View source for Astronomy:Sagittarius A

[[File:Labeled Map of the Milky Way Center.tif|thumb|Sgr A and environs, as seen at 90 cm wavelength by the [[Astronomy:Very Large Array|Very Large Array]]]]
{{Starbox begin
| name= Sagittarius A}}
{{Starbox observe
| epoch = J2000
| constell = Sagittarius
| pronounce = <!--Pronunciation guide-->
| ra ={{RA|17|45|40.0409}} <ref name="Reidetal" /><!--or {{Deg2HMS|ddd.dddd|sup=yes}}-->
| dec = {{DEC|-29|00|28.118}} <ref name="Reidetal" /><!--Declination −29° 00′ 28.118″ -->
| appmag_v = <!--Apparent magnitude (Johnson-Cousins V  system)-->
}}
{{Starbox astrometry
| radial_v=46
| prop_mo_ra=
| prop_mo_dec=
|  gal_lat=+00.147
|  gal_lon=000.064
| parallax=
| p_error=
| absmag_v= }}
{{Starbox detail
| mass=~4.1 million
| radius=31.6
| luminosity=
| temperature=
| metal=
| escape velocity= 801 194 075 m/s
| gravity=
| rotation=
| age=+10.000}}
{{Starbox catalog
| names= AX J1745.6-2900, SAGITTARIUS A, W 24, Cul 1742-28, SGR A, [DGW65] 96, EQ 1742-28, RORF 1742-289, [SKM2002] 28. }}
{{Starbox reference |
  Simbad=Sagittarius+A }}
{{Starbox end}}

'''Sagittarius A''' ('''Sgr A''') is a complex [[Astronomy:Astronomical radio source|radio source]] at the [[Galactic Center|center]] of the [[Astronomy:Milky Way|Milky Way]], which contains a [[Astronomy:Supermassive black hole|supermassive black hole]]. It is located in the [[Astronomy:Constellation|constellation]] [[Astronomy:Sagittarius (constellation)|Sagittarius]], and is [[Astronomy:Extinction|hidden]] from view at [[Physics:Visible spectrum|optical wavelengths]] by large [[Astronomy:Interstellar cloud|clouds]] of [[Astronomy:Cosmic dust|cosmic dust]] in the spiral arms of the Milky Way. The [[Astronomy:Dust lane|dust lane]] that obscures the Galactic Center from a vantage point around the [[Astronomy:Sun|Sun]] causes the [[Astronomy:Great Rift|Great Rift]] through the bright [[Astronomy:Galactic bulge|bulge]] of the galaxy.

The radio source consists of three components: the [[Astronomy:Supernova remnant|supernova remnant]] '''Sagittarius A East''', the [[Spiral|spiral]] structure '''Sagittarius A West''', and a very bright compact radio source at the center of the spiral, [[Astronomy:Sagittarius A*|Sagittarius A*]] (read "A-star"). These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West.

