Gravitational lens brings an ancient supernova into view

Earlier this year, an international research team reported a rare feat: the real-time tracking of a red supergiant star exploding in a supernova. Despite the use of new generations of telescopes with wide fields of view that scan the skies for transient cosmic events, it’s not often that astronomers get to see supernovae in action. Now Wenlei Chen of the University of Minnesota and colleagues have managed to do so by looking back in time. In analyzing archival images from the Hubble Space Telescope, the researchers spotted the echoes of the explosion of a red supergiant that occurred about 11.5 billion years ago.

The supernova sighting came via an automated transient search of images from Hubble‘s Frontier Fields project. The project’s goal was to use massive galaxy clusters as gravitational lenses that bend and magnify the light of more distant galaxies that otherwise would be too faint to detect. Some of the lensed galaxies appear as Einstein crosses—four images that surround the lensing galaxy, each with a distinct magnification and light travel time depending on the shape of the gravitational well created by the lens.

In December 2010 Hubble images, Chen and his colleagues found a lensing galaxy that is surrounded by three bright blobs (see panel c in the figure above) that far outshine the nondescript distant galaxy (shown in the circles in panel b) that appears in other archival shots of the same region. The researchers determined that they were seeing three of the four Einstein-cross images of a supernova (the fourth wasn’t magnified enough to be visible) that took place in that distant galaxy.

The Einstein-cross configuration gave the researchers three snapshots of the supernova over eight days, the time difference between the light’s shortest and longest routes through curved spacetime. By analyzing the spectra of the supernova images, the researchers measured a significant temperature drop in the emissions—from roughly 10⁵ K to 10⁴ K—over the eight days. They then compared the spectra with the predicted light curves of various kinds of core-collapse supernovae, in which a massive star explodes after collapsing under its own gravity to form a neutron star or black hole. As shown in the graph, the data were most consistent with a type IIP explosion of a red supergiant star, with the first of the images likely capturing the supernova just several hours after the star exploded. Chen and colleagues then used the light curve to calculate the size of the progenitor star, which they estimate at 414–687 solar radii. Red supergiants are the largest stars by volume in the universe.

Future discoveries of similar supernovae will help astronomers better understand the populations of massive stars that existed in the early universe. Surveys by the James Webb Space Telescope and the future Vera C. Rubin Observatory in Chile should add to the currently limited roster of high-redshift explosions. (W. Chen et al., Nature 611, 256, 2022 .)