Astronomers have spotted a black hole speeding through the Milky Way as it gobbles up the outer layers of a doomed companion star locked in a fatal gravitational embrace.
The discovery provides direct observational evidence that black holes with masses comparable to single stars can, as long theorized, form in supernova explosions, the catastrophic end result for the most massive stars in the universe.
"This is the first black hole found to be moving fast through the plane of our galaxy," Felix Mirabel, of the French Atomic Energy Commission and the Institute for Astronomy and Space Physics of Argentina, said in a statement. "This discovery is exciting because it shows the link of a black hole to a supernova."
The black hole, known as GRO J1655-40, is heading in the general direction of Earth, racing through space at about 250,000 mph in the constellation Scorpius. Lest anyone worry about some future collision, the black hole is 6,000 to 9,000 light years away, putting it more than 16 million years from Earth's vicinity in the Milky Way.
"It is much more likely that a normal star could produce discernible effects than a black hole, simply because they are much more numerous," Mirabel said in an e-mail from Madrid.
While GRO J1655-40 itself cannot be directly observed, the companion star is visible, as are the effects of the black hole's titanic gravity.
The hole is slowly consuming the companion sun, which whirls around GRO J1655-40 once every 2.6 days. Huge jets of debris stream away from the poles of the black hole at sizable fractions of the speed of light.
Mirabel says the black hole probably formed in the inner disk of the Milky Way galaxy, where star formation is at its highest. The supernova explosion that created the hole provided the energy to send it careening away through space at such high velocity.
It is that velocity that provides the best evidence yet that black holes with masses comparable to individual suns are formed in supernova explosions.
"What this does say is yes, it did form in a supernova because there's nothing else that can impart that kind of velocity," said Paul Hertz, a senior scientist at NASA headquarters in Washington.
Stars like our sun remain stable by balancing the inward pull of gravity with the outward pressure generated by nuclear fusion in their cores. When the fuel for nuclear fusion eventually runs out, gravity triumphs and the core collapses.
What happens at that point depends on the mass of the star and its core. For stars with up to about eight times the mass of Earth's sun, the core collapses to a point where gravity is held at bay by quantum mechanical effects that limit how tightly electrons can be crammed together.
The result is a white dwarf, a star about the size of Earth that slowly cools and fades away over billions of years. Such stars frequently blow off their outer layers as they run out of fuel, producing spectacular expanding clouds of glowing gas.
For stars with more than eight times the mass of the sun, the effects that produce white dwarf stars are not sufficient to hold off the inward pull of gravity.
If the planet-size core of the doomed star is between about 1.4 and three times the mass of the sun, the end result is a neutron star, an object about the size of a large city, the mass of a normal star and the density of an atomic nucleus. Gravity is held at bay by quantum nuclear forces that represent a star's last line of defense.
When the core reaches this enormous density, the collapse stops and material still falling inward hits what amounts to a brick wall and rebounds. The resulting shock wave blows the rest of the star apart in a supernova explosion.
For rare stars with more than 25 to 30 times the mass of the sun -- and with cores containing more than three solar masses -- no known force can stop gravity once the star's nuclear fuel runs out. Again, a supernova results, but in those cases the core collapses into a black hole, an object with such extreme gravity not even light can escape.
That's the theory, but observational evidence is slim, in large part because black holes are difficult, by definition, to detect. Finding a solar mass black hole like GRO J1655-40, one with a velocity of 250,000 mph, is strong evidence black holes do, in fact, form in supernova explosions.
Mirabel said a neutron star or black hole can be accelerated to high velocities in a supernova detonation by either an asymmetrical collapse of the core or because of the offset center of gravity in a binary star system.
GRO J1655-40 was observed by the Hubble Space Telescope in 1995 and 2001. Those observations were combined with ground-based measurements of its radial velocity to determine the binary star system's actual speed through space. Mirabel's results are discussed in the Nov. 19 issue of Astronomy and Astrophysics.