==Discovery==
In April 1933, [[Biography:Karl Guthe Jansky|Karl Jansky]], considered one of the fathers of radio astronomy, discovered that a radio signal was coming from a location in the direction of the constellation of Sagittarius, towards the center of the Milky Way.<ref>{{cite web |url=https://armaghplanet.com/karl-jansky-the-father-of-radio-astronomy.html |title=Karl Jansky: The Father of Radio Astronomy |date=29 August 2012 |access-date=2019-01-27 |archive-date=2019-06-28 |archive-url=https://web.archive.org/web/20190628163259/http://www.armaghplanet.com/karl-jansky-the-father-of-radio-astronomy.html |url-status=live }}</ref> His observations did not extend quite as far south as we now know to be the Galactic Center.<ref name="goss_history">{{cite journal |last1=Goss |first1=W. M. |last2=McGee |first2=R. X. |title=The Discovery of the Radio Source Sagittarius A (Sgr A) |journal=The Galactic Center, Astronomical Society of the Pacific Conference Series |date=1996 |volume=102 |pages=369 |bibcode=1996ASPC..102..369G |url=https://ui.adsabs.harvard.edu/abs/1996ASPC..102..369G/abstract |access-date=25 February 2021 |archive-date=3 March 2021 |archive-url=https://web.archive.org/web/20210303171739/https://ui.adsabs.harvard.edu/abs/1996ASPC..102..369G/abstract |url-status=live }}</ref> Observations by Jack Piddington and [[Biography:Harry Clive Minnett|Harry Minnett]] using the [[Earth:CSIRO|CSIRO]] radio telescope at Potts Hill Reservoir, in [[Place:Sydney|Sydney]] discovered a discrete and bright "Sagittarius-Scorpius" radio source,<ref name="piddington">{{cite journal |last1=Piddington |first1=J. H. |last2=Minnett |first2=H. C. |title=Observations of Galactic Radiation at Frequencies of 1200 and 3000 Mc/s. |journal=Australian Journal of Scientific Research A |date=1 December 1951 |volume=4 |issue=4 |pages=459 |doi=10.1071/CH9510459 |bibcode=1951AuSRA...4..459P |url=https://ui.adsabs.harvard.edu/abs/1951AuSRA...4..459P/abstract |access-date=25 February 2021 |archive-date=13 April 2021 |archive-url=https://web.archive.org/web/20210413090955/https://ui.adsabs.harvard.edu/abs/1951AuSRA...4..459P/abstract |url-status=live }}</ref> which after further observation with the {{Convert|80|ft|m|abbr=off|adj=on}} CSIRO radio telescope at Dover Heights was identified in a letter to ''Nature'' as the probable Galactic Center.<ref name="galactic_centre_mcgee">{{cite journal |last1=McGee |first1=R. X. |last2=Bolton |first2=J. G. |title=Probable observation of the galactic nucleus at 400 Mc./s. |journal=Nature |date=1 May 1954 |volume=173 |issue=4412 |pages=985–987 |doi=10.1038/173985b0 |bibcode=1954Natur.173..985M |s2cid=4188235 |url=https://ui.adsabs.harvard.edu/abs/1954Natur.173..985M/abstract |access-date=25 February 2021 |issn=0028-0836 |archive-date=30 January 2022 |archive-url=https://web.archive.org/web/20220130113909/https://ui.adsabs.harvard.edu/abs/1954Natur.173..985M/abstract |url-status=live }}</ref> The name Sagittarius A was first used in 1954 by John D. Kraus, Hsien-Ching Ko, and Sean Matt<ref>{{Cite journal |last1=Kraus |first1=J. D. |last2=Ko |first2=H. C. |last3=Matt |first3=S. |date=December 1954 |title=Galactic and localized source observations at 250 megacycles per second. |url=https://ui.adsabs.harvard.edu/abs/1954AJ.....59..439K |journal=[[Astronomy:The Astronomical Journal|The Astronomical Journal]] |volume=59 |pages=439&ndash;443 |bibcode=1954AJ.....59..439K |doi=10.1086/107059 |issn=0004-6256 |via=The ADS |access-date=May 16, 2022 |archive-date=June 10, 2022 |archive-url=https://web.archive.org/web/20220610145454/https://ui.adsabs.harvard.edu/abs/1954AJ.....59..439K |url-status=live }}</ref> when they included the object in the list of radio sources found with the [[Organization:Ohio State University|Ohio State University]] radio telescope at 250&nbsp;MHz. As was common practice at the time, sources were named by constellation with capital letters in order of brightness within each constellation, with A denoting the brightest radio source within the constellation.

==Sagittarius A East==
This feature is approximately 25 [[Astronomy:Light-year|light-year]]s in width and has the attributes of a supernova remnant from an explosive event that occurred between 35,000 and 100,000 BC. However, it would take 50 to 100 times more energy than a standard [[Astronomy:Supernova|supernova explosion]] to create a structure of this size and energy. It is conjectured that Sgr A East is the remnant of the explosion of a star that was gravitationally compressed as it made a close approach to the central [[Astronomy:Black hole|black hole]].<ref>{{cite journal |last1=Maeda |first1=Y. |last2=Baganoff |first2=F. K. |last3=Feigelson |first3=E. D. |last4=Morris |first4=M. |last5=Bautz |first5=M. W. |last6=Brandt |first6=W. N. |last7=Burrows |first7=D. N. |last8=Doty |first8=J. P. |last9=Garmire |first9=G. P. |last10=Pravdo |first10=S. H. |last11=Ricker |first11=G. R. |last12=Townsley |first12=L. K. |title=AChandra ''Study'' of Sagittarius a East: A Supernova Remnant Regulating the Activity of Our Galactic Center? |url=https://sirrah.troja.mff.cuni.cz/~schovan/diplomka/tyden29-32/CND_mass/Maeda_etal_2002__2002ApJ...570..671M.pdf |access-date=2020-08-29 |journal=The Astrophysical Journal |date=2002-05-10 |volume=570 |issue=2 |pages=671–687 |doi=10.1086/339773 |arxiv=astro-ph/0102183 |bibcode=2002ApJ...570..671M |s2cid=18482187 |archive-date=2022-01-20 |archive-url=https://web.archive.org/web/20220120205914/https://sirrah.troja.mff.cuni.cz/~schovan/diplomka/tyden29-32/CND_mass/Maeda_etal_2002__2002ApJ...570..671M.pdf |url-status=live }}</ref>

==Sagittarius A West==
[[Image:SgrAWest BEAR.jpg|thumb|Surface brightness and velocity field of the inner part of Sagittarius A West]]
Sgr A West has the appearance of a three-arm spiral, from the point of view of the Earth.  For this reason, it is also known as the "Minispiral". This appearance and nickname are misleading, though: the three-dimensional structure of the Minispiral is ''not'' that of a spiral. It is made of several [[Astronomy:Interstellar cloud|dust and gas clouds]], which orbit and fall onto Sagittarius A* at velocities as high as 1,000 kilometers per second. The surface layer of these clouds is ionized. The source of ionisation is the population of massive stars (more than one hundred [[Astronomy:OB star|OB star]]s have been identified so far) that also occupy the central [[Astronomy:Parsec|parsec]].

