Editing Interstellar medium (section)

===Structures===
[[File:The Local Interstellar Cloud and neighboring G-cloud complex.svg|thumb|200px|Map showing the [[Sun]] located near the edge of the Local Interstellar Cloud and [[Alpha Centauri]] about 4 [[light-year]]s away in the neighboring [[G-Cloud]] complex]]
[[File:Interstellar medium annotated.jpg|thumb|upright=2|Interstellar medium and [[astrosphere]] meeting]]

Astronomers describe the ISM as [[turbulence|turbulent]], meaning that the gas has quasi-random motions coherent over a large range of spatial scales. Unlike normal turbulence, in which the [[fluid]] motions are highly [[Subsonic speed|subsonic]], the bulk motions of the ISM are usually larger than the [[sound speed]]. Supersonic collisions between gas clouds cause [[shock waves]] which compress and heat the gas, increasing the sounds speed so that the flow is locally subsonic; thus supersonic turbulence has been described as 'a box of shocklets', and is inevitably associated with complex density and temperature structure. In the ISM this is further complicated by the magnetic field, which provides wave modes such as [[Alfvén wave]]s which are often faster than pure sound waves: if turbulent speeds are supersonic but below the Alfvén wave speed, the behaviour is more like subsonic turbulence.

[[Star formation|Stars are born]] deep inside large complexes of [[molecular clouds]], typically a few parsecs in size. During their lives and deaths, [[star]]s interact physically with the ISM.

Stellar winds from young clusters of stars (often with giant or supergiant [[HII region]]s surrounding them) and [[shock wave]]s created by supernovae inject enormous amounts of energy into their surroundings, which leads to hypersonic turbulence. The resultant structures – of varying sizes – can be observed, such as [[stellar wind bubble]]s and [[superbubble]]s of hot gas, seen by X-ray satellite telescopes or turbulent flows observed in [[radio telescope]] maps.

Stars and planets, once formed, are unaffected by pressure forces in the ISM, and so do not take part in the turbulent motions, although stars formed in molecular clouds in a galactic disk share their general orbital motion around the galaxy center. Thus stars are usually in motion relative to their surrounding ISM. The [[Sun]] is currently traveling through the [[Local Interstellar Cloud]], an irregular clump of the warm neutral medium a few parsecs  across, within the low-density [[Local Bubble]], a 100-parsec radius region of coronal gas.

In October 2020, astronomers reported a significant unexpected increase in density in the [[outer space|space]] beyond the [[Solar System]] as detected by the ''Voyager 1'' and ''[[Voyager 2]]'' [[space probe]]s. According to the researchers, this implies that "the density gradient is a large-scale feature of the [[#Structures|VLISM]] (very local interstellar medium) in the general direction of the [[Heliosphere#Outer structure|heliospheric nose]]".<ref name="SA-20201019">{{cite news |last=Starr |first=Michelle |title=Voyager Spacecraft Detect an Increase in The Density of Space Outside The Solar System |url=https://www.sciencealert.com/for-some-reason-the-density-of-space-is-higher-just-outside-the-solar-system |date=19 October 2020 |work=[[ScienceAlert]] |access-date=19 October 2020 }}</ref><ref name="AJL-20200825">{{cite journal |last1=Kurth |first1=W.S. |last2=Gurnett |first2=D.A. |title=Observations of a Radial Density Gradient in the Very Local Interstellar Medium by Voyager 2 |date=25 August 2020 |journal=[[The Astrophysical Journal Letters]] |volume=900 |number=1 |pages=L1 |doi=10.3847/2041-8213/abae58 |bibcode=2020ApJ...900L...1K |s2cid=225312823 |doi-access=free }}</ref>