This Is The Coldest Place In The Universe – And Now We Know Why

The Boomerang Nebula seen by ALMA (orange) and Hubble (blue). ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble; NRAO/AUI/NSF

Scientists think they may have discovered why the Boomerang Nebula, located 5,000 light-years from Earth, is so cold.

This huge nebula spans 3 trillion kilometers from end to end (21,000 times the Earth-Sun distance). But at just half a degree above absolute zero (-272.5°C, -458.5°F) it’s colder than anywhere else we’ve seen.

The reason might be that a small star has plunged into its heart. This nebula was created by a red giant star shedding its outer layers as its core collapses. Previously, it had been theorized it was losing mass 100 times faster than similar dying stars, and 100 billion times faster than our Sun.

In fact, the outflow of material is about 10 times faster than a star should be capable of on its own, which is why it’s so cold. The best assumption seems to be that another star is responsible for ejecting the matter.

“The only way to eject so much mass and at such extreme speeds is from the gravitational energy of two interacting stars, which would explain the puzzling properties of the ultra-cold outflow,” Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, California, said in a statement.

Sahai is lead author on a paper in The Astrophysical Journal describing these findings. They were made using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

ALMA's sole view of the Nebula. ALMA (ESO/NAOJ/NRAO), R. Sahai

The reason that the Boomerang Nebula has an hourglass-like shape is due to the central star firing out jets. These sweep up the inner regions of dust and gas, while the outer regions continue to expand. Material took 3,500 years to reach the outer edges traveling at 150 kilometers (93 miles) per second. It’s known as a reflection nebula, as the nebula’s glow is provided by the light of the star itself.

In 1995, when the Boomerang Nebula was first discovered, astronomers found that it was actually absorbing the light of the Cosmic Microwave Background. This radiation left over from the Big Bang is just 2.725 degrees above absolute zero, which meant the nebula must be colder. Its temperature was measured by observing its spectrum of light, and applying that to Wien’s law.

While this is the coldest place in space, it’s not the coldest temperature ever recorded. That honor goes to the National Laboratory in Gran Sasso, Italy, which reached a rather chilly 0.006 Kelvins (-273.144°C, -459.659°F).

The Boomerang Nebula's time as the coldest known place outside Earth is numbered, though. Eventually, the red giant at its core will shrink and get hotter, producing a planetary nebula where the dust and gas are separated from the star.

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