Blocking Light

Why is it that dust blocks our optical view of the Milky Way? It's due to the size of the dust grains. Let's think about this for a moment. A satellite dish can be made out of a wire mesh, perforated by small holes. Why doesn't this structure let the radio waves slip through, like water through a sieve? Because it's catching radio waves, and radio waves are big. So big, in fact, that as long as the holes are small enough, the radio waves don't even know the holes are there. The waves reflect off the surface of the dish as if it were solid. In fact, all electromagnetic radiation (light included) works this way. The waves interact only with things that are about the same size as their wavelength. As luck would have it, optical wavelengths are about the same size as the diameter of a typical dust grain. As a result, optical photons are absorbed or scattered by dust, while long-wavelength radio waves pass right through.

The combined effect of the scattering and absorption caused by the dust results in extinction. The dust absorbs blue light more than red and also scatters blue light more than red. As a result, the visible light that makes it through is reddened. Interstellar reddening is increased when objects are farther away, and astronomers must take this into account when determining the true color of a star.

You may picture interstellar dust as a kind of fog. Certainly, fog, which consists of water molecules and often particulate matter (a.k.a. dust), interferes with the transmission of light, as anyone who has driven in terror along a foggy mountain road can attest. We often speak of a thick fog and say "You couldn't see your hand in front of your face."

Gas and dust is thinly distributed throughout space, and is the matter from which the stars are formed. About 5 percent of our Galaxy's mass is contained in its gas and dust. The remaining 95 percent is in stars.

Astronomer's Notebook

Gas and dust is thinly distributed throughout space, and is the matter from which the stars are formed. About 5 percent of our Galaxy's mass is contained in its gas and dust. The remaining 95 percent is in stars.

Close Encounter

If a cloud of dust is between us and a distant star, the light from the star must pass through the dust before it can get to us. Dust allows the longer (redder) wavelengths to pass, but the shorter (bluer) wavelengths are scattered and absorbed. As a result, the light that makes it through is reddened. The setting sun looks redder for the very same reason. As the sunlight passes through a thicker slab of the atmosphere near the horizon, its blue frequencies are absorbed and scattered by the atmosphere and the sun looks red.

But the fact is, you usually have no trouble seeing such nearby objects as your hand. Indeed, fog seems to play with us. You never really walk into the fog. It always seems to be just ahead of you. This is because the obscuring effect of all those water molecules and dust particles is cumulative. The matter stacks up with depth. A cubic yard of thick fog (about the distance of a hand held at arm's length from your face) may not be especially impressive, but ten or twenty or fifty cubic yards, let alone a hundred or more, create the well-known pea-soup effect: a thick fog, and photons scatter before they can get to you.

Now, interstellar dust is so diffuse that many thousands of miles of it may not obscure anything. But stack billions upon billions of miles of material between you and a star, and it will certainly begin to have an effect on what you see.

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