We can’t see what’s in the center of galaxies, but over time we came up with a very accurate idea by noting many indirect clues in the vicinity. In fact, we are certain that Sagittarius A is the center of our galaxy, the Milky Way, a supermassive black hole with a mass 4 million times greater than the mass of the Sun.
But how can we be so sure?
For example, observations of “quasars” – the extremely bright cores of distant galaxies – show that they release up to a trillion times the energy of a typical star, all within a volume no larger than the size of the Solar System.
The only mechanism that could explain this massive release of energy is the conversion of gravitational energy into light by one of these massive black holes. Any other type of object, such as massive stars, simply cannot produce the observed amount of energy. Even if extremely massive stars were to form, their lifetimes would be too short (a few million years) to explain the number of quasars seen in the universe.
However, the best direct evidence for the existence of supermassive black holes at the center of galaxies comes from observations of the motions of stars near that region. Their very high orbital velocities can only be explained if the central body is extremely compact and extremely massive. As far as we know now it couldn’t be anything other than that.
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