Matter of Perspective

When we are able to see part of the interstellar medium visually, it is often because a nearby star has lit it up. But there are other ways to see. In some directions, if we are fortunate, a cold dark cloud of gas will fall between the earth and an emission nebula. When that happens, we can see the dark cloud as a black patch against the emission from the ionized gas. In other directions, we see small patches of the sky where there are few or no stars. It is unlikely that there are truly few stars in that direction. Almost certainly, a dense cloud of gas in our line of sight is absorbing the starlight from the stars that are indeed behind it.

These gas pillars in the Eagle Nebula, an emission nebula 7,000 light-years from the earth, were imaged by the Hubble Space Telescope. They consist primarily of dense molecular hydrogen gas and dust. This image shows both the emission from ionized hydrogen at the surface of the pillars, as well as the absorption caused by the gas and dust contained in the pillars.

(Image by Jeff Hester and Paul Scowen of Arizona State University and NASA)

But we don't have to depend on luck. Even if an interstellar cloud of gas does not fall between the earth and a background source, we have ways to detect it. Even at the relatively cold temperatures of these clouds (100 K as compared to the nearly 10,000 K in most emission nebulae), atoms and molecules are in motion. As molecules collide with one another, they are occasionally set spinning about.

It turns out (according to quantum mechanics) that molecules can spin only at very particular rates, like the different speeds of an electric fan. And when a molecule goes from spinning at one rate to spinning at another rate, it gives off (or absorbs) a small amount of energy. We can detect that energy in the form of electromagnetic radiation. For many molecules, these photons have wavelengths that are about a millimeter long. Special telescopes (called millimeter telescopes and millimeter interferometers) can detect these photons.

With the addition of millimeter-wave telescopes, there is no portion of the interstellar medium that can escape our notice. With radio telescopes we can image neutral (cold) hydrogen atoms. With optical, infrared, and radio telescopes, we get pictures of the hot gas near young stars. And with millimeter telescopes, we can even seek out the cold clouds of gas that contain molecular hydrogen and other molecules.

The Berkeley Illinois Maryland Association (BIMA) array has done extensive studies of the molecules in the Orion nebula. These images show where particular molecules are located in this star-forming region. From the molecules listed, it is apparent that the interstellar medium is teeming with complex organic molecules.

(Image from NCSA)

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