Blog

Oct 25 · 2018

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 thought 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.
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). 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.
As of April 2017, there have been direct radio images taken of Sagittarius A* with the Event Horizon Telescope, but the data is still being processed, and images have yet to be released. The Event Horizon Telescope uses interferometry to combine images taken from widely spaced observatories at different places on Earth in order to gain a higher picture resolution. It is hoped the measurements will test Einstein’s theory of relativity more rigorously than has previously been done. If discrepancies between the theory of relativity and actual observation are found, scientists may have identified physical circumstances under which the theory breaks down. source
Taking the First Picture of a Black Hole