What is the mass of Sagittarius A* in millions of solar masses?

The mass of Sagittarius A* is about 4.1 million solar masses. Sagittarius A*, or Sgr A*, is the supermassive black hole at the center of the Milky Way, roughly 27,000 light-years from Earth.

Astronomers determined its mass by tracking stars orbiting an extremely compact, invisible object. The star S2 is especially useful: it completes an elongated orbit in about 16 years and reaches extraordinary speeds near the black hole. Applying Kepler’s laws to these stellar paths reveals how much mass is concentrated there.

The 4.1-million-Sun figure comes from measurements of S2 and other stars; later studies have refined the estimate to roughly 4.3 million solar masses. Thus, 4.1 million is a legitimate rounded estimate, not a competing object or a measurement of the entire Galactic Center.

A common mix-up is confusing Sagittarius A* with the central bulge or the Milky Way’s total mass. The black hole contains only a tiny fraction of the Galaxy’s mass, although its gravity dominates the motions of nearby stars.