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Basic Astronomy Concepts

Interstellar Matter

elliptical galaxies, Milky Way Galaxy, Spiral galaxies, interstellar dust, chemical elements

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>  Distribution of Interstellar Matter

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Interstellar Matter, gas and dust between the stars in a galaxy. In our own galaxy, the Milky Way, we can see glowing gas and dark, obscuring dust between the galaxy’s many visible stars. This gas and dust makes up interstellar matter. Galaxies differ in the density of interstellar matter that they contain. Spiral galaxies, such as the Milky Way, have much more interstellar matter than elliptical galaxies, which have almost none. About 3 percent of the mass of the Milky Way Galaxy is interstellar gas, and 1 percent is interstellar dust. Stars make up the rest of the ordinary matter in the galaxy. Dark matter—a material that does not reflect or emit light or other forms of electromagnetic radiation—also makes up some of the mass of the galaxy. Astronomers consider interstellar matter separately from intergalactic matter, or matter between galaxies.

Hydrogen gas makes up most of the interstellar matter, but essentially all of the chemical elements occur in interstellar matter. About 90 percent of the atoms in space are hydrogen, about 9 percent helium, and less than 1 percent consists of all the other chemical elements. The interstellar matter is so spread out that the space it occupies would be considered a vacuum in laboratories on Earth.


Pasachoff, Jay. M.m A.B., A.M., Ph.D.

Field Memorial Professor of Astromony and Director of the Hopkins Observatory, Williams College. Author of "Astronomy: From the Earth to the Universe, 6th ed."; "Field Guide to the Stars and Planets, 4th ed."; "Fire in the Sky"; and "Nearest Star: The Exciting Science of Our Sun".

Article key phrases:

elliptical galaxies, Milky Way Galaxy, Spiral galaxies, interstellar dust, chemical elements, Hydrogen gas, Dark matter, atoms, percent, Astronomers, vacuum, mass, Earth, laboratories, space, rest, light, material

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