Was Einstein smarter than a sixth-grader? When it came to black holes, maybe not. For much of the 20th century, astronomers and physicists were unsure about whether black holes — which were predicted by Einstein’s General Relativity Theory — even existed. That was a fair enough doubt, since the idea of a superdense star remnant with a gravity field so powerful not even light could escape it did defy credibility. Even Einstein himself thought they were just a mathematical curiosity that couldn’t possibly exist in the real universe.
Today, every schoolchild knows better. Astronomers have found giant black holes lurking at the cores of galaxies, the Milky Way among them, and more modest black holes all over the place, including up to 60 in our home galaxy. Now, that cosmic census has expanded dramatically: using the orbiting Chandra X-Ray Observatory, astronomers have spotted 26 of the more modest variety of black hole in the Andromeda galaxy, which is not only the Milky Way’s closest neighbor, but also its near twin.
Along with nine black holes detected previously, that brings Andromeda’s total to 35. If that many are detectable, says Harvard astronomer Robin Barnard, lead author of a report on the new objects in The Astrophysical Journal, “we know there must be lots we can’t detect — maybe tens of thousands, maybe more.”
The reason: a star-size black hole like the ones Barnard and his colleagues found can be detected only if it’s gulping down gas, and that’s relatively rare. It only happens, in fact, when the black hole orbits an ordinary star and dines on its outer atmosphere. As it does, the gas heats up to millions of degrees as it tries to cram its way down the black hole’s voracious gravitational throat, and in so doing emits bursts of X-rays—sort of giant, electromagnetic burps.
In the handful of cases in which that’s happening, Chandra can easily spot a star’s digestive blast. “If you look at the central region of Andromeda in visible light,” says Barnard, “you see a huge mass of stars — billions upon billions of them.” If you look with Chandra, though you see just a few hundred bright dots — the ones emitting x-rays.
Some of those emissions are actually not produced in Andromeda, but rather, come from the giant, gas-sucking black holes at the cores of other galaxies which just happen to lie in the background, but much, much farther away. “These background galaxies are incredibly bright,” says Barnard, but they’re easy to identify because their X-ray light varies up and down relatively slowly as their gas supply waxes and wanes, a pattern different from that produced by the smaller black holes in Andromeda itself.
Other dots of X-ray brightness come from stars in Andromeda that orbit neutron stars, the dense remnants of dead stars that aren’t quite massive enough to collapse into black holes. These pull gas from companion stars as well, and while the gas merely collides with their surfaces, that impact heats it up enough to produce X-rays too. The difference, says Barnard, is that the mix of X-ray “colors,” or wavelengths, is quite different from those generated by black holes.
Once you weed out the X-rays from background galaxies and from neutron stars, says Barnard, what’s left is black holes, ranging from five to ten times the mass of the Sun. Getting a perfectly clear picture might, in theory, require the astronomers to weed out emissions from the giant black hole at Andromeda’s core too. Unlike those in the background galaxies Chandra sees, however, Andromeda’s black hole, says, Barnard, is “wimpy. It’s ten to a hundred times fainter than the small black holes we can see, even though it’s a million times more massive.” That’s because there doesn’t happen to be a lot of gas available for swallowing these days — which is the case for the Milky Way and most nearby galaxies as well, although that could change before too long, thanks to an interstellar gas cloud spotted in 2011 that’s hurtling toward our galactic center and could arrive in September or October. (Astronomers expect some x-ray fireworks, but little more.)
There’s actually one more class of black holes that physicists have speculated about: teeny tiny ones, smaller than an atom, which might have formed in the intense turbulence of the first moments of the Big Bang. These probably would long since have vanished, although there was some speculation that new ones might emerge from the Large Hadron Collider atom-smasher in Europe.
That would have been a triumph for science — but since it could in theory have destroyed the Earth, it’s just as well that it didn’t happen.