Sagittarius A*

Sagittarius A* (pronounced "Sagittarius A-star", standard abbreviation Sgr A*) is a bright and very compact astronomical radio source at the center of the Milky Way, near the border of the constellations Sagittarius and Scorpius. It is part of a larger astronomical feature known as Sagittarius A. Sagittarius A* is believed to be the location of a supermassive black hole, like those that are now generally accepted to be at the centers of most spiral and elliptical galaxies. Observations of the star S2 in orbit around Sagittarius A* have been used to show the presence of, and produce data about, the Milky Way's central supermassive black hole, and have led to the conclusion that Sagittarius A* is the site of that black hole.

Observation and description

Astronomers have been unable to observe Sgr A* in the optical spectrum because of the effect of 25 magnitudes of extinction by dust and gas between the source and Earth. 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 an angular diameter for the source of 37 μas. At a distance of 26,000 light-years, this yields a diameter of 44 million kilometers. For comparison, Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers from the Sun at perihelion. The proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination.

Sagittarius A

Sagittarius A or Sgr A is a complex radio source at the center of the Milky Way. It is located in the constellation Sagittarius, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the spiral arms of the Milky Way.

It consists of three components, the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A*. These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West.

Two-dimensional size

The two-dimensional size of Sagittarius A* has been determined, based on Very Long Baseline Array observations, which helps explain the context of jet disk and accretion disk models on the radio emission spectrum. These measurements were made through the use of a wavelength of 7 mm, enabling the size to be determined. The result was that Sagittarius A* was determined to be an elliptical Gaussian with a semi-major axis size of 35.4×12.6 Rs with an inclination of 95 degrees east of north. The observations for this finding were found from the detection of NIR (near infrared) flares, and X-ray flares that were spotted by NuSTAR. Both flares come from electron excitation, not an enhanced accretion rate onto the black hole, which means that not all high energy events produce variability at radio wavelengths.