About how massive is the Milky Way’s central black hole, measured in Suns?

The Milky Way’s central black hole has a mass of about 4.3 million Suns.

Astronomers determine this mass by tracking stars orbiting the compact radio source at the center of the Milky Way. The star S2, for example, follows a highly elongated orbit and completes one revolution in roughly 16 years. Its speed and path reveal the enormous mass concentrated in a very small region.

The measured object is far more massive than a typical stellar-mass black hole but much smaller than the largest known quasar engines. It is classified as a supermassive black hole. Despite its great mass, its event horizon would span only a small fraction of the Solar System’s scale.

The central black hole is relatively quiet today compared with actively feeding galactic nuclei. However, infrared observations show evidence that gas and stars orbit in its vicinity, allowing researchers to test general relativity in a strong-gravity environment.