Editing X-ray astronomy (section)

===Young, low-mass stars===
{{main|pre-main-sequence star}}
[[File:Chandra_X-ray_View_of_Orion.jpg|thumb|right|A Chandra X-ray image of the [[Trapezium Cluster|Cluster]] of [[star formation|newly formed stars]] in the [[Orion Nebula]].]]
Newly formed stars are known as [[pre-main-sequence star]]s during the stage of stellar evolution before they reach the [[main-sequence]]. Stars in this stage (ages <10 million years) produce X-rays in their stellar coronae. However, their X-ray emission is 10<sup>3</sup> to 10<sup>5</sup> times stronger than for main-sequence stars of similar masses.<ref name=preibisch05>{{Cite journal | last = Preibisch | first = T. | display-authors = etal | year= 2005| title = The Origin of T Tauri X-Ray Emission: New Insights from the Chandra Orion Ultradeep Project | journal = Astrophysical Journal Supplement | volume = 160 | issue =2 | pages = 401–422 | doi = 10.1086/432891| bibcode = 2005ApJS..160..401P|arxiv = astro-ph/0506526 | s2cid = 18155082 }}</ref>

X-ray emission for pre–main-sequence stars was discovered by the [[Einstein Observatory]].<ref name=feigelson81>{{Cite journal | last1 = Feigelson | first1 = E. D. | last2=Decampli| first2=W. M. | year= 1981| title = Observations of X-ray emission from T Tauri stars | journal = Astrophysical Journal Letters| volume = 243 | pages = L89–L93 | doi = 10.1086/183449| bibcode = 1981ApJ...243L..89F}}</ref><ref name=montmerle83>{{Cite journal | last = Montmerle | first = T. | year= 1983| title = Einstein observations of the Rho Ophiuchi dark cloud - an X-ray Christmas tree | journal = Astrophysical Journal, Part 1 | volume = 269 | pages = 182–201 | doi = 10.1086/161029| bibcode = 1983ApJ...269..182M}}</ref> This X-ray emission is primarily produced by magnetic reconnection flares in the stellar coronae, with many small flares contributing to the "quiescent" X-ray emission from these stars.<ref name=feigelson99>{{Cite journal | last1 = Feigelson | first1 = E. D. | last2=Montmerle| first2=T. | year= 1999| title = High-Energy Processes in Young Stellar Objects | journal = Annual Review of Astronomy and Astrophysics | volume = 37 | pages =363–408 | doi = 10.1146/annurev.astro.37.1.363 | bibcode = 1999ARA&A..37..363F}}</ref> Pre–main sequence stars have large convection zones, which in turn drive strong dynamos, producing strong surface magnetic fields. This leads to the high X-ray emission from these stars, which lie in the saturated X-ray regime, unlike main-sequence stars that show [[rotational modulation]] of X-ray emission. Other sources of X-ray emission include accretion hotspots<ref name=kastner01>{{Cite journal | last = Kastner | first = J. H. | display-authors=etal | year= 2001| title = Discovery of Extended X-Ray Emission from the Planetary Nebula NGC 7027 by the Chandra X-Ray Observatory | journal = Astrophysical Journal | volume = 550 | issue =2 | pages = L189–L192 | doi = 10.1086/319651| bibcode = 2001ApJ...550L.189K|arxiv = astro-ph/0102468| s2cid = 12306305 }}</ref> and collimated outflows.<ref name=pravdo10>{{Cite journal | last = Pravdo | first = S. H. |display-authors=etal | year= 2001| title = Discovery of X-rays from the protostellar outflow object HH2 | journal = Nature | volume = 413 | issue = 6857 | pages = 708–711 | doi = 10.1038/35099508| bibcode = 2001Natur.413..708P| pmid=11607024| s2cid = 13878953 }}</ref>

X-ray emission as an indicator of stellar youth is important for studies of star-forming regions. Most star-forming regions in the Milky Way Galaxy are projected on [[Zone of Avoidance|Galactic-Plane fields]] with numerous unrelated field stars. It is often impossible to distinguish members of a young stellar cluster from field-star contaminants using optical and infrared images alone. X-ray emission can easily penetrate moderate absorption from molecular clouds, and can be used to identify candidate cluster members.<ref name=feigelson13>{{Cite journal | last = Feigelson | first = E. D. | display-authors=etal | year= 2013| title = Overview of the Massive Young Star-Forming Complex Study in Infrared and X-Ray (MYStIX) Project | journal = Astrophysical Journal Supplement | volume = 209 | issue =2 | pages = 26 | doi =10.1088/0067-0049/209/2/26  | bibcode = 2013ApJS..209...26F |arxiv = 1309.4483 | s2cid = 56189137 }}</ref>