In the 1950s and 1960s, astronomers in Cambridge, England, began to use the most sensitive radio telescopes of the day to map the entire sky. There have been several “Cambridge catalogs,” each deeper and more detailed than the last. The common practice in modern astronomy is that, when an object is detected using one band of electromagnetic radiation, the same object is searched for in other bands as well to get a more comprehensive understanding of the object through all of its different types of light emission.

The third Cambridge (3C) catalog contains hundreds of radio sources, and astronomers took visible-light photographs of these sources to see what they would look like to our eyes. The 273rd object in the 3C catalog looked like a star. But when astronomers subsequently studied its emitted light more carefully, it was discovered that 3C 273 was actually an active galaxy far away from the Milky Way. In fact, 3C 273 was the first quasar ever discovered and identified as a distant “active galactic nucleus” (AGN) galaxy.