Quasars are the brightest objects in the universe. A single quasar can outshine a trillion stars, and it can be seen from billions of light-years away. The name is short for quasi-stellar object, because early astronomers thought they were stars. They are actually the active cores of distant galaxies, powered by supermassive black holes devouring matter.

The engine is an accretion disk. Gas and dust spiral into the black hole, heating up to millions of degrees and radiating across the electromagnetic spectrum. The black hole itself is invisible, but the disk around it is blindingly bright. Magnetic fields channel some of the material into jets that shoot outward at nearly the speed of light.

Quasars were more common in the early universe. Their peak activity was about 10 billion years ago, when galaxies were younger and gas was more abundant. Today, they are rare. The nearest quasar is hundreds of millions of light-years away. The most distant known quasar is over 13 billion light-years away, seen as it was when the universe was less than a billion years old.

Quasars are useful for cosmology. Their extreme brightness makes them visible across the universe, and their spectra reveal the composition of intergalactic gas. They are lighthouses that illuminate the cosmic dark ages.