Something massive hides at the center of the Milky Way — four million times heavier than the Sun. The star S2, like a racer on a steep curve, loops around this object every sixteen years in an elongated orbit, accelerating to 7650 kilometers per second. By tracking exactly how it moves, astronomers are trying to figure out what that 'something' is.

Usually, it's thought to be a black hole — a region where gravity is so strong that even light can't escape. But physicists have come up with other possibilities: a black hole with an electric charge, 'regular' black holes without a point of infinite density at their core, and even a 'naked singularity'—an exotic object with no horizon behind which a singularity could hide. The authors of the new study took the trajectory of the star S2 and calculated what it would look like in each of these scenarios.

Comparing with observations from the Very Large Telescope showed that an ordinary black hole, a charged black hole, and one of the regular models give almost identical predictions. No differences between them are visible yet. However, for the 'naked singularity' and other exotic options, constraints on their parameters were established. The more precise the measurements of the star's position, the tighter these limits will become.

The star S2 is like a probe launched by nature itself into the very heart of the Galaxy. Right now, its trajectory says: 'I can't yet tell one black hole from another.' But new observations with the VLT and the Keck telescopes promise greater precision, and then, perhaps, it will become clear what kind of 'beast' is lurking at the center of the Milky Way.