How to Find a Supermassive Black Hole

Deep in the night sky, in the constellation of Sagittarius, shrouded by interstellar gas and dust, lies a mysterious object.   So enigmatic is this object, it cannot be observed directly, not even light can escape its presence.  This object found at the core of our Milky Way galaxy is of incredible mass, equaling millions of suns and yet stretches across an area smaller than our solar system. It is of course, a supermassive black hole.

Intrigued by the notion of these gigantic black holes silently churning away at the center of our Milky Way galaxy, astronomers discovered the first supermassive black hole (SMBH) candidate through radio telescopes in the 1970’s. (Balick & Brown 1974)  Almost a decade later the object was officially entitled Sagittarius A* (Brown 1982).

The strongest evidence for our galaxy’s own SMBH was conducted decades later by astronomers who carefully observed the motion of stars around the Milky Ways’ center over a 15 year period.  They observed that stars were orbiting in an elliptical motion around an invisible object.

By measuring the orbits of these doomed stars around our SMBH, astronomers were able to determine the mass and area of this invisible object.    With the velocity and the orbit know, it was calculated that the density of our Milky Way’s SMBH is 4 Million solar masses in an area of 44 million kilometers.

Amazed by the discovery of Sagittarius A*, astronomers were curious to know if other galaxies hosted these supermassive objects.  Peering outside of our own Milky Way galaxy, scientists studied the spectral image of gas circling the center of distant galaxies.

Gas was moving so rapidly around their galactic centers, scientist could watch the redshift and blueshift of its motion.  As the gas was whirling towards our telescopes – the spectral lines would shift towards the blue, as they spirals away from our telescopes the spectral lines would shift to the red.