Editing Interstellar medium (section)

=== The ISM in different kinds of galaxy ===
[[File:Three-dim-pillars-creation.jpg|thumb|Three-dimensional structure in [[Pillars of Creation]].<ref>{{cite web|title=The Pillars of Creation Revealed in 3D|url=http://www.eso.org/public/news/eso1518/|access-date=14 June 2015 |publisher=European Southern Observatory |date=30 April 2015}}</ref>]]
Most discussion of the ISM concerns [[spiral galaxies]] like the [[Milky Way]], in which nearly all the mass in the ISM is confined to a relatively thin [[Galactic disc|disk]], typically with [[scale height]] about 100 [[parsec]]s (300 [[light year]]s), which can be compared to a  typical disk diameter of 30,000 parsecs. Gas and stars in the disk orbit the galactic centre with typical orbital speeds of 200&nbsp;km/s. This is much faster than the random motions of atoms in the ISM, but since the orbital motion of the gas is coherent, the average motion does not directly affect structure in the ISM.  The vertical scale height of the ISM is set in roughly the same way as the Earth's atmosphere, as a balance between the local gravitation field (dominated by the stars in the disk) and the pressure. Further from the disk plane, the ISM is mainly in the low-density warm and coronal phases, which extend at least several thousand parsecs away from the disk plane. This [[galactic halo]] or 'corona' also contains significant magnetic field and cosmic ray energy density.

The rotation of galaxy disks influences ISM structures in several ways. Since the [[angular velocity]] declines with increasing distance from the centre, any ISM feature, such as giant molecular clouds or magnetic field lines, that extend across a  range of radius are sheared by differential rotation, and so tend to become stretched out in the tangential direction; this tendency is opposed by interstellar turbulence (see below) which tends to randomize the structures.  [[Density wave theory|Spiral arms]] are due to perturbations in the disk orbits - essentially ripples in the disk, that cause orbits to alternately converge and diverge, compressing and then expanding the local ISM. The visible spiral arms are the regions of maximum density, and the compression often triggers star formation in molecular clouds, leading to an abundance of H&nbsp;II regions along the arms. [[Coriolis force]] also influences large ISM features.

Irregular galaxies such as the [[Magellanic Clouds]] have similar interstellar mediums to spirals, but less organized. In [[Elliptical galaxy|elliptical galaxies]] the ISM is almost entirely in the coronal phase, since there is no coherent disk motion to support cold gas far from the center: instead, the scale height of the ISM must be comperable to the radius of the galaxy. This is consistent with the observation that there is little sign of current star formation in ellipticals. Some elliptical galaxies do show evidence for a small disk component, with ISM similar to spirals, buried close to their centers. The ISM of [[Lenticular galaxy|lenticular galaxies]], as with their other properties, appear intermediate between spirals and ellipticals.

Very close to the center of most galaxies (within a few hundred light years at most), the ISM is profoundly modified by the central [[supermassive black hole]]: see [[Galactic Center]] for the Milky Way, and [[Active galactic nucleus]] for extreme examples in other galaxies. The rest of this article will focus on the ISM in the disk plane of spirals, far from the galactic center.