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. Number 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 (interstellar absorption lines), especially as noticed as common to stars close together within the celestial sphere, thus showing the effects of passing through material in front of them. There is a nesting of types of ISM:

• Most ISM: intercloud gas, warm-to-hot and very thin: ionized hydrogen (HII) or neutral atomic hydrogen (HI) at thousands of K, i.e., the warm neutral medium, WNM, aka warm intercloud medium, aka atomic ISM, or (hotter) warm ionized medium aka diffuse ionized gas, WIM or DIG, or (even hotter) coronal gas/hot coronal gas/hot ionized medium, HIM.
• Cooler, denser region of neutral hydrogen (HI and some CII) at 50-100 K (cold neutral medium or CNM, HI region).
• Even cooler, denser subregion: molecular hydrogen (H2, with CI and CO) at 10-20 K (molecular ISM or 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 a Strömgren sphere).

The hot ionized medium (HIM) often results from supernovae, and is referred to as coronal gas because its temperature (and perhaps other characteristics) are comparable to the gas of the Sun's corona.

The term cloud is used for regions with particular characteristics, such as a higher density, which may be termed an interstellar cloud, gas cloud, dust cloud, dense cloud, etc.

A diffuse interstellar band (DIB) is an absorption spectral band affecting observations of stars and other objects. Such bands are attributed to polycyclic aromatic hydrocarbons (PAHs) in the ISM.