In a distant galaxy about 290 million light-years away, the object HLX-1 lies hidden. It once flared brighter than hundreds of billions of suns, but now it has quieted down. From 2018 to 2022, the Hubble, Chandra, and Swift space telescopes tracked its fading. The X-ray emission from the black hole dropped by nearly a factor of 100 compared to its peak in 2010. But the visible and ultraviolet light dimmed much less.

This discrepancy is a real puzzle. If the light came from a hot disk around the black hole, the X-ray and optical fading would go hand in hand. Instead, the optical glow is too bright for such weak X-rays. Calculations show that the disk could not process enough energy, like trying to boil a bucket of water with a match. However, the observed pattern fits well with a cooling photosphere (the outer glowing layer) at a temperature of about 30,000 degrees. That is how the remnants of a star torn apart by a black hole behave.

The scientists also examined a mysterious ultraviolet source near HLX-1. New Hubble images resolved it into a ring-like structure where stars are being born. Perhaps the black hole itself and its host star cluster collided with this dwarf galaxy at tremendous speed. Such a head-on collision could trigger a burst of star formation, a cosmic fireworks display of young stars.