If Sagittarius A* is only 4 million solar mass, how can it be the centre of the MilkyWay which contains 100-400 billion stars?

Question

If Sagittarius A* is only 4 million solar mass, how can it be the centre of the MilkyWay which contains 100-400 billion stars?

In: 3

Answer 1

Sagittarius A* is *at* the center of the Milky Way, it’s not *the* center of the Milky Way. Every star in the galaxy contributes to the galaxy’s mass and to its gravitational field,† so it’s more accurate to say that we orbit the Milky Way’s barycenter (an astronomical term for an orbital system’s center of mass). Sagittarius A* happens to reside there because the region is dense with huge stars and interstellar matter, which are conducive to the formation and growth of black holes.

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†Most of a galaxy’s mass comes not from stars, but rather from invisible “dark matter,” so-called because we can see its gravitational effects on the speed of galactic rotation, but we cannot detect it directly since it does not interact with light or ‘solid’ matter.