Dying star generates the stuff of life
18:00 27 June 2007
NewScientist.com news service
One of the largest and most luminous stars in our galaxy is a surprisingly prolific building site for complex molecules important to life on Earth, new measurements reveal.
The discovery furthers an ongoing shift in astronomer's perceptions of where such molecules can form, and where to set the starting line for the chain of events that leads from raw atoms to true biology.
"Where we thought molecules could never form, we're finding them. Where we thought molecules could never survive, they're surviving," says Lucy Ziurys, an astronomer at the University of Arizona in Tucson, US.
Using the 10-metre radio dish atop Mount Graham in Arizona, Ziurys and her team searched the extended envelope of gas around VY Canis Majoris, a red hypergiant star estimated to be 25 times the Sun's mass and nearly half a million times the Sun's brightness.
There they found the telltale radio emissions of various compounds, including hydrogen cyanide (HCN), silicon monoxide (SiO), sodium chloride (NaCl) and a molecule, PN, in which a phosphorus atom and a nitrogen atom are bound together.
Even simple phosphorus-bearing molecules such as PN are of interest to astrobiologists because phosphorus is relatively rare in the universe – yet it is necessary for constructing both DNA and RNA molecules, as well as ATP, the key molecule in cellular metabolism.
For years astronomers have known that dense molecular clouds, which are pervasive in the plane of our Milky Way galaxy, are the repositories of a wide variety of chemicals that can later find their way into newborn solar systems. What has been less clear is exactly how those molecules form in the first place.
In the last few years, astronomers have turned their attention to ageing stars, which typically expel vast quantities of gas as they expand and turn into red giants. Until recently, it was expected that any molecules that condensed from the cooling, expelled gas would later be destroyed by the intense ultraviolet radiation emitted by the star.
Work by Ziurys and others demonstrates that this is not the case. The ejected material contains clumps of dust particles that apparently shield the molecules and can shepherd them safely into interstellar space.
The latest findings add a new twist to the story. Because VY Canis Majoris is an oxygen-rich star, it was not expected to harbour so many interesting molecules. Oxygen atoms easily outnumber carbon atoms around such stars and would be expected to take up the available carbon by forming carbon monoxide (CO).
The discovery of molecules such as HCN and a carbon sulphur compound (CS) around VY Canis Majoris suggests that chemical composition can vary greatly within a circumstellar envelope. It also implies that the chemistry that leads to life may be more widespread in the universe and more robust than previous studies have suggested.
"It shows that there is much more to be learnt about the use of old stars as laboratories for studying interstellar chemistry," comments Sun Kwok of the University of Hong Kong.
VY Canis Majoris is well known as an evolved star that expels large amounts of matter. But only within the past year have astronomers had the technology to detect the exceedingly faint radio emissions produced by molecules around the star.
The new technology comes in the form of an improved version of a device called the Superconductor-Isolator-Superconductor (SIS) Mixer, which can discern the energy emitted when molecules spontaneously shift from one rotational state to another.
The detector used by Ziurys and her team was developed for the Atacama Large Millimetre Array (ALMA), a high-altitude radio interferometer, consisting of 50 dishes – each 12 metres wide – currently under construction in Chile.
According to Ziurys, the fact that a single detector is already yielding such significant data bodes well for the future study of interstellar matter and its relationship to life in the universe.
"This is just the beginning of a new era in interstellar chemistry," she told New Scientist.
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Journal reference: Nature (vol 447, p 1094)
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