Sagittarius B2

Sagittarius B2 (Sgr B2) is a giant molecular cloud of gas and dust that is located about 120 parsecs from the center of the Milky Way. This complex is the largest molecular cloud in the vicinity of the core and one of the largest in the galaxy, spanning a region about 45 parsecs across.cite news
last=Chown | first=Marcus | date=November 27, 1999
title=Star attraction | publisher=New Scientist
url=http://www.newscientist.com/article/mg16422144.900-star-attraction.html
accessdate=2007-10-29] The total mass of Sgr B2 is about 3 million times the mass of the Sun. [cite book
first=P. M. | last=Solomon
editors=Giancarlo Setti and Giovanni G. Fazio
year=1978
title=Physics of Molecular Clouds from Millimeter Wave Length Observations
work=Infrared Astronomy | publisher=Springer
location=New York | id=ISBN 9027708711 ] The mean hydrogen density within the cloud is 3000 atoms per cm³, which is about 20–40 times denser than a typical molecular cloud. [cite journal
last=Goldsmith | first=Paul F.
coauthors=Lis, Dariusz C.; Hills, Richard; Lasenby, Joan
title=High angular resolution submillimeter observations of Sagittarius B2
journal=Astrophysical Journal
year=1990 | volume=350 | pages=186–194
url=http://adsabs.harvard.edu/cgi-bin/bib_query?1990ApJ...350..186G
accessdate=2007-10-31
doi=10.1086/168372 ]

The internal structure of this cloud is complex, with varying densities and temperatures. The cloud is divided into three main cores, designated north (N), middle or main (M) and south (S) respectively. Thus Sgr B2(N) represents the north core. The sites Sgr B2(M) and Sgr B2(N) are sites of massive star formation. The first 10 H II regions discovered were designated A through J. [cite journal
last=Lis
first=Dariusz C.
coauthors=Goldsmith, Paul F.
title=Modeling of the continuum and molecular line emission from the Sagittarius B2 molecular cloud
journal=Astrophysical Journal, Part 1
year=1990
volume=356
pages=195–210
url=http://adsabs.harvard.edu/cgi-bin/bib_query?1990ApJ...356..195L
accessdate=2007-10-25
doi=10.1086/168830] H II regions A-G, I and J lie within Sgr B2(M), while region K is in Sgr B2(N) and region H is in Sgr B2(S). [cite journal
last=Takagi | first=Shin-ichiro
coauthors=Murakami, Hiroshi; Koyama, Katsuji
title=X-Ray Sources and Star Formation Activity in the Sagittarius B2 Cloud Observed with Chandra
journal=The Astrophysical Journal
year=2002 | volume=573 | pages=275–282
url=http://adsabs.harvard.edu/abs/2002astro.ph..3035T
accessdate=2007-10-31
doi=10.1086/340499 ] The 5-parsec-wide core of the cloud is a star-forming region that is emitting about 10 million times the luminosity of the Sun. [cite book
first=Ramon D. | last=Wolstencroft
coauthors=William Butler Burton | year=1988
title=Millimetre and Submillimetre Astronomy
publisher=Springer | id=ISBN 9027727635 ]

Temperatures in the cloud vary from 300 K in dense star-forming regions to 40 K in the surrounding envelope. [cite conference
first =P.
last =de Vicente
coauthor=Martin-Pintado, J.; Wilson, T. L.
title =A Hot Ring in the SGR B2 Molecular Cloud
booktitle =Proceedings Astronomical Society of the Pacific Conference Series
pages =64–67
publisher =Astronomical Society of the Pacific.
date =March 10-15, 1996
location =La Serena, Chile
url =http://adsabs.harvard.edu/abs/1996ASPC..102...64D
accessdate = 2007-10-29 ] Because the average temperature and pressure in Sgr B2 are low, chemistry based on the direct interaction of atoms is exceedingly slow. However, the Sgr B2 complex contains cold dust grains consisting of a silicon core surrounded by a mantle of water ice and various carbon compounds. The surfaces of these grains allow chemical reactions to occur by accreting molecules that can then interact with neighboring compounds. The resulting compounds can thenevaporate from the surface and join the molecular cloud.

The molecular components of this cloud can be readily observed in the 10²–10³ m range of wavelengths. About half of all the known interstellar molecules were first found near Sgr B2, and nearly every other currently known molecule has since been detected in this feature. [cite journal
author=S. E. Cummins, R. A. Linke, P. Thaddeus
title=A survey of the millimeter-wave spectrum of Sagittarius B2
journal=Astrophysical Journal Supplement Series | year=1986 | volume=60
pages=819–878 | url=http://adsabs.harvard.edu/abs/1986ApJS...60..819C
accessdate=2007-02-06
doi=10.1086/191102 ]

The European Space Agency's gamma-ray observatory INTEGRAL has observed gamma rays interacting with Sgr B2, causing x-ray emission from the molecular cloud. This energy was emitted about 350 years before by the supermassive black hole (SMBH) at the galaxy's core. The total energyfrom this outburst is an estimated million times stronger than the current output from the SMBH. [cite news
author=Staff | date=January 28, 2005
title=Integral rolls back history of Milky Way's super-massive black hole
publisher=Hubble News Desk
url=http://www.esa.int/SPECIALS/Integral/SEMSKPO3E4E_0.html
accessdate=2007-10-31 ]

ee also

* List of molecules in interstellar space

References

External links

*cite web
author=R. M. Gaume "et al"
date=October 31, 2007
url=http://www.nrao.edu/imagegallery/php/level3.php?id=55
title=Sagittarius B2 (North)
publisher=National Radio Astronomy Observatory
accessdate=2007-10-31