Image: The Boomerang Nebula, called the “coldest place in the Universe,” reveals its true shape with ALMA. The background blue structure, as seen in visible light with the Hubble Space Telescope, shows a classic double-lobe shape with a very narrow central region. ALMA’s resolution and ability to see the cold molecular gas reveals the nebula’s more elongated shape, as seen in red.
It’s easy to guess where the hottest place in the universe might be—the core of a giant star, maybe, or a disk of gas heated to millions of degrees as it tries to cram its way into a super-massive black hole, or even, for a fraction of a second, a fusion reactor in New Jersey. It’s harder to imagine the coldest place, though.
You might think that the thermometer drops as far as it can go in the vast empty darkness between stars. But you’d be wrong. How about between galaxies? Wrong again. The coldest place, according to astronomers using the world’s newest giant telescope, is the Boomerang Nebula, a cloud of gas puffed out by a dying star some 5,000 light-years away. Intergalactic space is admittedly pretty cold, at -455°F (-270°C). But parts of the Boomerang have it beat, clocking in at -457.7°F (-272°C). That’s about 2°F (1.1°C) above absolute zero, the coldest temperature possible according to the laws of physics.
This literally chilling discovery isn’t what’s new here; that occurred back in 1995, when astronomers were trying to figure out how the cloud got its peculiar shape (it actually looks more like a bow tie than a boomerang). They did this by training a submillimeter-wave telescope on the object, which looks for the faint wisps of radiation emitted by matter at super-low temperatures, and found that the Boomerang was even colder than the cosmic microwave background radiation left over from the Big Bang—up until then, the coldest thing known to science.
When the ALMA telescope (short for the Atacama Large Millimeter/submillimeter Array) came on line last spring, astronomers realized they had a chance to probe more deeply into the secrets of the Boomeraring: the 66-dish array, located in the thin, crisp air of northern Chile’s Atacama Desert, is by far the world’s most powerful of its kind.
What the astronomers found is that the nebula’s bow tie shape is an illusion: the actual cloud surrounding the dying star is roughly spherical. ALMA managed to figure that out by looking for the radiation emitted by vibrating molecules of carbon monoxide that are spread throughout the dust and gas that make up the nebula. The flaps of the tie are created by starlight reflecting off dust grains, but a relatively thick band of dust close in blocks that light from illuminating those grains in all directions.
“Using ALMA,” said Raghvendra Sahai, of NASA’s Jet Propulsion Laboraty, who led the observations, “we were quite literally and figuratively able to shed new light on the death throes of a Sun-like star.”
Why is the nebula so cold? For the same reason, it seems, that a refrigerator can keep milk from spoiling, or an air conditioner can keep you from doing the same. They work by letting gas expand, which automatically forces it to cool (Albert Einstein patented a similar but simpler technology in 1926, but unlike most of his other ideas, this one didn’t take off). The science is nifty, but sometimes it can come in second to the visuals. And for most of us, what’s really cool (so to speak) about the Boomerang is the ghostly, glowing appearance of this bizarre object in ALMA’s extraordinary new images.