A. J. Smith

When a star wanders too close to a black hole, the black hole starts pulling it in. If the star doesn't fall in entirely but only loses some material, it can survive several such 'bites.' Each time, a chunk of gas is ripped off, heats up, and flares — astronomers see a bright spot in the sky.

Dark matter is like an invisible glue that holds galaxies together. It emits no light, but if its particles collide, gamma rays can flash. Scientists hunted for such flashes in dwarf galaxies — small stellar clusters with lots of dark matter and almost no ordinary matter.

A spotlight shines ever brighter, but at some limit the bulb suddenly burns out, and no more light comes through. Astrophysicists have seen something similar in the distant blazar Markarian 421. This is an active galaxy whose jet of superheated plasma points almost straight at Earth, like the beam of a cosmic lighthouse.