At the center of the Milky Way lurks something invisible but extremely massive — about four million times heavier than the Sun. It is usually called a black hole and designated Sagittarius A*. But astronomers are not letting their guard down: what if it is not a hole but something more exotic, like a giant ball of ultralight particles — a boson star? Such a star has no hard surface; rather, it resembles a loose ball of cotton that gradually fades away at the edges.
To test this idea, scientists used data from the GRAVITY instrument, which tracks infrared flares near the horizon of Sagittarius A*. These flares look like bright sparks racing in a circle around the central object. Their motion was compared with predictions from various models — from a classical black hole to five versions of boson stars with different degrees of fuzziness. Statistical methods, including spectroscopy and Markov chain Monte Carlo, were applied to analyze the trajectories.
The result was telling: for most boson stars, the derived mass was noticeably larger than the accepted value — about 4.3 million solar masses. The fuzzier the model (i.e., the fewer sharp boundaries it had), the closer its behavior to a black hole and the more accurate the mass match. This places serious constraints on the boson star hypothesis. However, the fluffiest variants are not yet off the table.
If you replaced the Sun with a boson star of the same mass, its radius would be comparable to Neptune's orbit, yet it would remain almost transparent to light.
In Bruce Sterling's novel 'Schismatrix', intelligent clouds of Bose condensate are described, reminiscent of such stars.