Sherry Suyu can’t wait for the Sun to get out of the way. Sometime after June, when Earth’s turn around the Sun brings a distant galaxy into view, she expects to see a giant star explode at the end of its life. Supernovae usually don’t send out invitations in advance. But this one, called SN Requiem, has already gone off three times before. Suyu, a cosmologist at the Max Planck Institute for Astro-physics, predicts it will detonate yet again—as if the star is refusing to stay dead.
It’s an illusion. The star exploded just once, but its death is replayed through the magic of gravitational lensing. A huge cluster of galaxies sits between the shattered star and Earth, and its potent gravity is bending the supernova’s light, forcing it to follow different paths with different travel times. As a result, the flash of the explosion appears repeatedly, at different locations and times. The lens produced three bursts of light in 2016. After precisely mapping the mass in the galaxy cluster, Suyu and her colleagues calculated it would also steer light along a longer path—with a fourth reappearance coming sometime in the next year or two. “We made our predictions, we published them,” Suyu says. Now, “We just wait.”
Starting in June, they will check on SN Requiem’s galaxy once a month with the Hubble Space Telescope. If it spots anything, they will enlist the sharper vision of JWST, NASA’s infrared observatory. They hope to watch the light show unfold in real time, and not only to confirm their prediction. By clocking the time delay, they can accurately measure the galaxy’s distance. And by combining its distance with how quickly it is receding from Earth, indicated by the “redshift” of its light, they will get a reading of the Hubble constant: the expansion rate of the universe.
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