The Boomerang Nebula, at a crisp one degree Kelvin (-458 degrees Fahrenheit), is the coldest known object in the Universe. In fact, the Boomerang Nebula is colder than the faint afterglow of the Big Bang, which is the natural background temperature of space.
To learn more about this intriguing object, a team of astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) telescope to investigate its frigid properties and determine its true shape, which is eerily ghost-like.
The nebula appeared lopsided when originally observed with ground-based telescopes. This lopsidedness is how this nebula got its name. A bow-tie-like structure was revealed when astronomers later used Hubble Space Telescope observations. According to the new ALMA observations, the Hubble data only told part of the story. The twin lobes seen in the Hubble image may actually be a trick of the light as seen at visible wavelengths.
“This ultra-cold object is extremely intriguing and we’re learning much more about its true nature with ALMA,” saidRaghvendra Sahai, a researcher and principal scientist at NASA’s Jet Propulsion Laboratory (JPL). “What seemed like a double lobe, or ‘boomerang’ shape, from Earth-based optical telescopes, is actually a much broader structure that is expanding rapidly into space.”
The results of this study were published in the Astrophysical Journal.
Located approximately 5,000 light-years away in the constellation Centaurus, the Boomerang Nebula is a relatively young example of an object known as a planetary nebula. Contrary to their name, planetary nebulae are actually the end-of-life phases of stars like our Sun that have sloughed off their outer layers. The remaining centers are white dwarf stars, emitting intense ultraviolet radiation that causes the gas in the nebulae to glow and emit light in brilliant colors.
The Boomerang is an example of a pre-planetary nebula, which is a stage in a star’s life immediately preceding the planetary nebula phase. At this point, the central star is not yet hot enough to emit enough electrical ultraviolet radiation to produce its characteristic glow. A pre-planetary nebula is seen by starlight reflecting off its dust grains.
The gas outflow from the Boomerang Nebula’s star is expanding rapidly and cooling itself in the process, in a way similar in principle to the way refrigerators use expanding gas to produce cold temperatures. The astronomers were able to gauge the temperature of the gas in the nebula by seeing how it absorbed the cosmic microwave background radiation, which has a very uniform temperature of 2.8 degrees Kelvin (-455 degrees Fahrenheit).
“When astronomers looked at this object in 2003 with Hubble, they saw a very classic ‘hourglass’ shape,” commented Sahai. “Many planetary nebulae have this same double-lobe appearance, which is the result of streams of high-speed gas being jettisoned from the star. The jets then excavate holes in a surrounding cloud of gas that was ejected by the star even earlier in its lifetime as a red giant.”
Single-dish millimeter wavelength telescopes, however, did not detect the narrow waist observed by the Hubble. Rather, they found a more uniform and nearly spherical outflow of material.
ALMA has unprecedented resolution, which allowed the astronomers to reconcile the discrepancy. The team was able to detect the double-lobe structure that is seen in the Hubble image, but only in the inner regions of the nebula, by observing the distribution of carbon monoxide molecules, which glow brightly at millimeter wavelengths. A more elongated cloud of cold gas that is roughly round was observed farther out.
A dense lane of millimeter-sized dust grains were found to be surrounding the star, providing an explanation for why the outer cloud has an hourglass shape in visible light. The dust grains form a mask that shade a portion of the central star, allowing its light to leak out in narrow but opposite directions in the cloud. This forms the hourglass appearance.
“This is important for the understanding of how stars die and become planetary nebulae,” said Sahai. “Using ALMA, we were quite literally and figuratively able to shed new light on the death throes of a Sun-like star.”
The current findings also suggest that the outer fringes of the nebula are beginning to warm, even though they are slightly colder than the cosmic microwave background. The warming could be caused by the photoelectric effect — an effect first proposed by Albert Einstein in which light is absorbed by solid material, which then re-emits electrons.