Sagittarius A*

**Sagittarius A\*** (abbreviated **Sgr A\***) is a bright and complex astronomical radio source at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, visually close to the Butterfly Cluster (M6). Sagittarius A\* is the location of a supermassive black hole, similar to massive objects at the centers of most, if not all, spiral and elliptical galaxies.

Observations of several stars orbiting Sagittarius A\*, particularly star S2, have been used to determine the mass and upper limits on the radius of the object. Based on mass and increasingly precise radius limits, astronomers have concluded that Sagittarius A\* is the Milky Way's central supermassive black hole. The current value of its mass is slightly in excess of 4 million solar masses.

## Central black hole

In July 2018, it was reported that S2 orbiting Sgr A\* had been recorded at 7,650 km/s (17.1 million mph), or 2.55% the speed of light, leading up to the pericenter approach, in May 2018, at about 120 AU (18 billion km; 11 billion mi) (approximately 1,400 Schwarzschild radii) from Sgr A\*. At that close distance to the black hole, Einstein's theory of general relativity (GR) predicts that S2 would show a discernible gravitational redshift in addition to the usual velocity redshift; the gravitational redshift was detected, in agreement with the GR prediction within the 10 percent measurement precision.

Assuming that general relativity is still a valid description of gravity near the event horizon, the Sagittarius A\* radio emissions are not centered on the black 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. If the apparent position of Sagittarius A\* were exactly centered on the black hole, it would be possible to see it magnified beyond its size, because of gravitational lensing of the black hole. According to general relativity, this would result in a ring-like structure, which has a diameter about 5.2 times the black hole's Schwarzschild radius. For a black hole of around 4 million solar masses, this corresponds to a size of approximately 52 μas, which is consistent with the observed overall size of about 50 μas.

### Evidence

This is compatible with, and strong evidence in support of, the hypothesis that Sagittarius A\* is a supermassive black hole. Several teams of researchers, including groups at the National Radio Astronomy Observatory and at the Shanghai Astronomical Observatory, have attempted to image Sagittarius A\* in the radio spectrum using Very Long Baseline Interferometry. The images obtained have been consistent with the Sagittarius A\* radio emissions being associated with the accretion disc and polar jets of a supermassive black hole.

Sagittarius A\* is "associated" with the supermassive black hole; what is seen is not strictly the black hole itself. 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 emits only Hawking radiation. In the near future, astronomical interferometers may allow the direct imaging of the event horizon.

## See also

* List of nearest black holes
* Galactic Center gEV excess