Astronomers have been misinterpreting the shapes of a dying star for decades, tricked into believing billowing dust was actually permanent structural features, a new study has found. The conclusion was reached with the help of possibly the coolest instrument for stellar astronomy of all time: the rings of Saturn acting as a filter for the Cassini spacecraft.
CW Leo, also known as IRC+10216 is a star at the very end of its life. With a mass less than the Sun's, it has blown out to an estimated 700 times our star's size and shrouded itself in a gigantic cloud of dust. Exceptionally faint in visible light, it is the brightest star in the sky at certain infrared wavelengths, thanks to the heat imparted to the dust cloud.
All this makes it hard to study, which prompted to the University of Sydney's Professor Peter Tuthill and his PhD student Paul Stewart to seek a radical solution. “If you are trying to work out whether a star is a single point or has a companion you can watch it go behind another object,” Tuthill explained to IFLScience. “If it is a single star it will just wink out. But if it is a binary it will disappear in steps.”
Tuthill noted this technique has been used when the Moon passes in front of a star to study objects, but he and Stewart took it up several levels. They used the way CW Leo appeared and disappeared behind Saturn's rings, as seen from the Cassini spacecraft, to reconstruct a map of its light. “I think Cassini has a 20-centimeter [8-inch] telescope,” Tuthill told IFLScience. “We produced an image that would normally require a telescope many thousands of times larger.
Cassini originally took some of the images of CW Leo for other reasons. As Tuthill explained, “One way to study the rings is to watch the way they extinguish stars.” However, the idea of using Cassini for stellar astronomy sufficiently impressed NASA that Stewart got to briefly fly Cassini to position it for additional photos, which he described to IFLScience as “the most fun I have had professionally”.
Tuthill and Stewart combined the results obtained with those from the more conventional sources of the Keck Observatory and Very Large Telescope to produce maps of CW Leo's recent changes. They found that what astronomers have been interpreting as “structural” jets are actually a case of parediolia, the process where we see faces or whales in clouds, or for that matter coffins on Mars.
Tuthill told IFLScience that a structural feature of a dying star would be something that was effectively permanent, but the clouds around CW Leo are more similar to the random movements of rain clouds, leading the paper to be titled “The weather report from IRC+10216.”
Previous observations had revealed bright spots, which astronomers interpreted as being where the star itself was located. Tuthill told IFLScience this was a case of astronomers fooling themselves into believing they had found what they expected to see. The bright spots have moved while we have been watching and the distances of the apparent shifts are several times the diameter of the Solar System. “Stars don't just hop around like that," Tuthill said. Instead, he thinks there are times where windows open up that allow us to see deeper into the clouds, viewing spots that are further in and therefore more lit up.
Animation of the changes seen on CW Leo over almost a decade combining data from several telescopes. Credit: Paul Stewart.
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