Is there a supermassive black hole at the center our galaxy, asks Mark Reid from the Harvard Smithsonian Center for Astrophysics in Cambridge.

There sure is and Reid gives a good account of the evidence to prove it.

How can astronomers be so sure?  The first evidence began to emerge in the 1950s when the first radio telescopes spotted a mysterious source of radio waves from the center of the Milky Way in the constellation of Sagittarius. This source was given the name Sagittarius A.

Better observations in the 1970s led to estimations that this source was small, less than the size of our solar system.

Towards the end of the 70s other evidence emerged. The movement of gas clouds near Sagittarius A indicated they it must circling a a compact mass several million times greater than the Sun. Then various stars were observed circling this mass, the most recent being a star called S2 which orbits Sagittarius A every 15.8 years. This implies that Sagittarius A is a point mass some 6 million times the sun’s mass

Then astronomers proved that the radio source and the gravitational  mass were centred at the same point. Other evidence of a weak infrared source at the same point also turned up.

And the clincher was that Sagittarius A is motionless relative to the rest of the Milky Way. In other words, the entire galaxy revolves around this object.

That kind of evidence is overwhelming. It’s hard to think of anything with such a dim optical signature that could create such a high mass density (although there are some possibilities, the most obvious being a cluster of dark stars).

Nevertheless, most astronomers are convinced we’ve got a super massive black hole on our doorstep and it’ll take some spectacular contradictory evidence to change their minds.

Ref:   arxiv.org/abs/0808.2624: Is There a Supermassive Black Hole at the Center our Galaxy

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