huge galaxies ­ have been clocked at speeds of tens of thousands of miles per second, or hundreds of millions of miles per hour. The discovery of galaxies moving at such amazing speeds, and the fact that they were all moving away from us, is what led to the formulation of the Big Bang theory. The Big Bang theory is the foundational theory of modern astronomy and cosmology, and it is owed completely to the power of spectral analysis. Visual observing or taking conventional photos of stars and galaxies would never have revealed the motions of galaxies; it's been only by spectrographic analysis that we have been able to discover not only their rapid motions but also their detailed chemical make-up and even much of their histories.

And you can do it yourself

Spectral analysis may seem the exclusive realm of Ph.D.s using exotic equipment, but in fact some forms of spectral analysis require nothing more than a bit of clever knowledge. See, for example, this image of the Orion nebula:

Pink-red color of the Orion Nebula indicates ionized hydrogen. Blue in NGC 1973-75-77 indicates gas and dust that is not ionized, but which reflects light. Image copyright © Wil Milan.
Note that much of the nebula is a pinkish red color, the color of ionized hydrogen; anytime you see that color in a nebula you know not only that it is mostly made of hydrogen, but that the hydrogen is being irradiated by nearby stars so that it is ionized and glowing just like a neon sign. (A neon sign glows because of ionized neon ­ the same chemical principle applied to neon gas instead of hydrogen.)
The peculiar shade of pink you see in this nebula is not a pure pink, but actually has a faintly purple tinge to it. That's because mixed in with the reddish color of hydrogen is a bit of blue-green, the color of ionized oxygen, which is the other primary component in the light of emission nebulae like this. So now anytime you see a pinkish-red nebula with just a wee hint of purple, you will know you are looking at the color signature of ionized hydrogen and oxygen.
What about the blue portions? Blue indicates clouds of gas and dust that are not close enough to bright stars to be ionized by their radiation, but close enough to reflect the light from such stars. These nebulae are therefore known as reflection nebulae, because they are reflecting the light of nearby stars rather than emitting their own light. And because the hottest, brightest stars are usually blue, most reflection nebulae are blue as well.
So now you have the key: Next time you see a photo of a nebula, you can amaze everyone by doing a bit of on-the-spot spectral analysis. And reflect for a moment on how remarkable that is, how it's one of the great fortunate coincidences of creation that the delicate colors that stir the soul also bring us answers to a great quest of the mind.

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