What do the orbits of stars around Sgr A* reveal about the Milky Way’s central black hole?

• A. The pericenter distances provide the exact distance to the Galactic center only.
• B. Precise measurements of stars' orbits, especially pericenters, yield the enclosed mass and constrain the black hole's mass and distance.
• C. The orbits show a diffuse mass distribution rather than a compact object.
• D. They indicate a supermassive star rather than a black hole.

Answer: (B)

Studying the orbits of stars that closely approach the central radio source Sgr A* lets us map the gravitational pull there. By tracking how long their orbits take and how close they get to the center (pericenter), we can determine the mass enclosed inside each orbit. Using the orbital size and period together, we apply orbital dynamics to estimate the mass concentrated at the center and, with angular measurements on the sky, constrain the distance to the Galactic center. The key point is that the observed stellar motions require a very large mass packed into a tiny volume, which points to a supermassive black hole rather than a diffuse mass or a luminous star. The pericenter data are especially powerful because they reveal the strongest gravitational effects and thus the tightest constraints on both the central mass and its distance.