Sagittarius A*, thought to be the location of a supermassive black hole

Sagittarius A*, thought to be the location of a supermassive black hole Culture's Ways

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Sagittarius 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.


What is Sagittarius A*?

Sagittarius A* 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.


Karl Jansky was the first person to determine that a radio signal was coming from a location at the center of the Milky Way, in the direction of the constellation of Sagittarius. Sgr A* was discovered on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory. The name Sgr A* was coined by Brown in a 1982 paper because the radio source was “exciting”, and excited states of atoms are denoted with asterisks.

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 diameterfor 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. 

There are plans to image Sagittarius A* in much greater detail than before using interferometry combining images from widely spaced observatories at different places on Earth. It is hoped the measurements will test Einstein’s theory of relativity more rigorously than has previously been done. Discrepancies may be found between the theory and actual observation or, if there are no discrepancies relativity may be confirmed.

Image: By Alain r – Own work, CC BY-SA 2.5,
Text: Wikipedia contributors. “Sagittarius A*.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 25 Jul. 2017. Web. 9 Aug. 2017.


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