Editing Outer space (section)

=== Interstellar space ===
{{Redirect|Interstellar space|the album|Interstellar Space}}
{{Main|Interstellar medium}}
[[File:52706main hstorion lg.jpg|right|thumb|[[Bow shock]] formed by the [[magnetosphere]] of the young star [[LL Orionis]] (center) as it collides with the [[Orion Nebula]] flow|alt=Patchy orange and blue nebulosity against a black background, with a curved orange arc wrapping around a star at the center.]]

Interstellar space is the physical space outside of the bubbles of plasma known as [[astrosphere]]s, formed by [[stellar wind]]s originating from individual stars, or formed by solar wind emanating from the Sun.<ref name=jpl_interstellarspace /> It is the space between the stars or [[stellar systems]] within a nebula or galaxy.<ref name=Cooper_2023/> Interstellar space contains an [[interstellar medium]] of sparse matter and radiation. The boundary between an astrosphere and interstellar space is known as an [[astropause]]. For the Sun, the astrosphere and astropause are called the heliosphere and heliopause, respectively.<ref name=Garcia-Sage_et_al_2023/>

Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms. This is enriched with trace amounts of heavier atoms formed through [[stellar nucleosynthesis]]. These atoms are ejected into the interstellar medium by stellar winds or when evolved stars begin to shed their outer envelopes such as during the formation of a [[planetary nebula]].<ref name="Ferrière2001"/> The cataclysmic explosion of a [[supernova]] propagates [[shock waves]] of stellar ejecta outward, distributing it throughout the interstellar medium, including the heavy elements previously formed within the star's core.<ref name="witt2001"/> The density of matter in the interstellar medium can vary considerably: the average is around 10<sup>6</sup> particles per m<sup>3</sup>,<ref name=Boulares1990/> but cold [[molecular clouds]] can hold 10<sup>8</sup>–10<sup>12</sup> per m<sup>3</sup>.{{sfn|Prialnik|2000|pp=195–196}}<ref name="Ferrière2001"/>

A [[List of interstellar and circumstellar molecules|number of molecules]] exist in interstellar space, which can form dust particles as tiny as 0.1&nbsp;[[Micrometre|μm]].{{sfn|Rauchfuss|2008|pp=72–81}} The tally of molecules discovered through [[radio astronomy]] is steadily increasing at the rate of about four new species per year. Large regions of higher density matter known as molecular clouds allow chemical reactions to occur, including the formation of organic polyatomic species. Much of this chemistry is driven by collisions. Energetic cosmic rays penetrate the cold, dense clouds and ionize hydrogen and helium, resulting, for example, in the [[trihydrogen cation]]. An ionized helium atom can then split relatively abundant [[carbon monoxide]] to produce ionized carbon, which in turn can lead to organic chemical reactions.<ref name="PNAS103_33_12232"/>

The local interstellar medium is a region of space within 100&nbsp;[[parsec|pc]] of the Sun, which is of interest both for its proximity and for its interaction with the Solar System. This volume nearly coincides with a region of space known as the [[Local Bubble]], which is characterized by a lack of dense, cold clouds. It forms a cavity in the [[Orion Arm]] of the Milky Way Galaxy, with dense molecular clouds lying along the borders, such as those in the [[constellation]]s of [[Ophiuchus]] and [[Taurus (constellation)|Taurus]]. The actual distance to the border of this cavity varies from 60 to 250&nbsp;pc or more. This volume contains about 10<sup>4</sup>–10<sup>5</sup> stars and the local interstellar gas counterbalances the [[Stellar-wind bubble|astrospheres]] that surround these stars, with the volume of each sphere varying depending on the local density of the interstellar medium. The Local Bubble contains dozens of warm interstellar clouds with temperatures of up to 7,000&nbsp;K and radii of 0.5–5&nbsp;pc.<ref name=redfield2006/>

When stars are moving at sufficiently high [[peculiar velocities]], their astrospheres can generate [[bow shock]]s as they collide with the interstellar medium. For decades it was assumed that the Sun had a bow shock. In 2012, data from [[Interstellar Boundary Explorer|Interstellar Boundary Explorer (IBEX)]] and NASA's [[Voyager program|Voyager]] probes showed that the Sun's bow shock does not exist. Instead, these authors argue that a [[Mach number|subsonic]] bow wave defines the transition from the solar wind flow to the interstellar medium.<ref name=bow_science/><ref name=bow/> A bow shock is a third boundary characteristic of an astrosphere, lying outside the [[heliosphere#Termination shock|termination shock]] and the astropause.<ref name=bow/>