Scientists have discovered a telltale clue that a star is about to go supernova. The new discovery could help astronomers develop an "early warning system" before one of these cosmic explosions happens that would allow scientists to watch one in real time, a new study suggests.
"With this early-warning system we can get ready to observe them real-time, to point the world's best telescopes at the precursor stars," said study lead author Benjamin Davies. With this early warning, scientists can then "watch them getting literally ripped apart in front of our eyes."
Published Oct. 13 in the journal Monthly Notices of the Royal Astronomical Society, the study simulated data on what red supergiant stars would look like in the year before they go supernova. Such stars swell in size and can self-destruct in a type II-P supernova, or a violent explosion that occurs in stars that are between eight and 40 to 50 solar masses. The simulations reveal that before this explosion occurs, a cocoon of circumstellar dust builds-up around the star.
"Recent studies of supernovae have shown that the star that blew up was nested inside a thick cocoon of material, presumably ejected from the star just before it died," Davies, an astrophysicist at Liverpool John Moores University in the U.K,. told Live Science in an email.
The paper determined that stars between eight and 20 times the mass of the sun and in the final red supergiant phase undergo dramatic changes in their last few months.
"We have no idea why stars do this — it wasn't expected, and images of stars taken approximately one year before they died show nothing untoward," Davies said.
Such stars suddenly become around 100 times fainter in visible light in their final few months before they die. This dimming may be caused by a sudden accumulation of material around the star, which obscures its light, though scientists aren’t sure how this happens.
"Our best bet is that the very late stages of nuclear burning trigger gravity/pressure waves that propagate out to the surface" of the collapsing star, Davies said. Scientists had previously theorized that this occurs for the most extreme supernovas. "But we won't know until we can watch it happening," Davies added.
The opportunity to watch a supernova explosion in real-time has so far eluded astronomers. The closest observation so far has been SN 2013fs, a supernova explosion of a red supergiant star in the spiral galaxy NGC 7610 about 160 million light-years from Earth in the constellation of Pegasus. It was observed in October 2013, just three hours after its light reached Earth.
"When you see the supernova this early you see the outer edge of the 'cocoon' illuminated from the inside by the on-rushing explosion," Davies said. "By continuously monitoring it [the astronomers] were able to determine the time at which the supernova over-ran the cocoon, and hence determine how far from the star it extended."
Because the dense material almost completely obscures the star just before an explosion, astronomers hoping to catch a supernova in real-time will require telescopes that can alert them to a star that has become about 100 times fainter in the visible part of the spectrum, according to the paper.
That will be possible with the upcoming Vera Rubin Observatory (VRO), slated to go online in 2023, whose 3.2 gigapixel camera-powered all-sky survey will look for tiny changes in the entire visible sky every three nights.
"The VRO will be able to see around half the red supergiant stars within around 10 million light-years, allowing us to monitor them every few days or so," Davies said. "If we see one suddenly start to dim dramatically this could be an indication that the countdown to a supernova has begun."
With that information, astronomers will be able to point other telescopes at the target to study the physical conditions of the star's atmosphere as it's ejected and as it morphs into the cocoon around it.
The closest red supergiant star to the solar system is Betelgeuse in the constellation Orion, which in February 2020 was observed to have lost two-thirds of its normal luminosity after an enormous ejection from its surface. However, this dimming was not the result of an impending supernova.
"A red supergiant star on its 'supernova countdown' would be like Betelgeuse on steroids," Davies said. "It would become much fainter, much faster, possibly disappearing from view at visual wavelengths altogether within a matter of weeks."
Other red supergiant stars in the night sky include Antares in the constellation Scorpius, about 555 light-years distant, and Aldebaran in the constellation Taurus, about 65 light-years away.
