English: This spectacular starry field of view shows the globular cluster NGC 6553 which is located approximately 19 000 light-years away in the constellation of Sagittarius. In this field, astronomers discovered a mysterious microlensing event.

Microlensing is a form of gravitational lensing in which the light from a background source is bent by the gravitational field of a foreground object, creating an amplified image of the background object. The object causing the microlensing in NGC 6553 bent the light of a red giant star in the background (marked with an arrow). If this object lies in the cluster — something the scientists believe might only have a 50/50 chance of being correct — the object could be a black hole with a mass twice that of the Sun, making it the first of its kind to be discovered in a globular cluster. It would also be the oldest known stellar-mass black hole ever discovered. However, further observations are needed to determine the true nature of this lensing object for sure.

This cosmological curiosity was detected by ESO's VISTA telescope at the Paranal Observatory in Chile as part of the VISTA Variables in the Vıa Lactea Survey (VVV) — a near-infrared survey aimed at scanning the central parts of the Milky Way.