M33 X-7
M33 X-7 is a black hole binary located in the Triangulum Galaxy, about 2.7 million light-years from Earth. The system contains a stellar-mass black hole and a massive blue companion star.
The black hole weighs about 15.6 solar masses, while the companion star is around 70 solar masses (a hot O-type giant). The two objects orbit each other every 3.45 days, and the companion star periodically blocks the light from the black hole, allowing scientists to measure the system’s properties.
Together, the two stars have a total mass of about 86 solar masses. The orbit is nearly circular and tilted about 75 degrees to our line of sight, with a separation of roughly 42 solar radii.
M33 X-7 has been studied with NASA’s Chandra X-ray Observatory and the Gemini telescope. It was once thought to hold the most massive stellar black hole known, but newer measurements have found heavier black holes elsewhere.
How such a system forms is puzzling. The black hole’s progenitor star far outshines its companion and would have needed to transfer a lot of mass to the companion while shedding its outer layers. This close interaction likely led to a Wolf–Rayet star before the collapse into a black hole, after which the companion continued to gain mass. In some scenarios, this setup could eventually become a binary black hole.
This page was last edited on 3 February 2026, at 15:44 (CET).