Until NASA's Cold Lab comes online in 2016, the coldest place in the universe is a faint scattering of gas and dust 5,000 light years away in the Centaurus constellation. The Boomerang Nebula is just 1 degree above absolute zero, colder than the natural background temperature of space.
Top image: Boomerang Nebula as seen by the Hubble Space Telescope (blue) and the Alma Obervatory (red). Credit: Bill Saxton/NRAO/AUI/NSF/NASA/Hubble/Raghvendra Sahai
The Boomerang Nebula is 1°K (- 272.15°C or -458°F): that's just one shivering degree above the absolute zero, a point so cold even atoms stop vibrating and the temperature cannot drop any farther even in theory. While we can get incredibly close to absolute zero in lab experiments, nothing we've done lasts nearly long enough to be a "place" instead of a brief, chilly moment in time. The Boomerang Nebula is so cold, the only way to measure the temperature is by watching how it absorbed the cosmic microwave background radiation, which is a relatively almost-toasty 2.8 Kelvin.
The Boomerang Nebula is a pre-planetary nebula, a dying star shrouded in sloughed-off gas. In the last 1,500 years, enough gas to make up a sun-and-a-half has been shed by the star, bipolar outflow where the gas cools as it expands. The central star hasn't yet compacted and heated up enough to start spewing out the intense ultraviolet radiation characteristic of white dwarfs in planetary nebulas, irradiating and exciting the gas. Instead, the entire nebula is seen only from starlight reflecting off dust grains. The very edges of the nebula are starting to warm up, possibly by the photoelectric effect as light is absorbed then re-emitted.
Pinning down the shape of the Boomerang Nebula has been a multi-stage process. When it was first spotted by ground-based telescopes, we thought it was lopsided and vaguely boomerang-like. In 2003, the Hubble Space Telescope tracked out a bow-tie structure and more hourglass shape to the nebula. Only last year did the ALMA Observatory catch the Boomerang Nebula in all its ghosty glory.
Many planetary nebula have an hourglass shape with double-lobes of gas streaming out of the star. But in millimeter wavelengths, the gas wasn't divided into lobes with a narrow waist, instead flowing out in a nearly-uniform sphere. This apparent conflict was resolved by ALMA's greater resolution, as the array was able to track carbon monoxide distribution in greater detail and reveal lobes closer to the star that elongated into a rounded cloud of cold gas farther out. Dense lanes of millimeter-sized dust grains close to the star block visible light, creating an hourglass appearance in visible light.
Read more at the Alma Observatory.