As cosmic events go, supernova explosions epitomize the saying, “Live fast, die young, and leave a good-looking corpse.” They’re the deaths of stars so massive that they tear through their fuel in a short time. Then, they explode and create gorgeous scenes of stellar destruction. These seminal events enrich the universe with chemical elements for new generations of stars and planets.
Hubble Space Telescope (HST) witnessed the death throes of a very distant core-collapse supernova, one that occurred more than 11 billion years ago. Astronomers found evidence of that explosion in archival data while looking for transient events in the distant universe. Those faint flickers often turn out to be something as common as a star dimming and brightening, or as spectacular as the destruction of a star.
Imaging Star Death
HST’s image shows a scene of destruction. And, it contains three different views of the same event at different times of the same explosion. The views were made possible by gravitational lensing. That’s good because capturing the entire sequence of a supernova is tough. These explosions happen so quickly that spotting the earliest onset is incredibly fortuitous. “It is quite rare that a supernova can be detected at a very early stage because that stage is really short,” explained Wenlei Chen, a postdoctoral researcher at the University of Minnesota. “It only lasts for hours to a few days, and it can be easily missed even for a nearby detection. In the same exposure, we are able to see a sequence of the images—like multiple faces of a supernova.”
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This isn’t the first time HST has caught multiple images of a supernova thanks to gravitational lensing. In 2021, scientists announced they’d seen a “quadruply lensed” stellar explosion thanks to another galaxy cluster.
Lensing the Supernova
Gravitational lensing helped astronomers suss out the distant star thanks to the immense gravity of the galaxy cluster Abell 370. It lies some 5 billion light-years away from us. The combined gravitational field of its hundreds of galaxies, plus associated (but invisible) dark matter is the secret sauce. It bends and magnifies light from more distant objects. That includes the supernova that popped off in a galaxy located behind the cluster.
The warping produced by the Abell 370 gravitational lens produced multiple images of the explosion over different time periods. That was possible only because the magnified images took different routes through the cluster. That was due to the differences in the length of the pathways the supernova light followed. Also, the slowing of time and curvature of space due to gravity affected light travel time. The result is like getting three postcards of the explosion, with each postcard showing a different part of the event. The postcards also show how the supernova changed color over time. The blue colors show the supernova during a very hot phase. As it cooled, it turned red.
“You see different colors in the three different images,” said Patrick Kelly, study leader and an assistant professor at the University of Minnesota’s School of Physics and Astronomy. “You’ve got the massive star, the core collapses, it produces a shock, it heats up, and then you’re seeing it cool over a week. I think that’s probably one of the most amazing things I’ve ever seen!”
What About the Progenitor Star?
The HST observation of the lensed supernova is the first time astronomers have gotten the chance to measure the size of a dying star so early in cosmic history. They used the brightness and cooling rate to estimate its size. But, astronomers also study supernova explosions to understand how the progenitor star dies. That gives them insight into the evolution of the star. There are challenges, of course. Once it explodes, it’s hard to know how big the original star’s radius was. Also, it’s extremely difficult to get good information about the progress of the explosion in the very earliest moments after it occurs. The best information comes through ultraviolet observations. The biggest challenge is distance, and the farther away a supernova occurs, the harder it is to get any sense of the progenitor star or the progress of its death.
Distant populations of stars in the early universe contain some of the most massive stars ever detected. They are the supernova progenitors of the early universe. Pre- and post-explosion views give a better idea about how these earliest behemoth stars formed, lived, and died. In the case of the supernova caught in the gravitational web of Abell 370, astronomers figured out quite a bit about it from the HST data. The star was most likely a red supergiant with a pre-explosion radius about 533 times the radius of the Sun. Large stars like that burn their nuclear fuel at very high rates. They live fast and die young. And, as HST showed in its gravitationally lensed postcards from the past, they leave behind beautiful and data-rich scenes of stellar destruction.