Editing Astronomy (section)

=== Galactic astronomy ===
[[File:Galactic longitude.JPG|thumb|A diagram of the Sun's location in the Milky Way, the angles represent longitudes in the [[galactic coordinate system]]]]

{{Main|Galactic astronomy}}
The [[Solar System]] orbits within the [[Milky Way]], a [[barred spiral galaxy]] that is a prominent member of the [[Local Group]] of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.<ref name=":0" />{{Rp|pages=837–842,944}}

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a [[supermassive black hole]] at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, [[Stellar population|population I]] stars. The disk is surrounded by a [[Galactic spheroid|spheroid halo]] of older, [[population II]] stars, as well as relatively dense concentrations of stars known as [[globular cluster]]s.<ref>{{cite web|url=http://www.mpe.mpg.de/ir/GC/index.php|title=The Galactic Centre|last=Ott|first=Thomas|date=24 August 2006|publisher=Max-Planck-Institut für extraterrestrische Physik|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060904140550/http://www.mpe.mpg.de/ir/GC/index.php|archive-date=4 September 2006 }}</ref>

Between the stars lies the [[interstellar medium]], a region of sparse matter. In the densest regions, [[molecular cloud]]s of [[Hydrogen|molecular hydrogen]] and other elements create star-forming regions. These begin as a compact [[pre-stellar core]] or [[dark nebula]]e, which concentrate and collapse (in volumes determined by the [[Jeans length]]) to form compact protostars.<ref name=Smith2004>{{cite book|first=Michael David|last=Smith|date=2004|pages=53–86|title=The Origin of Stars|chapter=Cloud formation, Evolution and Destruction|publisher=Imperial College Press|isbn=978-1-86094-501-4|chapter-url=https://books.google.com/books?id=UVgBoqZg8a4C|access-date=26 August 2020|archive-date=13 August 2021|archive-url=https://web.archive.org/web/20210813210429/https://books.google.com/books?id=UVgBoqZg8a4C|url-status=live}}</ref>

As the more massive stars appear, they transform the cloud into an [[H II region]] (ionized atomic hydrogen) of glowing gas and plasma. The [[Solar wind|stellar wind]] and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young [[open cluster]]s of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.<ref>{{cite book|first=Michael David|last=Smith|date=2004|pages=185–99|title=The Origin of Stars|chapter=Massive stars|publisher=Imperial College Press|isbn=978-1-86094-501-4|chapter-url=https://books.google.com/books?id=UVgBoqZg8a4C|access-date=26 August 2020|archive-date=13 August 2021|archive-url=https://web.archive.org/web/20210813210429/https://books.google.com/books?id=UVgBoqZg8a4C|url-status=live}}</ref>

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A [[dark matter halo]] appears to dominate the mass, although the nature of this dark matter remains undetermined.<ref>{{cite journal|author=Van den Bergh, Sidney|title=The Early History of Dark Matter|journal=Publications of the Astronomical Society of the Pacific|date=1999|volume=111|issue=760|pages=657–60|doi=10.1086/316369|arxiv = astro-ph/9904251 |bibcode = 1999PASP..111..657V |s2cid=5640064}}</ref>