ALMA Captures Ghostly Shape of the Boomerang Nebula, Coldest Place in the Universe

Astronomers have caught a ghostly image of one of the coldest places in the universe. Using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, they've taken a closer look at the Boomerang Nebula, which plunges to an astonishing minus 458 degrees Fahrenheit.

The Boomerang Nebula was originally observed with ground-based telescopes. Appearing slightly lopsided from that vantage point, astronomers later used the Hubble Space telescope to get a closer look at the nebula. This new look at the object revealed a bow-tie-like structure. Yet now with the new ALMA data, astronomers have realized that the Hubble observations tell only part of the story. The twin lobes seen in the image may actually be a trick of the light as seen at visible wavelengths.

The Boomerang Nebula is located about 5,000 light-years away from Earth in the constellation Centaurus. It's a relatively young example of a planetary nebula, which is an end-of-life phase for stars like our own Sun that have sloughed off their outer layers. Because the Boomerang Nebula is so young, though, it's really a pre-planetary nebula; it represents a time when the central star is not yet hot enough to emit sufficient ultraviolet radiation to produce the characteristic glow. At this stage, the nebula is seen by starlight reflecting off its dust grains.

This particular nebula has an outflow of gas that's expanding rapidly and cooling itself in the process. The occurrence is similar to the way refrigerators use expanding gas to produce cold temperatures.

In this case, the researchers made some new findings with the ALMA telescope. They observed a double-lobe structure as seen in the Hubble image, but only in the inner regions of the nebula. Further out, they saw a more elongated cloud of cold gas that is roughly round. In addition, the scientists also discovered a dense lane of millimeter-sized dust grains surrounding the star, which explains why this outer cloud has an hourglass shape in visible light.

"This is important for the understanding of how stars die and become planetary nebulae," said Raghvendra Sahai, one of the researchers, in a news release. "Using ALMA, we were quite literally and figuratively able to shed new light on the death throes of a Sun-like star."

The findings are published in The Astrophysical Journal.