Most of the ISM is extremely diffuse. The average density is just one particle per cubic centimeter! (In comparison, a typical cubic centimeter of air at sea level contains about — that’s 30,000,000,000,000,000,000 — particles.) Even the densest regions of the ISM contain only about 1,000,000 particles per cubic centimeter — which is almost as close as modern lab equipment can get to a perfect vacuum. One interesting effect of these incredibly low densities is that particles don’t encounter each other very often, which gives individual atoms enough time to undergo improbable energy transitions between collisions, producing “forbidden line” emission. These “forbidden” spectral lines are important to astronomers because they help reveal the composition of the ISM.
The interstellar medium, or ISM, is the formal term for everything that fills the spaces between the stars in a galaxy. The ISM of the Milky Way and other nearby galaxies has been extensively studied by astronomers. It consists mostly of hydrogen, along with other gases and dust grains. Some of this material will eventually coalesce to form stars. When these stars reach the end of their lives, the material will return to the ISM, enriched by nuclear fusion processes. Scientists can therefore study the ISM to learn how stars form and how previous generations of stars in the galaxy lived and died.
As we will see on the rest of the tour, the ISM consists of mostly hydrogen gas. We can divide the ISM into “phases” based on its temperature, density, and ionization state:
- Coronal Gas (Hot Ionized Medium, HIM): Very hot (1,000,000 K), very diffuse ionized gas, heated by supernovae and other shocks.
- HII Gas: Ionized hydrogen at temperatures of about 10,000 K. Either “diffuse HII” if not surrounded by a molecular cloud, or “HII region” if bounded by neutral gas.
- Warm and Cool HI (Warm and Cool Neutral Medium, WNM and CNM): neutral atomic hydrogen at “warm” temperatures (5,000-10,000 K) and “cool” temperatures (50-100 K). The existence of the WNM is debated.
- : cold (17-20K) molecular hydrogen, protected from ionizing radiation by a surrounding layer of neutral atomic hydrogen.
- Dust: very small solid particles, typically less than m. The composition of interstellar dust is discussed here.
All of these phases are observed on the tour, with the exception of the coronal gas, which is too diffuse to easily observe in our own galaxy. Often, multiple phases can exist at the same time; with no sharp boundary between, say, regions of molecular hydrogen gas and regions of dust.
The ISM also includes dark matter, photons (light) from stars and other sources, cosmic rays (particles that have been accelerated to extremely high speeds), and gravitational and magnetic fields. All of these things can interact with the gas and dust we see, contributing to the rich complexity of the ISM as seen in images like the Hubble Space Telescope photo above.