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SYFY WIRE Bad Astronomy

The darkness and the light

By Phil Plait

The sky is not as it seems.

Certainly, gazing upon it on a clear night you see so much: stars, planets, the glow of hot gas here and there... but there's also darkness. Look at the Milky Way, its stream split down the middle by a rift of black. Gape at a gaudy nebula, and you'll see it pocked here and there by pools of black.

But what is inky pitch to our eyes glows with a cold light to those attuned to it.

Tell me, what do you see here?


The bright star is obvious enough, but you can also, dimly, see a feathered stripe of black splashed across the vista, blocking, absorbing the light from stars behind it. Details are muted, structure difficult to ascertain, and you strain to see features that your brain cannot interpret.

But that's with your eyes. Try again, look at it, but this time, widen your view. See it now?


Well done! Where before you saw material absorbing light, now it emits! Of course, unbeknownst to you, you had some help: the ESO APEX telescope in Chile. It sees into the far, far infrared, where light is so stretched out it is entirely invisible to humans. In fact, the wavelength of light is so wide there that if it were a vibrating string, you could physically see the crests and troughs, since each would be separated by the next by nearly a millimeter. The light your eye can see has a wavelength only a thousandth that wide.

When APEX looked at this ribbon of dark, frigidly cold dust, it sees the material glowing. What we see as dark, it sees as bright. You can even compare the two directly, using a slider over the two versions of this picture, unveiling precisely what your now-expanded vision can take in.

Cold dust is the bane of the astronomer who uses merely visible light, since it blocks the view behind it. But one person's poison is another's meat, and if you study the material that wends its way between the stars -- and sometimes comes together to form them -- then the view from APEX is sustenance for you. This material is barely above the ultimate freezing point of absolute zero, and you might think it dead and useless. But from such stuff are you and I descended, and everything you see around you.

So when you do peer around you, and take in your environment, your surroundings, your home, look again. You are surrounded by the invisible, permeated by it... but always remember, it was invisible only until we chose to look for it. We created the means necessary to do so, and when we did the Universe opened up before us.

Image credit: ESO/APEX (MPIfR/ESO/OSO)/A. Hacar et al./Digitized Sky Survey 2. Acknowledgment: Davide De Martin.

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