Sgr A West is surrounded by a massive, clumpy [[Torus|torus]] of cooler molecular gas, the Circumnuclear Disk (CND). The nature and [[Physics:Kinematics|kinematics]] of the Northern Arm cloud of Sgr A West suggest that it once was a clump in the CND, which fell due to some perturbation, perhaps the supernova explosion responsible for Sgr A East. The Northern Arm appears as a very bright North—South ridge of emission, but it extends far to the East and can be detected as a dim extended source.

The Western Arc (outside the field of view of the image shown in the right) is interpreted as the ionized inner surface of the CND. The Eastern Arm and the Bar seem to be two additional large clouds similar to the Northern Arm, although they do not share the same [[Astronomy:Orbital plane|orbital plane]]. They have been estimated to amount for about 20 solar masses each.

On top of these large scale structures (of the order of a few light-years in size), many smaller cloudlets and holes inside the large clouds can be seen. The most prominent of these perturbations is the Minicavity, which is interpreted as a bubble blown inside the Northern Arm by the [[Astronomy:Stellar wind|stellar wind]] of a massive star, which is not clearly identified.

==Sagittarius A*==
{{main|Astronomy:Sagittarius A*}}
[[File:EHT Saggitarius A black hole.tif|thumb|The  supermassive black hole Sagittarius A*, imaged by the Event Horizon Telescope.<ref>{{cite news |title=Astronomers reveal first image of the black hole at the heart of our galaxy |url=https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy |access-date=2022-05-12 |newspaper=Event Horizon Telescope |url-status=live |archive-date=2022-05-12 |archive-url=https://web.archive.org/web/20220512132514/https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy }}</ref>]]
Astronomers now have evidence that there is a [[Astronomy:Supermassive black hole|supermassive black hole]] at the center of the galaxy.<ref>[http://news.bbc.co.uk/1/hi/sci/tech/7774287.stm Black hole confirmed in Milky Way] {{Webarchive|url=https://web.archive.org/web/20090215113111/http://news.bbc.co.uk/1/hi/sci/tech/7774287.stm |date=2009-02-15 }}, Ghosh, Pallab, BBC News. 2008-12-09. Retrieved 2008-12-10.</ref> Sagittarius A* (abbreviated Sgr A*) is agreed to be the most plausible candidate for the location of this supermassive black hole. The [[Astronomy:Very Large Telescope|Very Large Telescope]] at Chile and Keck Telescope at Hawaii have detected stars orbiting Sgr A* at speeds greater than that of any other stars in the galaxy. One star, designated [[Astronomy:S2 (star)|S2]], was calculated to orbit Sgr A* at speeds of over 5,000 [[Kilometre|kilometers]] per second at its closest approach.<ref>{{cite web |url=http://www.eso.org/public/news/eso0226/ |title=Surfing a Black Hole - Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours |last= |website=www.eso.org |access-date=2011-05-16 |archive-date=2012-10-16 |archive-url=https://web.archive.org/web/20121016000125/http://www.eso.org/public/news/eso0226/ |url-status=live }}</ref>

A gas cloud, G2, passed through the Sagittarius A* region in 2014 and managed to do so without disappearing beyond the [[Astronomy:Event horizon|event horizon]], as theorists predicted would happen. Rather, it disintegrated, suggesting that G2 and a previous gas cloud, G1, were star remnants with larger gravitational fields than gas clouds.<ref>{{cite news |last1=Lemonick |first1=Michael |title=Black Hole Fails to Destroy Mystery Cosmic Cloud |url=https://www.nationalgeographic.com/adventure/article/141106-space-g2-black-hole-science |access-date=26 December 2022 |work=National Geographic |date=6 November 2014 |archive-url=https://ghostarchive.org/archive/20221226/https://www.nationalgeographic.com/adventure/article/141106-space-g2-black-hole-science |archive-date=2022-12-27}}</ref><ref>{{cite news |last1=Cowen |first1=Ron |title=It's Snack Time in the Cosmos |url=https://www.nytimes.com/2014/02/18/science/its-snack-time-in-the-cosmos.html?_r=0 |access-date=26 December 2022 |work={{wipe|The New York Times}} |date=2017-03-02 |archive-url=https://web.archive.org/web/20170302053406/https://www.nytimes.com/2014/02/18/science/its-snack-time-in-the-cosmos.html?_r=0 |archive-date=2017-03-02}}</ref>

