This image, taken by NASA's Spitzer Space Telescope, shows the mysterious gas cloud G0.253+0.016, at the center of the Milky Way, which is the black object on the left.
Here’s how star formation is supposed to work, according the astronomers who specialize in the topic: You start with a giant molecular cloud, a billowing mass of gas and dust, tens of light-years across or more, drifting among the stars. Then something comes along to make the cloud start collapsing — the shockwave from nearby supernova will do nicely. The collapse accelerates as the cloud’s own gravity begins to kick in. When the density rises high enough, the resulting knot of matter bursts into nuclear fire and begins to shine as a newborn star (or stars, since there are often multiple knots).
All good so far. But then there’s the case of G0.253+0.016, and suddenly things get more complicated. G0.253+0.016, which sits near the Milky Way’s center, is just the sort of cloud that should be manufacturing stars like crazy. It’s made of the right stuff, and it’s plenty dense enough — some 25 times denser than the nearby cloud known as the Orion Nebula, where astronomers have spotted hundreds of young stars in various stages of emerging from the interstellar haze.
But G0.253+0.016 appears to have churned out only a handful of small stars — about 45 times fewer than the time-honored theory would have predicted. In a burst of J. Conan Doyle-esque inspiration, Caltech astronomers, who have been puzzling over this cosmic oddball, have dubbed it “The Mystery of the Curiously Dense Cloud.” They’ve come up with some the answers, but not all, and they recently laid out the facts of the case so far for their colleagues at the American Astronomical Society’s winter meeting in Long Beach, California.
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The lack of young stars in the G0.253+0.016 formation was only the first clue that something was amiss. Observations with an array of eight radio telescopes on the summit of Hawaii’s extinct Mauna Kea volcano next revealed something else: the “dense cores” of gas and dust that would normally turn into stars weren’t there either.
That’s not all: when the astronomers swiveled an array of 23 radio telescopes in California toward G0.253+0.016, it became clear why the cloud might not be an ideal place for starbirth. Some 30 light-years across, the formation is whirling so fast — about ten times faster than such a cloud would normally spin — that it’s on the verge of flying apart. This may be due in part to its location close to the monster three-million-solar-mass black hole that lurks at the Milky Way’s core, exposing it to a gravitational maelstrom that whips whatever comes near it around at breakneck speed.
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The California array also detected a surprisingly large amount of silicon monoxide in the cloud, a substance usually trapped in dust grains and liberated only by shock waves. This suggests that G0.253+0.016 is really a mashup of two separate clouds — again, not such a surprising thing in the galactic center.
Now the question is whether other giant clouds in the vicinity are similarly short on newborn stars. There are plenty of young, bright stars in the area, so they must have come from somewhere — but why should one cloud be comparatively barren and not another? In the continuing hunt for answers, the researchers are turning to the brand-new 66-dish ALMA radio telescope array in the high desert of northern Chile, where they may finally be able to figure out why the clouds at the core of the Milky Way seem to be misbehaving so badly.
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