The space between the stars is filled with two basic components, gas and dust. The vast majority (99 percent) of interstellar matter is what we call gas, and the majority of the gas consists of the most abundant element in the universe, hydrogen. The remainder of the material is what is referred to as "dust," though smoke might be a more accurate description. In fact, we'll see that interstellar dust puts up quite a smoke screen, keeping us from viewing many regions in our own Galaxy optically.
A multifrequency view of our own Milky Way galaxy, viewed from our perspective within it, makes it clear that there is more than meets the eye. The gas and dust, apparent in the radio and infrared wavelength views of the galaxy, all but disappear at optical wavelengths. The individual wavelengths are labeled, with low frequencies (long wavelengths) at the top and high frequencies (short wavelengths) at the bottom.
(Image from NASA)
The material between the stars is not evenly distributed. Since gas and dust have mass, the material pulls together into clouds and clumps via its own gravity. The patchy distribution of the interstellar medium means that in some regions of the sky, astronomers can observe objects that are very distant. In other regions, where the interstellar matter is more concentrated, our range of vision is more limited. Think of
the last time you looked out an airplane window. If the clouds were patchy, there were some directions in which you could see the ground and others in which you could not.
The lowest-density interstellar clouds consist mostly of atomic hydrogen (called HI—no, not "Hi," but H-one, an H followed by a roman numeral I). Until the advent of radio astronomy, this atomic material was impossible to detect. Cooler, higher-density clouds contain molecular hydrogen (H2). These regions are the fuel tanks of the universe, ready to produce new stars. The regions closest to young stars are ionized—their electrons stripped away. The 10,000 K gas in these regions (called HII—"H-two"—regions) emits strongly in the optical portion of the spectrum.
Let's look more closely at these different types of matter between the stars.
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