Sagittarius A*

Sagittarius A*

Sgr A* (centre) and two light echoes from a recent explosion (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	17h 45m 40.045s[1]
Declination	-29° 0′ 27.9″[1]
Details
Mass	(4.31 ± 0.38) × 106[2] M☉ or (4.1 ± 0.6) × 106[3] M☉
Astrometry
Distance	25,900 ± 1,400 ly (7,940 ± 420[4] pc)

Sagittarius A* (pronounced "A-star", standard abbreviation Sgr A*) is a bright and very compact astronomical radio source at the center of the Milky Way Galaxy, part of a larger astronomical feature at that location (Sagittarius A). Sagittarius A* is likely to be the location of a supermassive black hole,^[5] as is hypothesized to be at the centers of many spiral and elliptical galaxies.

Supermassive black hole hypothesis

Several teams of researchers have attempted to image Sagittarius A* in the radio spectrum using Very Long Baseline Interferometry (VLBI). The current highest-resolution measurement, made at a wavelength of 1.3 mm, indicated a size for the source of 37 μas.^[6] At a 26,000 light-year distance, this yields a diameter of 44 million kilometers. For comparison, the Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers from the Sun at its closest.

If Sagittarius A* were exactly centered on the black hole, we would see it magnified beyond its actual size, due to gravitational lensing. According to general relativity, this would result in a minimum observed size of at least 5.2 times the black hole's Schwarzschild radius, which, for a black hole of around 4 million solar masses, corresponds to a minimum observed size of approximately 52 μas. This is much larger than the observed size of 37 μas and so suggests that the Sagittarius A* radio emissions are not centered on the hole but arise from a bright spot in the region around the black hole, close to the event horizon, possibly in the accretion disc or a relativistic jet of material ejected from the disc.^[6]

The mass of Sagittarius A* has been estimated by Gillessen et al. to be 4.31 ± 0.38 million solar masses,^{[2]:(12), §6.2.6} and by Ghez et al. to be 4.1 ± 0.6 million solar masses.^[3] Given that this mass is confined inside a 44 million km diameter sphere, this yields a density ten times higher than previous estimates. While, strictly speaking, there are other mass configurations that would explain the measured mass and size, such an arrangement would collapse into a single supermassive black hole on a timescale much shorter than the life of the Milky Way.^[6]

Ultimately, what is seen is not the black hole itself, but observations that are consistent only if there is a black hole present near Sgr A*. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole. The black hole itself is believed to emit only Hawking radiation at a negligible temperature, on the order of 10⁻¹⁴ kelvin.

Observational history

On October 16, 2002, an international team led by Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near to Sagittarius A* for a period of ten years, and obtained evidence that Sagittarius A* is a highly massive compact object.^[7] From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 2.6 ± 0.2 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU).^[8] Later observations determined the mass of the object to be about 4.1 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU) or about 6.7 billion kilometres.^[9]

In November 2004 a team of astronomers reported the discovery of a potential intermediate-mass black hole, referred to as GCIRS 13E, orbiting three light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimate the object's mass at 4.31 ± 0.38 million solar masses. The result was announced in 2008 and published in the Astrophysical Journal in 2009.^[2] Reinhard Genzel, team leader of the research said the study has delivered "what is now considered to be the best empirical evidence that super-massive black holes do really exist. The stellar orbits in the galactic centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt." ^[10]

References

Further reading

• Melia, Fulvio, The Black Hole in the Center of Our Galaxy, Princeton U Press, 2003
• Melia, Fulvio, The Galactic Supermassive Black Hole, Princeton U Press, 2007
• Eckart, A., Schödel, R., Straubmeier, C., The Black Hole at the Center of the Milky Way, Imperial College Press, London, 2005

External links

• UCLA Faculty Research presentation on Sagittarius A* (Video)
• Is there a Supermassive Black Hole at the Center of the Milky Way? (arxiv preprint)
• 2004 paper deducing mass of central black hole from orbits of 7 stars (arxiv preprint)
• ESO video clip of orbiting star (533 KB MPEG Video)
• Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours ESO Press Release, October 16, 2002
• Max Planck page on the galactic center, with animation
• The Proper Motion of Sgr A* and the Mass of Sgr A* (PDF)
• NRAO article regarding VLBI radio imaging of Sgr A*

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