The James Webb Space Telescope has observed the oldest known supernova—the explosive death of a star that lived when the universe was only 730 million years old.
The ancient blast occurred when the cosmos was just 5 percent of its current age, and the supernova’s light has been traveling through space ever since. Astronomers were surprised to find that this primeval explosion strongly resembles today’s supernovae, which occur when massive stars run out of fuel for the nuclear fusion reactions that power them and then collapse under their own gravity.
Scientists had suspected that the universe’s earliest supernovae might look different because they represent the deaths of some of the first stars. Compared with today’s stars, they formed in a smaller, denser cosmos and contained lighter elements—mainly hydrogen, helium and trace amounts of lithium. And they were more massive than modern stars.
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Astronomers first spotted this primordial supernova in March, thanks to a 10-second-long flash of high-energy light known as a gamma-ray burst. Such a burst can be caused by a collision between a black hole and a dense object called a neutron star or—as in this case—by the death of a large star.
A telescope that scans for ephemeral events in space called the Space-Based Multi-Band Astronomical Variable Objects Monitor (SVOM) first detected the flash on March 14. That detection quickly set off a chain of observations around the world, including by NASA’s Neil Gehrels Swift Observatory, which pinpointed the flash’s location on the sky, the Nordic Optical Telescope on the Canary Islands, which suggested it was very far away, and the European Southern Observatory’s Very Large Telescope in Chile, which determined its age.
Because the object was so ancient, its light has been stretched as space has expanded over time. As a result, the light from the initial supernova that caused the gamma-ray burst was expected to become brightest a few months after the burst was sighted. The James Webb Space Telescope saw it in July, confirming that the flash was caused by a supernova, which has been designated GRB 250314A. The powerful observatory was also able to spot the galaxy that hosted the exploded star, although it appeared only as a small red smudge.