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Part 2 of my 4-part series on black holes for Medium members is up; you can read part 1 here. If enough of you read, they may keep me around to write more, so please read and share!
In the late 1950s, astronomers began spotting a number of bright sources of radio waves and visible light. These sources were pinpoints resembling blue stars, but further investigation showed they had to be something very different. For one thing, these quasi-stellar objects, as they were known then, were extraordinarily distant, much farther than any single star would be visible.
The spectra of these new quasi-stellar objects, or quasars, as physicist Hong-Yee Chiu abbreviated their name in 1964, showed they were emitting light through a completely different mechanism than starlight. The quantity of light quasars emitted to be visible across the universe meant they had to be driven by gravity.
Based on the data, astronomers concluded that each quasar was powered by a black hole millions or billions of times the mass of our sun. These supermassive black holes pull huge amounts of matter onto themselves, accelerating it until it glows very brightly. Additionally, the black hole jets a lot of matter away from itself rather than eating it, and those jets also glow intensely. These processes turn the ordinarily invisible black hole into something bright enough to see from billions of light-years away, outshining whole galaxies.