What is Active galactic nucleus?
[[Image:M87 jet.jpg|right|thumb| 300px|[[Hubble Space Telescope]] image of a 5000-[[light-year]]-long (1.5-[[parsec|kiloparsec]]-long) jet being ejected from the active nucleus of the active [[Messier 87|galaxy M87]], a [[radio galaxy]]. The blue [[synchrotron radiation]] of the jet contrasts with the yellow starlight from the host galaxy.]] An '''active galactic nucleus''' ('''AGN''') is a compact region at the centre of a [[galaxy]]. This central region radiates huge [[luminosity]] over the [[electromagnetic spectrum]]. The radiation from an AGN is thought to be caused by [[mass]] dragged in by the [[gravitation]]al pull of a [[supermassive black hole]] at the centre of the host galaxy. This [[electromagnetic radiation]] is observed in the [[radio waves|radio]], [[microwaves]], [[infrared]], [[visible spectrum|optical]], [[ultra-violet]], [[X-ray]] and [[gamma ray]] wavebands. A galaxy hosting an AGN is called an '''active galaxy'''. AGN are the most luminous and persistent sources of electromagnetic radiation in the universe. They can be used to discover distant objects. All theories about [[cosmology|the cosmos]] have to account for AGNs. They are one of the basic facts about the [[universe]]. So-called [[relativistic jet]]s are extremely powerful jets of [[plasma]] which come from some AGNs, notably [[radio galaxy|radio galaxies]] and [[quasar]]s.<ref>{{cite journal|title=What is the structure of relativistic jets in AGN on scales of light days?|author=Wehrle A.E. |display-authors = etal |date= 2009 |url=http://www.nrao.edu/A2010/whitepapers/rac/Wehrle_AGN_jets_GCT.pdf}}</ref> Their lengths can reach several thousand or even hundreds of thousands of light years.<ref>Biretta J. 1999. Hubble detects faster-than-llight motion in Galaxy M87 (http://www.stsci.edu/ftp/science/m87/m87.html)</ref><ref>Yale University - Office of Public Affairs 2006. Evidence for ultra-energetic particles in jet from black hole (https://web.archive.org/web/20080513034113/http://www.yale.edu/opa/newsr/06-06-20-01.all.html)</ref