A powerful flashlight shines sideways, and we see the glow: that's how AT 2019ijn was discovered. Tidal disruption of a star by a black hole launched a jet — a beam of matter traveling at near-light speed. It missed Earth, but a year after the optical flare, radio telescopes detected a long-lasting glow: the jet, slamming into gas, gave itself away. Unlike a supernova, the star wasn't exploded but torn apart.

In a dwarf galaxy, where star formation is boiling, the distance was calculated from the redshift (Hubble's law). The star's remnants, swirling into a superheated disk, glowed like a perfect heated body — according to Planck's law, its temperature exceeded ten thousand degrees. The main surprise: the black hole turned out to be of intermediate mass, though it was previously thought that only giants launch jets. And the second: had this beam been aimed at Earth, we would have seen one of the brightest gamma-ray bursts — fortunately, it missed. Radio observations will now help search for such hidden threats.