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Interstellar Medium

The interstellar medium (or ISM) is the gas and dust that pervade interstellar space and blends smoothly into the surrounding intergalactic space. For centuries, scientists believed that the space between the stars was empty. It was only in the last century that observations of interstellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure. The interstellar medium consists of an extremely dilute mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. Densities range from a few thousand to a few hundred million particles per cubic meter with an average value in the Milky Way Galaxy of a million particles per cubic meter. The gas is roughly 89% hydrogen and 9% helium and 2% elements heavier than hydrogen or helium by number of nuclei.

Recently Updated
A New 3-D Map of the Interstellar Gas Within 300 Parsecs from the Sun Last Updated on 2010-02-10 00:00:00 PARIS (Feb. 10, 2010) – Astronomy & Astrophysics is publishing new 3D maps of the interstellar gas in the local area around our Sun. A French-American team of astronomers presents new absorption measurements towards more than 1800 stars. They were able to characterize the properties of the interstellar gas within each sight line. FIGURE CAPTION -  Map of partially ionized interstellar gas within 300 parsecs around the Sun, as viewed in the Galactic plane. Triangles represent the sight-line positions of the stars used to produce the map. White to dark shading represents the low to high values of the gas density, and orange shading is for areas with no reliable measurement. The Local Cavity is shown as the white area of low density gas that surrounds the Sun at about 80 parsecs. This week, Astronomy & Astrophysics publishes new 3D maps of the interstellar gas... More »
Hot Interstellar Matter Last Updated on 2007-11-20 00:00:00 Very hot gases, with temperatures of a few million degrees, are also observed in a variety of galaxies; we are beginning to understand how these hot gases may be produced and evolve in different galaxies. These hot gases are important for the ecology of the universe, because hot winds may spread the metals formed in stars (the ‘elements of life’) outside of the parent galaxy, to be recycled in future episodes of star formation. Hot gaseous halos are trapped by the gravitation potential in some galaxies or groups of galaxies, thus providing the means for measuring the mass of the dark matter in these systems. Hot gases interact with the super-massive black holes at the nuclei of galaxies and are important for the creation and evolution of Active Galactic Nuclei (AGN) and quasars. This X-ray image is from the ROSAT All-Sky Survey. It shows X-rays at energies of .75 keV. This image is... More »