How do measurements of stellar orbits around Sgr A* constrain the presence of alternative compact objects (like boson stars) versus a BH?

• A. Orbits indicate a diffuse, luminous mass at the center.
• B. Orbits cannot constrain the nature of the central object.
• C. Observations rule out any black hole near Sgr A*.
• D. Orbits and gravitational potential match a compact, dark mass; deviations would imply different mass distributions, but observations favor a BH.

Answer: (D)

Stellar orbits around Sgr A* reveal how mass is arranged at the Galactic center. If the central object were something like a boson star or another alternative compact form, its mass would have to be spread out over a larger radius rather than packed into an almost point-like lump. That difference in how mass is distributed changes the gravitational field experienced by stars moving close to the center, especially for those that swing by at high speed.

The orbits we observe, for example of the star S2, indicate a mass of about 4 million solar masses confined within a very small region. The orbital shapes, speeds, and the relativistic redshift entering at pericenter match what you’d expect from a single, compact, dark mass—essentially a black hole. If the mass were distributed more diffusely, the gravitational potential would leave detectable signatures in the orbital motion, such as deviations from Keplerian, or measurable precession that doesn’t align with a point mass plus general relativity.

So the data point to a compact, dark mass at the center, and any alternative mass distributions would have to produce observable differences in the orbits that we don’t see. That’s why the observations strongly favor a black hole over other possibilities.