What are quasars?

The answer

Quasars are supermassive black holes at the centers of distant galaxies that are actively devouring enormous amounts of matter. As gas spirals into the black hole at nearly the speed of light, it heats up to billions of degrees and radiates more energy than the entire galaxy surrounding it - some quasars are 100 trillion times more luminous than our Sun. They are among the most distant objects visible, with the farthest known quasar existing when the universe was only 670 million years old. Quasars were crucial in proving that the universe evolved over time.