Editing Interstellar cloud (section)

==High-velocity cloud==
{{Main|High-velocity cloud}}
[[File:IRAS 10082-5647.jpg|thumb|[[Reflection nebula]] IRAS 10082-5647 observed by the [[Hubble Space Telescope]].]]

These interstellar clouds possess a velocity higher than can be explained by the rotation of the [[Milky Way]].<ref>Navarro, J. F., Frenk, C. S., & White, S. D. M. 1995, [[Monthly Notices of the Royal Astronomical Society]], 275, 720</ref> By definition, these clouds must have a v<sub>lsr</sub> greater than 90&nbsp;km s<sup>−1</sup>, where v<sub>lsr</sub> is the local standard rest velocity. They are detected primarily in the [[hydrogen line|21 cm line]] of neutral [[hydrogen]],<ref>{{Cite web |title=Dark Matter- More Than Meets the Eye |url=http://genesismission.jpl.nasa.gov/educate/scimodule/Cosmogony/CosmogonyPDF/MilkyWaySurpriseST.pdf |archive-url=https://web.archive.org/web/20190611235414/http://genesismission.jpl.nasa.gov/educate/scimodule/Cosmogony/CosmogonyPDF/MilkyWaySurpriseST.pdf |archive-date=Jun 11, 2019 |access-date=12 February 2010 |publisher=[[NASA]]}}</ref> and typically have a lower portion of heavy elements than is normal for interstellar clouds in the Milky Way.

Theories intended to explain these unusual clouds include materials left over from the formation of the galaxy, or [[tidal force|tidally-displaced]] [[matter]] drawn away from other galaxies or members of the [[Local Group]]. An example of the latter is the [[Magellanic Stream]]. To narrow down the origin of these clouds, a better understanding of their distances and [[metallicity]] is needed.

High-velocity clouds are identified with an HVC prefix, as with [[HVC 127-41-330]].