A brightening star pointed to something dark
In 2011, the MOA and OGLE surveys found a star toward the Galactic bulge growing brighter for months. An unseen object had passed close to the line of sight and bent the star’s light toward Earth, acting as a gravitational lens. Years of follow-up would give that dark object a mass of about seven Suns, with no detectable light of its own: evidence for an isolated stellar-mass black hole.
The event, called OGLE-2011-BLG-0462 or MOA-2011-BLG-191, lasted unusually long. The lens took about 270 days to move by one characteristic angular radius, called the Einstein radius. A perfectly aligned point source would form a ring of that radius on the sky. A long event was worth following, but duration alone could not weigh the lens. Mass, distances and relative speed all affect how long the brightening lasts.
Hubble followed a single unresolved spot
A point-mass lens makes two images of a background star. They were too close together for Hubble to separate in this event. Instead, it measured the centre of their combined light, weighted toward whichever image was brighter. As the lens moved, that light-centre wandered slightly away from the star’s otherwise expected path.
The size and evolution of this shift constrain the Einstein angular radius. Another measurement comes from Earth’s yearly orbit: our changing vantage point slightly alters the brightness history. This is microlensing parallax. The angular radius and parallax together give the lens’s mass. With the background star’s distance, the parallax also gives the lens’s distance. The changing brightness and shifting position supply different pieces of the same geometry.
A nearby star made the first mass estimates disagree
In 2022, Kailash Sahu and colleagues combined eight Hubble observing epochs over six years with ground-based brightness measurements. They reported 7.1 solar masses, with an uncertainty of 1.3, at a distance of 1.58 kiloparsecs, with an uncertainty of 0.18. A separate analysis by Casey Lam and colleagues found a range of 1.6 to 4.4 solar masses, allowing a neutron star as well as a black hole. Mróz, Udalski and Gould examined systematic errors and obtained a higher estimate of 7.88 solar masses, with an uncertainty of 0.82.
The target has a bright neighbour only 0.4 arcseconds away. The wings of that neighbour’s image spill light into the target and can bias a tiny position measurement. Lam and Jessica Lu reprocessed the images with newer software and revised the brightness data. Their 2023 re-analysis gave 6.0 solar masses, with upper and lower uncertainties of 1.2 and 1.0. The earlier low-mass result had already been revised before the 2025 analysis appeared.
Eleven years of positions strengthened the black-hole case
Sahu’s team added three more Hubble epochs, extending the record to eleven years. Their 2025 analysis carefully subtracted the bright neighbour’s contribution in individual exposures. It found 7.15 solar masses, with an uncertainty of 0.83, at 1.52 kiloparsecs, with an uncertainty of 0.15: roughly five thousand light-years away.
That much mass, together with no detected light from the lens, supports the black-hole interpretation. Searches also found no companion over the separations the observations could test. The conclusion rests on fitting brightness and positions together while controlling the nearby star’s contamination. An orbiting companion supplied no clock or ruler here; the background star’s bent light supplied both an angular scale and a record of motion.