The most recent discovery, an object called IC 10 X-1, was found by a team led by Andrea H. Prestwich, a Harvard astronomer.
The object, announced in a paper released last week, has not been identified with absolute certainty as a black hole. But if further observations support the team’s initial conclusions, then IC 10 X-1 will be confirmed as a black hole 20 to 30 times more massive than the sun.
Black holes are objects that form when massive stars collapse at the end of their lives. They are so dense that nothing, not even light, can escape from them.
“It’s really hard for a single star to make a black hole of more than 20 solar masses,” Prestwich said.
“My best guess is that this guy really is more massive than 20 solar masses. And that’s why he’s interesting,” she added.
Conventional theory says that black holes formed by collapsing stars cannot have a mass more than about 15 times that of the Sun.
A team of Harvard researchers led by San Diego State professor Jerome A. Orosz separately found an object they called M33 X-7, which is about 15.7 times more massive than the sun and researchers are confident is a black hole.
In addition to its sheer size, M33 X-7 is unusual for having another star in an extremely close orbit around it, Orosz said.
He added that the discovery meant current theory would have to be rethought, since it states that black holes near stars should be smaller than average, rather than larger, as M33 X-7 is.
“The assumptions about how stars form black holes may need to be revised or revisited,” Orosz said.
Daniel Steeghs, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who was not involved with either team, said that these discoveries should generate a lot of interest among both theorists and observers.
“It’s very exciting—it always is—when you initially discover something you can’t explain. One should always see that as a challenge,” he said.
According to Prestwich, the availability of high-quality x-ray telescopes made the discoveries of two such large, stellar black holes outside our galaxy in rapid succession possible.