Editing Interstellar medium (section)

===Interstellar extinction===
[[File:Short, narrated video about IBEX's interstellar matter observations.ogv|thumb|left|350px|Short, narrated video about [[Interstellar Boundary Explorer|IBEX's]] interstellar matter observations.]]
[[Cosmic dust|Dust grains]] in the ISM are responsible for [[Extinction (astronomy)|extinction]] and [[interstellar reddening|reddening]], the decreasing [[Radiance|light intensity]] and shift in the dominant observable [[wavelength]]s of light from a star. These effects are caused by scattering and absorption of [[photon]]s and allow the ISM to be observed with the naked eye in a dark sky. The apparent rifts that can be seen in the band of the [[Milky Way]] – a uniform disk of stars – are caused by absorption of background starlight by dust in molecular clouds within a few thousand light years from Earth. This effect decreases rapidly with increasing wavelength ("reddening" is caused by greater absorption of blue than red light), and becomes almost negligible at mid-[[infrared]] wavelengths (>&nbsp;5&nbsp;μm).

Extinction provides one of the best ways of mapping the three-dimensional structure of the ISM, especially since the advent of accurate distances to millions of stars from the [[Gaia (spacecraft)|''Gaia'' mission]]. The total amount of dust in front of each star is determined from its reddening, and the dust is then located along the line of sight by comparing the dust [[Area density|column density]] in front of stars projected close together on the sky, but at different distances. By 2022 it was possible to generate a map of ISM structures within 3&nbsp;kpc (10,000 light years) of the Sun.<ref>{{Cite journal |last1=Vergely |first1=J. L. |last2=Lallement |first2=R. |last3=Cox |first3=N. L. J. |date=August 2022 |title=Three-dimensional extinction maps: Inverting inter-calibrated extinction catalogues |url=https://www.aanda.org/10.1051/0004-6361/202243319 |journal=Astronomy & Astrophysics |volume=664 |pages=A174 |doi=10.1051/0004-6361/202243319 |s2cid=248863272 |issn=0004-6361|arxiv=2205.09087 |bibcode=2022A&A...664A.174V }}</ref>

[[Far ultraviolet|Far ultraviolet light]] is absorbed effectively by the neutral hydrogen gas in the ISM. Specifically, atomic [[hydrogen]] absorbs very strongly at about 121.5 nanometers, the [[Lyman series|Lyman-alpha]] transition, and also at the other Lyman series lines. Therefore, it is nearly impossible to see light emitted at those wavelengths from a star farther than a few hundred light years from Earth, because most of it is absorbed during the trip to Earth by intervening neutral hydrogen. All photons with wavelength <&nbsp;91.6&nbsp;nm, the Lyman limit, can ionize hydrogen and are also very strongly absorbed. The absorption gradually decreases with increasing photon energy, and the ISM begins to become transparent again in [[soft X-ray]]s, with wavelengths shorter than about 1&nbsp;nm.