The Interstellar Medium

Staring across the vastness of space to neighboring stars it is easy for us to miss a very important part of the galaxy. This is the interstellar medium (or ISM) which is a collective name that astronomers give to all of the stuff that exists between the stars. On average there is only 1 atom per cubic centimeter making up a tenuous, cold gas having a density of about 10^-20 of the density of Earth's atmosphere at sea level. Despite this the total mass contained in the ISM is about 5 billion times the mass of the sun! It has taken astronomers most of this century to piece together a "picture" of what the interstellar medium is but now we think that we have a pretty good understanding of this tenuous but important component of our galaxy.

The Main Components of the ISM

The ISM is a collective term that refers to:

• Cold Atomic Clouds which have densities of 10 - 20 atoms per cubic centimeter and temperatures of about 50 K. Typical clouds measure 10 pc (30 ly) across and have masses of about 1000 suns. On average, these clouds are about 500 ly apart.
• Warm Diffuse Atomic Gas clouds have temperatures of about 5000 K but densities of only about 0.1 atoms/cc. The clouds are mostly hydrogen (90%) and along with the cold atomic clouds make up about 50% of the mass of the ISM.
• Molecular Clouds are large regions of cold gas with densities from 1000 to 10 000 hydrogen atoms/cc. Dust mixed into these clouds help block out star light and the result is clouds that are dark and very cold (only a few 10's K). It is the combination of high density and cold that makes it possible to form complex molecules in these clouds. Radio astronomy has revealed a wealth of complex and in many cases organic molecules in molecular clouds.
• HII Regions are bright glowing regions of ionized hydrogen gas surrounding luminous, hot stars. These regions have temperatures of about 10 000 K and densities of a few 100 to 1000 ions/cc.
• Dust is also an important part of the ISM. The dust in the ISM consists of small (less than a micron or millionth of a meter) "pellets" consisting of elements heavier than hydrogen and helium. We think that most of the dust grains in the ISM are being produced in the cool atmospheres of red giant stars. Mass loss from these stars spreads the dust into the ISM.

And now the quiz ...

Click here to see a beautiful image of a large nebula 1200 pc away in the direction of Sagittarius. This nebula has a mass of about 2600 solar masses and an average density of 80 particles/cc. How many features of the ISM can you identify in this picture?

The answers:

You have just looked at M8 or the Lagoon nebula. It is called an emission nebula because of the very bright reddish glow produced by the HII regions surrounding many young bright stars. Also, you see that the nebula is "crisscrossed" by dark lanes and curlicues. These are dark, dusty regions that would also include molecular clouds. The entire nebula (as we see it) is embedded in the cold atomic gas of the ISM. Click on again and see if you can see these features.