Star's shells show Sun's fate
by Dr Emma Rigby
for ASTRONOMY NOW
Posted: 30 September 2011
Giant dust shells around CW Leonis, an elderly giant star in the constellation of Leo studied by the Herschel Space Telescope, are providing clues on how our own Sun will behave when it nears the end of its life, around five billion years from now.
The dust shells surrounding CW Leonis. The star has been removed from the image to reveal the rings, and the 6-pronged diffraction spikes are artifacts caused by the telescope structure. Image: ESA/PACS/MESS & ESO/VLT
In this star the nuclear reactions needed to power it now occur in thin layers of hydrogen and helium, and have ceased within the now-inert carbon-oxygen core. As a result it cools and expands, allowing dust to form in its outer envelope, and begins to pulsate. This in turn expels the dust into the surrounding area at speeds of up to 30,000 miles per hour. New observations from ESA's Herschel Space Telescope have revealed a series of these shells, ejected at intervals of 500 to 1700 years. The most distant shell in the images set off 16,000 years ago and there are also likely to be farther, older, ones that are too faint to be seen.
"Until recently, it was thought that giant star's surroundings were homogenous: evenly distributed matter without any exceptionally large clumps, but there are more and more indicators suggesting that this is not a reliable picture", says Leen Decin an astronomer from K. U. Leuven, who led the research. "New images from the Herschel satellite confirm this in a spectacular way: We discovered more than a dozen shells expelled throughout the star's life as a giant. The weakest shell we found is 7,000 billion kilometres from the star."
CW Leonis is also the subject of another astronomical mystery: in 2010 the same team of astronomers, using a previous set of Herschel observations, found warm water vapour in its outer envelope. "This detection came as a surprise, since the deduced water abundance was four orders of magnitude larger than expected" says Decin.
However, she explains, the existence of these dust shells may provide an explanation: "One of the reasons we thought on that could produce water in this carbon-rich environment is the fact that the envelope is NOT homogeneous. In case of a homogeneous envelope, the interstellar ultraviolet radiation can not penetrate very deep into the envelope. However, in case of an inhomogeneous envelope the interstellar ultraviolet radiation can penetrate deep [and] will photodissociate SiO and 13CO. The liberated oxygen atoms, will then form [water] very fast. The new Herschel images of the envelope around CW Leo indeed demonstrate very clearly that [it] is NOT homogeneous, giving support to our idea of explaining the presence of water in this carbon-rich envelope."
When our own Sun swells into a red giant in around five billion years time and cools, it too will produce dust in its outermost layers. Monitoring the behaviour of CW Leonis will therefore provide insight into our star's fate.
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