Editing Interstellar medium (section)

==Observations of the ISM==
Despite its extremely low density, photons generated in the ISM are prominent in nearly all bands of the electromagnetic spectrum. In fact the optical band, on which astronomers relied until well into the 20th century, is the one in which the ISM is least obvious.
* Ionized gas radiates at a broad range of energies via [[bremsstrahlung]]. For gas in the warm phase (10<sup>4</sup>&nbsp;K) this is mostly detected in microwaves, while bremsstrahlung from the million-kelvin coronal gas is prominent in soft X-rays. In addition, many [[spectral lines]] are produced, including the ones significant for cooling mentioned in the previous section. One of these, a [[forbidden line]] of doubly-ionized oxygen, gives many nebulae their apparent green colour in visual observations, and was once thought to be a new element, [[nebulium]]. Spectral lines from highly excited states of hydrogen are detectable at infra-red and longer wavelengths, down to [[radio recombination line]]s which, unlike optical lines, are not absorbed by dust and so can trace ionized regions throughout the disk of the Galaxy. Coronal gas emits a different set of lines, since atoms are stripped of a larger fraction of their electrons at its high temperature.
* The warm neutral medium produces most of the [[21-cm line]] emission from hydrogen detected by radio telescopes, although atomic hydrogen in the cold neutral medium also contributes, both in emission and by absorption of photons from background warm gas ('H&nbsp;I self-absorption', HISA). While not important for cooling, the 21-cm line is easily observable at high spectral and angular resolution, giving us our most detailed view of the WNM.<ref>{{Cite journal |last1=Peek |first1=J. E. G. |last2=Babler |first2=Brian L. |last3=Zheng |first3=Yong |last4=Clark |first4=S. E. |last5=Douglas |first5=Kevin A. |last6=Korpela |first6=Eric J. |last7=Putman |first7=M. E. |last8=Stanimirović |first8=Snežana|author8-link=Snežana Stanimirović |last9=Gibson |first9=Steven J. |last10=Heiles |first10=Carl |date=2017-12-27 |title=The GALFA-H i Survey Data Release 2 |journal=The Astrophysical Journal Supplement Series |volume=234 |issue=1 |pages=2 |doi=10.3847/1538-4365/aa91d3 |s2cid=126210617 |issn=1538-4365|arxiv=1101.1879 |doi-access=free }}</ref><ref>{{Cite journal |last1=Ben Bekhti |first1=N. |last2=Flöer |first2=L. |last3=Keller |first3=R. |last4=Kerp |first4=J. |last5=Lenz |first5=D. |last6=Winkel |first6=B. |last7=Bailin |first7=J. |last8=Calabretta |first8=M. R. |last9=Dedes |first9=L. |last10=Ford |first10=H. A. |last11=Gibson |first11=B. K. |last12=Haud |first12=U. |last13=Janowiecki |first13=S. |last14=Kalberla |first14=P. M. W. |date=2016 |title=HI4PI: a full-sky H i survey based on EBHIS and GASS |url=http://www.aanda.org/10.1051/0004-6361/201629178 |journal=Astronomy & Astrophysics |volume=594 |pages=A116 |doi=10.1051/0004-6361/201629178 |arxiv=1610.06175 |bibcode=2016A&A...594A.116H |s2cid=118612998 |issn=0004-6361|hdl=10150/622791 |hdl-access=free }}</ref>
* Molecular clouds are detected via spectral lines produced by changes in the rotational quantum state of small molecules, especially [[carbon monoxide]], CO. The most widely used line is at 115&nbsp;GHz, corresponding to the change from 1 to 0 quanta of [[angular momentum]]. [[List of interstellar and circumstellar molecules|Hundreds of other molecules]] have been detected, each with many lines, which allows physical and chemical processes in molecular clouds to be traced in some detail. These lines are most common at millimetre and sub-mm wavelengths. By far the most common molecule in molecular clouds, H<sub>2</sub>, is usually not directly observable, as it stays in its ground state except when excited by rare events such as interstellar shock waves. There is some 'dark gas', regions where hydrogen is in molecular form and therefore does not emit the 21-cm line, but CO molecules are broken up so the CO lines are also not present. These regions are inferred from the presence of dust grains with no matching line emission from gas.<ref>{{Cite journal |last1=Planck Collaboration |last2=Ade |first2=P. A. R. |last3=Aghanim |first3=N. |last4=Arnaud |first4=M. |last5=Ashdown |first5=M. |last6=Aumont |first6=J. |last7=Baccigalupi |first7=C. |last8=Balbi |first8=A. |last9=Banday |first9=A. J. |last10=Barreiro |first10=R. B. |last11=Bartlett |first11=J. G. |last12=Battaner |first12=E. |last13=Benabed |first13=K. |last14=Benoît |first14=A. |last15=Bernard |first15=J.-P. |date=2011 |title=Planck early results. XIX. All-sky temperature and dust optical depth from Planck and IRAS. Constraints on the "dark gas" in our Galaxy |url=http://www.aanda.org/10.1051/0004-6361/201116479 |journal=Astronomy & Astrophysics |volume=536 |pages=A19 |doi=10.1051/0004-6361/201116479 |arxiv=1101.2029 |bibcode=2011A&A...536A..19P |s2cid=664726 |issn=0004-6361|hdl=10138/233735 |hdl-access=free }}</ref>
* Interstellar dust grains re-emit the energy they absorb from starlight as quasi-blackbody emission in the far infrared, corresponding to typical dust grain temperatures of 20–100&nbsp;K. Very small grains, essentially fragments of [[graphene]] bonded to hydrogen atoms around their edges ([[polycyclic aromatic hydrocarbons]], PAHs), emit numerous spectral lines in the mid-infrared, at wavelengths around 10&nbsp;micron. Nanometre-sized grains can be spun up to rotate at GHz frequencies by a collision with a single ultraviolet photon, and [[dipole radiation]] from such spinning grains is believed to be the source of [[anomalous microwave emission]].
* Cosmic rays generate gamma-ray photons when they collide with atomic nuclei in ISM clouds. The electrons amongst cosmic ray particles collide with a small fraction of photons in the interstellar radiation field and the [[cosmic microwave background]] and bump up the photon energies to X-rays and gamma-rays, via [[inverse Compton scattering]]. Due to the galactic magnetic field, charged particles follow spiral paths, and for cosmic-ray electrons this spiralling motion generates [[synchrotron radiation]] which is very bright at low radio frequencies.