In September 2019, scientists found that Sagittarius A* had been consuming nearby matter at a much faster rate than usual over the previous year. Researchers speculated that this could mean that the black hole is entering a new phase, or that Sagittarius A* had stripped the outer layer of G2 when it passed through.<ref>{{cite news |last=Devlin |first=Hannah |title=Black hole at centre of galaxy is getting hungrier, say scientists |url=https://www.theguardian.com/science/2019/sep/13/black-hole-at-centre-of-our-galaxy-is-getting-hungrier-say-scientists |work=The Guardian |date=2019-09-13 |access-date=2019-09-18 |issn=0261-3077 |language=en-GB |archive-date=2019-09-17 |archive-url=https://web.archive.org/web/20190917181608/https://www.theguardian.com/science/2019/sep/13/black-hole-at-centre-of-our-galaxy-is-getting-hungrier-say-scientists |url-status=live }}</ref>

== Sagittarius A in popular culture ==
* In the 2014 space-sim videogame ''Elite: Dangerous'' players are able to travel to Sagittarius A*, with an [[Software:Achievement (video gaming)|achievement]] tied to it in the [[Engineering:Xbox One|Xbox One]] and [[Engineering:PlayStation (console)|PlayStation]] versions of the game.
* In the television show ''Community'', Pierce Hawthorne mentions that in his opinion, Sagittarius A is the only black hole worth studying.
* In the final arc of the ''Sailor Moon'' manga series, "Sagittarius Zero Star" is the location of the Galaxy Cauldron, a fictional artifact that serves as the birthplace of all life in the Milky Way.<ref name="vol18">{{cite book |last1=Takeuchi |first1=Naoko |title=Prety Soldier Sailor Moon: Volume 18 |publisher=Kodansha |location=Tokyo |date=1997 |isbn=4061788582}}</ref>{{rp|Act 59}}

==References==
{{reflist|refs=<ref name="Reidetal">{{cite journal | last1=Reid | first1=M.J. | last2=Brunthaler | first2=A. | title=The Proper Motion of Sagittarius A* | journal=The Astrophysical Journal | volume=616 | issue=2 | pages=874, 883 | date=2004 | bibcode=2004ApJ...616..872R|arxiv = astro-ph/0408107 |doi = 10.1086/424960 | s2cid=16568545 }}</ref>}}

==Further reading==
* {{cite book |last=Melia |first=Fulvio |title=The Black Hole in the Center of Our Galaxy |url=https://archive.org/details/blackholeatcente0000meli |url-access=registration |publisher=Princeton University Press |location=Princeton, NJ / Oxford |date=2003 |isbn=978-0-691-09505-9}}

==External links==
* [http://www.mpe.mpg.de/www_ir/ir_recent.php Recent Results of the MPE Infrared/Submillimeter Group] {{Webarchive|url=https://web.archive.org/web/20021107121511/http://www.mpe.mpg.de/www_ir/ir_recent.php |date=2002-11-07 }}
* [http://www.mpe.mpg.de/ir/GC/ Galactic Center Research at MPE]
* [http://www.nature.com/nature/links/021017/021017-5.html ''Nature'' report, with link to the Schödel et al. paper]
* [http://chandra.harvard.edu/photo/2001/sgr_a/index.html Sagittarius A East]
* [https://arxiv.org/abs/astro-ph/0405197 Kinematic and structural analysis of the Minispiral in the Galactic Center from BEAR spectro-imagery (preprint)]
* [http://chandra.harvard.edu/photo/2003/0203long/index.html Chandra Photo Album Sagittarius A]
* [http://archive.ncsa.uiuc.edu/Cyberia/Bima/GalCntr.html The Galactic Center (outdated)] {{Webarchive|url=https://web.archive.org/web/20090131204105/http://archive.ncsa.uiuc.edu/Cyberia/Bima/GalCntr.html |date=2009-01-31 }}
* [[Organization:NASA|NASA]] Image of the Day Gallery, January 6, 2010 [http://www.nasa.gov/multimedia/imagegallery/image_feature_1559.html Into the Heart of Darkness] - [[Astronomy:Chandra X-ray Observatory|Chandra X-ray Observatory]] image.
* [http://www.constellation-guide.com/sagittarius-a/ Sagittarius A at Constellation Guide]

{{Milky Way}}

[[Category:Astronomical radio sources]]
[[Category:Supernova remnants|Sagittarius A East]]
[[Category:Sagittarius (constellation)]]

{{Sourceattribution|Sagittarius A}}