The material between the stars, also called the interstellar medium. The ISM in our Galaxy consists of 99 % gas and 1 % fine dust particles by mass. There are also a few very energetic cosmic-ray particles. The ISM is spread very thinly, with an average density of only 1 particle/cm3. However, there are enormous variations about this average. The densest parts, which include maser regions containing 1010 hydrogen molecules/cm3, are also the coldest, with temperatures close to absolute zero. The hottest material has temperatures of 108 K, and densities of 1 particle/m3 or less. The ISM is the reservoir of matter from which new stars form.
Subjects: Astronomy and Astrophysics.