interstellar medium

The interstellar medium (ISM) is matter such as gas and dust between stars. (Matter between entire galaxies is called the intergalactic medium.) Depending upon circumstance, the gas may be ionized, neutral, or molecular. Density varies from as little as 1 per 10,000 cm³ to 1,000,000 per cm³ (compared to about 2.5×10¹⁹ per cm³ for dry air at sea level). The ISM's existence was first deduced through observed absorption lines.

There is a nesting of types of ISM:

• Most ISM: hot and very thin: ionized hydrogen (HII) or neutral atomic hydrogen (HI) at thousands of K (warm neutral medium, WNM, or warm ionized medium, WIM, or coronal gas/hot ionized medium, HIM).
• Cooler, denser region of neutral hydrogen (HI and some CII) at 50-100K (cold neutral medium or CNM, HI region).
• Even cooler, denser subregion: molecular hydrogen (H2, with CI and CO) at 10-20K (molecular cloud, the site of possible star formation).
• Subregion reheated/dissociated/ionized by recently-formed, hot stars, to thousands of K, but still dense: HII, CII, and NII (HII region, modeled by Strömgren sphere).