Astronomers Capture A Rare Wolf-Rayet Star Just Hours After It Dramatically Explodes

Avishay Gal-Yam, Weizmann Institute of Science. Left: image of the UGC 9739 galaxy from the Sloan Digital Sky Survey (SDSS). Right: A bright blue spot which marked the supernova explosion.

For the first time ever, scientists have gathered direct evidence that demonstrates a massive object with the characteristics of a Wolf-Rayet star met its demise in a ferocious Type IIb supernova explosion. Furthermore, they were able to capture the event a mere 6 hours after the explosion was initiated. The observations have been published in Nature.

Our Sun may seem like a bit of a beast; after all, it makes up over 99% of the total mass of our Solar System and churns out a whopping 400 trillion watts of power per second. But compared with some stellar big boys, it’s a bit of a pansy. One example of a massive type of star is a Wolf-Rayet star; these stars have masses around 20 times greater than our Sun.

Little is known about these stars because they’re rare and often obscured from view by our telescopes, but astronomers are interested in them because they supply galaxies with vast amounts of heavy elements that will eventually give rise to planets. “We are gradually determining which kinds of stars explode, and why, and what kinds of elements they produce,” said Alex Filippenko, Professor of Astronomy at UC Berkeley and co-author of the paper. “These elements are crucial to the existence of life. In a very real sense, we are figuring out our own stellar origins.”

Researchers were able to capture a dramatic Wolf-Rayet supernova called SN 2013cu just a few hours after the explosion began thanks to the intermediate Palomar Transient Factory (iPTF) collaboration which makes use of resources based at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory.

“This is the smoking gun. For the first time, we can directly point to an observation and say that this type of Wolf-Rayet star leads to this kind of Type IIb supernova,” said co-author of the paper Peter Nugent, head of Berkeley Lab’s Computational Cosmology Center (C3).  

Throughout their lives, stars produce energy by continually fusing atoms of hydrogen to produce helium. This process is faster in bigger stars to prevent them from collapsing under their own gravity. As supermassive stars age and begin to run out of hydrogen, they turn to fusing heavier elements such as carbon and magnesium which will ultimately cause the core to turn into iron. After the core reaches a certain size the star will collapse and release a huge amount of energy into space, triggering a shockwave that spews out the remains of the star as a supernova.

Prior to this superb explosion, some supermassive stars will enter a Wolf-Rayet phase where the heavy elements from the star’s core rise to the surface, triggering stellar winds that slough off material from the star into space. This is what muddies our vision of these stars, making studying them problematic.

“When a Wolf-Rayet star goes supernova, the explosion typically overtakes the stellar wind and all information about the progenitor star is gone,” said Nugent. “We got lucky with SN 2013cu- we caught the supernova before it overtook the wind. Shortly after the star exploded, it let out an ultraviolet flash from the shock wave that heated and lit up the wind. The conditions that we observed in this moment were very similar to what was there before the supernova.”

After initial observations revealed that the star was likely a Wolf-Rayet, telescopes around the world were alerted to follow the event. Because it was spotted fairly early on, researchers were able to take a series of observations of the supernova which detailed its spectra, or chemical signature, through the use of technique called “flash spectroscopy”. From this information, the team were able to deduce that SN 2013cu was a Type IIb supernova because of the weak hydrogen and strong helium signatures that appeared after it cooled.

According to Nugent, the exciting discovery raises the possibility that some of the world’s largest telescopes could pick up the spectrum of a Wolf-Rayet star in our neighboring galaxies, perhaps even those as near as 4 million light years away. SN 2013cu is located a whopping 360 million light years away.

Alongside shedding light on these elusive stars, this research could also provide insight into the origin of important elements such as carbon and oxygen. 

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