The LIGO and Virgo teams soon went to work extracting every bit of information possible.


At the most fundamental level, the signal gave them an existence proof: the fact that the objects came so close to each other before merging meant that they had to be black holes, because ordinary stars would need to be much bigger.

"It is, I think, the clearest indication that black holes are really there," says Penrose.

The signal also provided researchers with the first empirical test of general relativity beyond regions - including the space around the binary pulsar - where there is comparatively little space-time warping.


There was no empirical evidence that the theory would keep its validity at the extreme energies of merging black holes, says Shapiro - but it did.


The signal held a trove of more-detailed information as well. By scrutinizing its shape just before the final cataclysm, the scientists found that it closely approximated a simple sine wave with a steadily increasing frequency and amplitude.


According to B.S. Sathyaprakash, a theoretical physicist at Cardiff University, UK, and a senior LIGO researcher, this pattern suggests that the orbits of the black holes were nearly circular, and that LIGO probably had a bird's-eye view of the circles, looking almost straight down on them rather than edge-on.


In addition, the LIGO and Virgo teams were able to use the frequency of the observed wave, along with its rate of acceleration, to estimate the masses of the two black holes:

because heavier objects radiate energy in the form of gravitational waves at a faster rate than do lighter objects, their pitch rises more quickly.

By recreating 'the Event' with computer simulations, the scientists calculated that the two black holes weighed about 36 times and 29 times the mass of the Sun, respectively, and that the combined black hole weighed about 62 solar masses. 1


The lost difference, about three Suns' worth, was dispersed as gravitational radiation - much of it during what physicists call the 'ringdown' phase, when the merged black hole was settling into a spherical shape.


(For comparison, the most powerful thermonuclear bomb ever detonated converted only about 2 kilograms of matter into energy - roughly 1030 times less.)


The teams also suspect that the final black hole was spinning at perhaps 100 revolutions per second, although the margin of error on that estimate is large.


The inferred masses of the two black holes are also revealing. Each object was presumably the remnant of a very massive star, with the larger star approaching 100 times the mass of the Sun and the smaller one a little less.


Thermonuclear reactions are known to convert hydrogen in the cores of such stars into helium much faster than in lighter stars, which leads them to collapse under their own weight only a few million years after they are born.


The energy released by this collapse causes an explosion called a type II supernova, which leaves behind a residual core that turns into a neutron star or, if it's massive enough, a black hole.


Scientists say that type II supernovae should not produce black holes much bigger than about 30 solar masses - and both black holes were at the high end of that range.


This could mean that the system formed from interstellar gas clouds that were richer in hydrogen and helium than the ones typically found in our Galaxy, and that were poorer in heavy elements - which astronomers call metals.


Astrophysicists have calculated that stars formed from such low-metallicity clouds should have an easier time forming massive black holes when they explode, explains Gijs Nelemans, an astronomer at Radboud University Nijmegen in the Netherlands and a member of the Advanced Virgo collaboration.


That's because during a supernova explosion, smaller atoms are less likely to be blown away by the blast.


Low-metallicity stars thus,

"lose less mass, so more of it goes into the black hole, for the same initial mass", Nelemans says.




Two by two


But how did these two black holes end up in a binary system?


In a paper 2 published at the same time as the one reporting the discovery, the LIGO and Virgo teams described two commonly accepted scenarios.


The simplest one is that two massive stars were born as a binary-star system, forming from the same interstellar gas cloud like a double-yolked egg, and orbiting each other ever since. (Such binary stars are common in our Galaxy; singletons such as the Sun are the exception, rather than the rule.)


After a few million years, one of the stars would have burned out and gone supernova, soon to be followed by the other. The result would be a binary black hole.


The second scenario is that the stars formed independently, but still inside the same dense stellar cluster - perhaps one similar to the globular clusters that orbit the Milky Way.


In such a cluster, massive stars would sink towards the centre and, through complex interactions with lighter stars, form binary systems, possibly long after their transformation into black holes.