01 July 2009

Goldilocks and the Black Hole

It’s like Goldilocks and her three bears. For decades, astronomers have identified either supermassive black holes, each a huge entity containing the mass of millions to billions of Suns and sitting smack dab in the heart of a galaxy, or far smaller ones weighing anywhere from three to twenty solar masses. But now astronomers in France, using the European Space Agency’s XMM-Newton X-Ray Space Telescope, have spotted a black hole that’s more toward the middle. Goldilocks might even say it’s “just right.” In a galaxy far, far away—some 290 million light-years from Earth—the observers detected the unique x-ray signature of a medium-sized hole that contains more than 500 times the mass of our Sun. Labeled HLX-1, this source offers the best evidence yet for what theorists have long suspected: that black holes come in a wide range of sizes, not just extra large or petite.

The first hint that black holes might exist in the real universe arrived in 1939 when J. Robert Oppenheimer (who later headed the Manhattan Project that constructed the first atomic bomb) and his student Hartland Snyder published a paper showing that if a dying star had enough mass it could gravitationally collapse in a wink and be crushed to a singular point. The result: space-time gets so warped around this collapsed star that it literally closes itself off from the rest of the universe. “Only its gravitational field persists,” reported Oppenheimer and Snyder.

But astronomers at the time weren’t ready to believe that such bizarre objects could possibly inhabit, what seemed at the time, their calm and serene universe. Even Einstein wrote a paper attempting to prove that they were impossible to form.

We have the late Princeton physicist John Archibald Wheeler to thank for reinvigorating this line of research back in the 1960s, while all others ignored it. Confident at first that some kind of force would surely step in to halt this horrific stellar contraction, Wheeler saw in his equations that nothing—absolutely nothing—could prevent the collapse. The plummet into the abyss was inevitable.

Wheeler went on the lecture circuit to discuss his theoretical findings, aided by the discovery of pulsars, not yet understood in 1967 to be spinning neutron stars but surely proving that strange celestial objects lurk in the dark envelope of space. Wheeler at the time lectured that astronomers should consider the possibility that the pulsars were “gravitationally collapsed objects,” as he then awkwardly called them. “Well, after I used that phrase four or five times, somebody in the audience said, ‘Why don’t you call it a black hole?’ So I adopted that,” Wheeler told me as I was researching my book Einstein’s Unfinished Symphony. Some of Wheeler's associates, though, suspect he carefully crafted the term himself after years of thought. Whatever the origin, the name became official and went into the scientific lexicon. Nature’s weirdest object, resisted by astronomers for so many decades, is now taken for granted. And, as witnessed by the latest data from the XXM-Newton space telescope, the evidence for the existence of black holes is piling up daily.

Above images: (1) Artist's impression of the new source HLX-1 (represented by the light blue object to the top left of the galactic bulge) in the periphery of the edge-on spiral galaxy ESO 243-49 (Credit: Heidi Sagerud); (2) John Wheeler in 1967, at the time he coined the term "black hole." (New York Times)

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