The Interstellar Medium
You may think there's not much of anything in between the stars, but of course you would be wrong. We speak of an interstellar medium, full of interesting atoms, molecules, and dust grains. The study of the atoms and molecules found in interstellar space is essential to the understanding of how stars are born, how they live, and how they die.
To start off, here are just a few of the broad categories of things found out between the stars of our galaxy.
- Clouds of Hydrogen
Neutral hydrogen can be detected because of its emissions at the radio wavelength of 21 centimeters. This 21 cm emission is due to the single electron of hydrogen changing its quantum spin direction. Otherwise, there would be virtually no way to detect neutral interstellar hydrogen.
If the hydrogen is hot enough, the electron is lost and we observe ionized hydrogen in HII regions. One example of a nearby HII region is the Orion Nebula.
- Molecular Clouds
Interstellar gas clouds also have a variety of interesting organic molecules which also emit radio energy at specific wavelengths. The National Radio Astronomy Observatory keeps track of these molecules which include something as simple as carbon dioxide to molecules with more than a dozen atoms.
- Interstellar Dust
This dust is just about what you would think dust is; small grains of silicate material with perhaps a coating of ice. The two main effects of interstellar dust are to decrease the intensity of starlight and to make those stars appear redder. If you have ever overestimated the distance to a stoplight on a foggy night, you can appreciate how interstellar dust may make a star appear to be farther away than it really is. And those of us who have seen bright red sunsets know that particles in between us and the light source can cause objects in the sky to appear red. The Coal Sack is a very good example of a dark nebula.
Nebulæ
Generally, in Astronomy, anything that is not a star is a nebula. In this section, we are interested in the nebulae that are truly clouds of gas and dust. These regions of gas and dust are seen from Earth in a variety of forms, all of which are called nebulae. One of the nearest of these nebulae is the famous Orion Nebula, seen here in a pleasing animation. There are a number of different kinds of nebulae. We find emission nebulae, reflection nebulae and dark nebulae. Planetary nebulae are all emission nebulae. In the constellation of Cygnus, there is a very nice nebula known as the Cocoon Nebula. Here we see a stellar nursery where new stars are forming.
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Updated April 10, 2010