Astronomers have found the biggest stellar black hole so far, a monster with a mass 15.65 times that of our Sun, lurking in a nearby spiral-shaped galaxy.
The find, located in a galaxy called Messier 33, has an even bigger companion -- a close-orbiting star that is 70 times the mass of the Sun, according to an investigation led by Jerome Orosz of San Diego University, California.
Black holes are among the most powerful forces in the Universe. They are believed to be concentrated fields of gravity which are so powerful that nothing, not even light, can escape them.
Stellar black holes derive from the collapse of stars, but typically range from about three to 14 or 15 solar masses.
Another category of black holes are "supermassive" holes, spotted at the centre of galaxies, that have masses millions, even billions, times that of the Sun.
The paper appears on Thursday in Nature, the weekly British science journal.
The Orosz team were able to make their calculations with unusual accuracy -- the estimate is plus or minus 1.45 solar masses -- because the Messier 33 phenomenon is a so-called "eclipsing binary."
This means that the companion star passes directly in front of the black hole in its 3.45-day orbit, blocking out X-ray emissions from the hole.
The regular fall and then rise in the X-ray signal provides the key indicator for calculating the "weight" of the hole.