Black holes are among the more exotic residents of the cosmic zoo. These bodies are so massive and dense that not even light can escape their gravitational influence (this explains why black holes are black). The behaviour of matter inside black holes defies physical explanation. The very blackness of the holes means that they are impossible to see directly, but there is increasing evidence for their existence from observations of nearby objects influenced by their gravitational fields.
One such object is the companion star to a candidate black hole orbiting each other in a binary star-system known as GRO J1655-40, or Nova Scorpii 1994. Rafael Rebolo at the Instituto de Astrofisica de Canarias, Tenerife, Spain and colleagues have been studying this star-system, and their results – reported in the 9 September issue of Nature
In theory, black holes form from the gravitational collapse of large quantities of matter. Stars represent suitable starting-points, but they have to be big ones: one scenario for black-hole formation starts with a star at least 25 times the mass of our own Sun running out of nuclear fuel. Unable to generate the energy to sustain itself against its own gravity, the star collapses without limit to form a black hole.
A second, slightly more complicated scenario has a massive star first exploding as a supernova, leaving a small, dense neutron star. These tiny stars are not as dense as black holes – however, some of the débris left over after the supernova explosion might fall back onto the neutron star, adding to its mass sufficiently so that it undergoes a second phase of collapse into the finality that is the black hole. Rebolo and colleagues think they have evidence for this second mechanism.
Close spectral study of the companion star (the one that isn't the black hole) in the Nova Scorpii 1994 system shows it to be relatively small – less than about three times the mass of the Sun – but it contains unusually high abundance of certain 'alpha' chemical elements, namely oxygen, magnesium, silicon and sulphur. Other elements, such as iron, are no more abundant than one would expect from a star such as this – but not even the nuclear furnace of an everyday run-of-the-mill star can create alpha elements in large quantities. High abundances of alpha elements are only formed in truly extreme conditions: within the inner cores of supergiant stars, 25-40 times the mass of the Sun, at temperatures in excess of three billion degrees, in the last few seconds before the star explodes as a supernova.
The conclusion is obvious – alpha elements were created in the explosion of the giant star that once accompanied the present, visible companion. The explosion plastered the companion with alpha-element-rich débris, while the dying star itself collapsed into a black hole, possibly by way of a brief stage as a neutron star.
The story is not quite over yet, however. Still to be resolved is how the companion star was not itself destroyed by the explosion, let alone managing to retain significant quantities of supernova débris. Nevertheless, this report presents the first convincing evidence that black holes might form from supernovae. Astronomers will now look closely at the spectra of other companion stars to candidate black holes in other binary systems to assess whether this mechanism is common, or if Nova Scorpii 1994 is an intriguing one-off example.