More Than a Star, Less Than a Galaxy
1963: Maarten Schmidt
Seen through a backyard telescope, 3c273 is a puny sight. It looks like a faint star in the constellation Virgo. In the 1950s, it had been cataloged as a strong source of radio waves (hence its name: object 273 in the third Cambridge catalog of radio sources). Later, astronomers also discovered a star-like point of light at the same position in the sky. It and several similar objects were described as quasi-stellar objects, which was later shortened to quasar.
When they analyzed the light of 3c273, though, it looked like no other star astronomers had ever seen. The "fingerprints" of various chemical elements found in ordinary stars were nowhere to be seen. Instead, the patterns of bright and dark lines in its spectrum appeared to be random and indecipherable.
In February 1963, though, Caltech astronomer Maarten Schmidt recognized the patterns of elements in the spectrum of 3c273, but they were drastically offset from their usual positions. The patterns were shifted to longer wavelengths, known as a redshift.
A redshift is produced by an object's motion away from Earth; the faster the object is moving, the greater the redshift. The redshift in the spectrum of 3c273 was so severe that it indicated the object was moving away from us at 16 percent of the speed of light — about 30,000 miles (48,000 km) per second.
Such high-speed motion means that the object is far away, and is moving because of the expansion of the universe itself. A redshift of 16 percent of the speed of light tells us that the object is about 2.5 billion light-years away. For it to be visible from that distance, it must be trillions of times brighter than the Sun!
That discovery alone was remarkable, because no one had ever seen such an energetic object anywhere in the universe. 3c273 was producing many times more energy than an entire galaxy of stars. Yet the story quickly got stranger, because 3c273 was changing brightness over a period of just a few days or weeks. Such rapid change told astronomers that the object must be quite small — around the size of our own solar system — because it would take light longer to travel across a longer object, so the flickering would not be as rapid.
And that left astronomers completely baffled. Cramming the energy of an entire galaxy, which can span hundreds of thousands of light-years, into a space that is only a few light-days across, seemed impossible.
As astronomers pondered this conundrum, they eventually came across a remarkable idea: 3c273 and other quasars could be powered by giant accretion disks encircling supermassive black holes. As matter spiraled into such a black hole it would be heated to billions of degrees. It also would emit energy across the spectrum as electrons spiraled through magnetic fields and as radiation warmed grains of dust in the outer regions of the accretion disk. The model fit the observations perfectly.
Since then, astronomers have cataloged thousands of quasars scattered around the entire sky. Some of them are among the most distant objects we can see, with redshifts indicating they are more than 12 billion light-years from Earth, which means we are seeing them as they looked about one billion years after the Big Bang.
Visible (left) and X-ray images of 3c273, showing a long jet shooting from the galaxy’s core. [NASA (2)]