The Moon passing in front of 3C 273 cracked the quasar mystery
In 1962 astronomers timed the Moon's passage across a baffling radio source called 3C 273 to pin down exactly where it sat. That fix let Maarten Schmidt find its faint visible counterpart and read its spectrum, revealing ordinary hydrogen shifted so far toward red that the object had to be enormously distant.
Radio surveys of the late 1950s turned up sources such as 3C 48 and 3C 273 that seemed to have no visible partner. By 1960 hundreds were listed in the Third Cambridge Catalogue. When Allan Sandage and Thomas Matthews matched 3C 48 with what looked like a faint blue star in 1963, its spectrum was full of broad, unrecognisable lines. Some of these objects brightened and dimmed within short spans, implying they could be no bigger than the Solar System.
The lunar occultation measurements that Cyril Hazard and John Bolton made with the Parkes Radio Telescope allowed Schmidt to take a spectrum of 3C 273 with the 200-inch Hale Telescope at Palomar. He showed its strange lines were hydrogen redshifted by 15.8 percent, and his colleagues soon identified 3C 48's lines as hydrogen and magnesium shifted by 37 percent. If the redshift meant distance, these tiny sources were pouring out staggering amounts of energy. In 1964 the astrophysicist Hong-Yee Chiu, finding quasi-stellar radio sources clumsily long, shortened the name to quasar.
Today a quasar is understood as an extremely luminous active galactic nucleus. Gas in a disk swirling around a supermassive black hole, weighing millions to tens of billions of Suns, heats up as it falls inward and radiates fiercely; the brightest outshine a galaxy like the Milky Way thousands of times over. Hubble images show quasars sitting in the centres of galaxies, some of them colliding or merging.
About a million quasars now have reliable spectroscopic redshifts. The closest lies about 600 million light-years away, while the most distant active nucleus on record, at redshift 10.1, is seen as it was 13.2 billion years ago. Quasar activity peaked roughly 10 billion years ago, and clusters of them, called large quasar groups, may rank among the biggest structures known.
Source: Quasar
