Researchers have found what they call the best evidence yet for the existence of a black hole. They found it in studying an unseen object orbited by a star in our Milky Way galaxy.
The researchers calculated that the mass of the compact unseen object is large enough that it must be a black hole, unless it is some other kind of exotic object.
Other scientists called the evidence strong or intriguing but said they did not yet consider it the strongest case for a black hole.
Black holes are, in theory, extremely dense objects whose gravitational pull is so strong that not even light can escape.
The latest work is reported in Thursday's issue of the journal Nature.
"I think it's almost certainly a black hole," said study co-author Phil Charles, head of the astronomy group at the Royal Greenwich Observatory in Spain's Canary Islands.
The researchers analyzed light from the orbiting star, which with the unseen object makes up a feature called V404 Cygni, at least 5,000 light-years away in the direction of the constellation Cygnus. A light-year is the distance light travels in one year, about 5.9-trillion miles.
By determining characteristics of the star's orbit, researchers calculated that the central object's mass must be at least about six times that of the sun.
That is too massive for a compact object to be anything other than a black hole, according to standard theory.
Some theoretical work suggests that other hypothetical objects might have masses like those of black holes, but "we have no physical justification" for believing such objects exist, Charles said.
"If I were a betting man, I'd put even odds" on V404 Cygni being a black hole, said astrophysicist Joseph Dolan of NASA's Goddard Space Flight Center in Greenbelt, Md.
He said he would put the same odds on another black hole candidate, Cygnus X-1, and that he considered the two objects to be the best candidates so far.