Distributed gas, dust and other matter that is found throughout interstellar space. It is generally considered a separate medium from the much denser concentrations of material found in relatively close proximity to deep gravity wells such as stars, brown dwarfs, and stellar remnants.
The interstellar medium is made up of a mixture of particles and elements in different combinations and sizes. Considering the constituent particles, from the largest to the smallest:
Interstellar Snowballs Mostly ice-based interstellar particles larger than an interstellar grain. May range in size from several microns to Kuiper-size objects.
Interstellar Dust Particles of carbon, iron, and silicates coated with ice, making up about 1% of the mass of the interstellar medium. Each grain is about a micron in diameter. They are often associated with nebula and young stars. The average distance between dust grains is about 150 meters. A nuisance to starfaring vessels, because the impact at relativistic velocity tends to seriously erode their protective shielding, interstellar dust is highly prized by cloudharvesters and other hider phyles.
Interstellar Grain Microscopic solid grain in interstellar space; the component parts of interstellar dust.
Interstellar Gas Diffuse atoms or molecules of gas in interstellar space, making up about 99% of the interstellar medium. Depending on density, the number of atoms range from 0.1 to 1 atoms per cubic centimetre of normal space, with up to 1000 per cubic centimetre in gas clouds. About 75% is hydrogen, the remainder being helium, with traces of other elements.
Interstellar Molecule Molecule of gas in interstellar space.
Interstellar Atom Individual atoms drifting in interstellar space. Mostly hydrogen or helium, although heavier elements are occasionally encountered.