Why do astronomers hypothesize that there is a super massive black hole at the center of the milky way?

Answer 1

Astronomers hypothesize that there's a supermassive black hole at the center of our galaxy, the Milky Way, both because of physical theory about the formation of galaxies, and direct observation from our own and other galaxies. Recent observations, focused on the behavior of stars very close to the nucleus of the Milky Way, led to a somewhat compelling conclusion: various stars at the galactic core could be tracked orbiting an object which was itself not directly observable (consistent with a black hole). The simple fact they were in orbit indicated the presence of a massive object. The maximum radius of the object they were orbiting had to be inside the closest approach of the nearest star in orbit, hence placing an upper limit on the radius of the object. The orbital radius and orbital period of the stars provided limits for the measurement of the amount of mass present in the object they orbited. Since the lower limit of the mass had to fit inside the maximum radius that the stars didn't collide with, the only object fitting the observations would be a supermassive black hole, of a little over four million solar masses.