Boomerang Nebula
The coldest natural place in the known universe.
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The Boomerang Nebula, a young bipolar reflection nebula, sits about 5,000 light-years away in Centaurus. It holds the record as the coldest known natural spot in the universe, with a temperature below that of the cosmic microwave background radiation.
The object was first noted in a 1976 or earlier survey by Holmberg and Lauberts (Uppsala Observatory) and Schuster and West (European Southern Observatory). Before or during 1978, I.S. Glass and G. Wegner (South African Astronomical Observatory) identified it as a nebula using data from the ESO Quick Blue Survey. In their 1979 paper, Wegner and Glass described a "butterfly" or "bow-tie" shape. Observations on July 17, 1979, by K.N.R. Taylor (University of New South Wales) and S.M. Scarrott (Durham University) led to the name "boomerang."
The central star is old, with a maximum surface temperature estimated at 6,000 K (Wegner and Glass, 1978 or earlier) or 7,000 K (Bujarrabal and Bachiller, before July 1990). The nebula is thought to be a star system moving toward the planetary nebula phase.
Gas flows out from the core, where a late-stage star sheds mass and its light illuminates surrounding dust. Millimeter-scale dust grains block much of the center, so most visible light escapes through two opposing lobes, creating an hourglass shape in space telescope images. The outflowing gas expands rapidly at about 164 km/s, and this expansion causes the nebula's extreme cold.
Using the 15-metre Swedish-ESO Submillimetre Telescope in Chile during 1994 and 1995, astronomers Sahai and Nyman found that carbon monoxide (CO) molecules in the outflow—produced after stellar absorption in a binary system—were less kinetically excited than the surrounding cosmic microwave background radiation. Radiation transfer of that background into the CO parts of the wind showed those regions must have a temperature lower than any other observed natural location. The kinetic energy of the CO outflow is theorized to come from common-envelope evolution, where the smaller star in a binary system is absorbed into the larger star's core, forcing the outer envelope away. Cooling below the cosmic microwave background temperature happens through adiabatic expansion.
Lore & Background
The Boomerang Nebula was first identified as an object in a survey by Holmberg & Lauberts and Schuster & West in 1976 or earlier. Before or during 1978, I.S. Glass and G. Wegner discovered it as a nebula from data of the ESO Quick Blue Survey. In their 1979 paper, Wegner and Glass described a 'butterfly' or 'bow-tie' shape. K. N. R. Taylor and S. M. Scarrott made observations on July 17, 1979, and named it after the boomerang.
The nebula is believed to be a star system evolving toward the planetary nebula phase. It continues to form and develop due to the outflow of gas from its core, where a star in its late stage sheds mass and emits starlight, illuminating dust in the nebula. Millimeter-scale dust grains obscure portions of the nebula's center, so most escaping visible light is in two opposing lobes forming a distinctive hourglass shape. The outflowing gas expands rapidly into space at about 164 km/s, and this expansion results in the nebula's unusual low temperature.
Modelling of measurements published in 1997 by Sahai and Nyman indicated that the nebula's temperature is less than the cosmic microwave background radiation, making it the coldest natural place currently known. Observations from 1994 and 1995 with the 15-metre Swedish-ESO Submillimetre Telescope in Chile led to the conclusion that carbon monoxide molecules in the outflow were less kinetically excited than the local outer space. The cooling to sub-cmbr temperature is by adiabatic expansion.
Reader's Guide
The Boomerang Nebula holds significance as the coldest natural place currently known, with a temperature below that of the cosmic microwave background radiation. This extreme cold results from the rapid adiabatic expansion of outflowing gas at about 164 km/s, which cools the carbon monoxide molecules in the nebula's wind to a state less kinetically excited than the surrounding space. The nebula's unique temperature was established through modelling of measurements published in 1997 by Sahai and Nyman, using observations from 1994 and 1995 with the 15-metre Swedish-ESO Submillimetre Telescope in Chile.
Its legacy includes being a key object for studying the physics of extreme cooling in astrophysical outflows. The nebula is also a valuable example of a star system in transition toward the planetary nebula phase, with a central star that is an old star, possibly a dying red giant as of mid-2017. Successive observations from November 2011 to June 2012, along with archived Hubble data from 1998 and 2005, revealed that the visible double lobe is surrounded by a larger spherical region of cold gas seen only in sub-millimeter radio wavelengths, and that the nebula's outer fringes appear to be gradually warming.
More in Emission, Dark and Reflection Nebulae
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Boomerang Nebula (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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