Fifty Years of Quasars
Radio astronomy was booming in the 1950s. After World War II, a slew of slightly used 7.5-meter-diameter antiaircraft radar antennas (called Würzburg dishes, so you can guess where they came from) were suddenly available for civilian use. Larger, purpose-built dishes also had sprung up in England, Australia, and North America. The so-called Third Cambridge catalog (3C for short), published in 1959 by a consortium of British radio astronomers, listed several hundred bright radio sources in the northern-hemisphere sky.
In those days, a radio telescope recorded a wavy line on a roll of chart paper. In order to figure out what the source was, you had to train an optical telescope on the same point in the sky. But as seen through a really large telescope, such as Caltech's 200-inch Hale Telescope atop Palomar Mountain, the sky is a very crowded place. Picking out the radio source from the myriad of small blots on a photographic plate requires extremely accurate coordinates.
Up at Caltech's Owens Valley Radio Observatory, sandwiched between the Sierra and Inyo National Forests and some five hours' drive north of civilization (or at least Pasadena), Thomas Matthews was using a pair of brand-new, 90-foot-diameter dishes to calculate such coordinates accurately enough for the Hale. He would map these onto photo prints from Palomar's 48-inch survey telescope and pass them on to a young astronomy professor named Maarten Schmidt, who would take the spectrum of that particular blot.
Every chemical element, when sufficiently heated, emits visible light at a few specific, well-known wavelengths. (Imagine a picket fence painted in all the hues of a rainbow from red to blue, but with most of the boards missing.) This spectral "fingerprint" is revealed when the light is separated into its constituent colors by an instrument called a spectrometer. The more distant a galaxy is, the more its light gets stretched, en route to us, by the relentless expansion of the universe. Measuring how much a given line has been shifted toward redder, longer wavelengths tells you how long the light's been traveling and thus how far it has come.
Most of the 3C radio sources were elliptical galaxies—featureless blobs lying at distances up to about three billion light-years away. But every now and then the source would be a starlike pinprick of light. True divas, these "stars" had spectra that could not be deciphered. Worse, they couldn't even be matched to one another. Each one was different; unique.
One such "star" was 3C 273, one of the brightest radio sources in the 3C catalog—and it lies only two degrees north of the celestial equator, which means it's visible from much of Earth's southern hemisphere as well. And this is a very good thing, because in 1962, Australian radio astronomers Cyril Hazard, M. B. Mackey, and Albert Shimmins aimed the 210-foot Parkes radio telescope, some 200 miles west of Sydney, at 3C 273 on three occasions when the moon passed in front of it. Matthews was using interferometry, triangulating the source's position from the very slight difference in the arrival times of its wave peaks at his two radio dishes. By noting the exact instant that 3C 273 winked out or reemerged, the Aussies nailed its position far more accurately. They also discovered that 3C 273 was really two radio sources very close together. They passed their findings on to Matthews, who worked them up and gave them to Schmidt, and when Schmidt went to Palomar in late December 1962, 3C 273 was on his to-do list.
The spectrograph Schmidt was using was mounted in the Hale's prime focus cage—a barrel six feet in diameter built into the telescope's steel skeleton some 50 feet above the mirror. The cage is accessible by an elevator that is moved out of the way during observations; anyone in the cage is marooned for the duration.
Schmidt had to be up there to keep the pinpoint of light he was seeking visually lined up on the spectrograph's slit, using a set of push buttons at his fingertips to control the stately motion of the telescope. "It was romantic!" he recalls fondly. "Once in a while you just had to stop and look around you. At times it could be damned cold, but I had a hot suit"—a pair of electrically heated coveralls also apparently left over from World War II. "It said 'Army Air Forces' on the front, and it did a rather good job of keeping you warm."
Schmidt's workday began in the darkroom, where he prepared his plates for the night. Each one was a thin sheet of glass, about an inch long and a third of an inch wide, cut down from the standard five-by-seven-inch plates used for full-field photographic work, and mounted in a plate holder. The holders, in turn, were packed in a light-tight box about the size of a cigar box for their journey to and from the spectrograph. As sensitive as they were, the plates were only about 2 percent efficient, Schmidt recalls, "which means that 98 percent of the light essentially wasn't used. Therefore the observations were very slow. I spent my night, typically, on one object that I observed for seven or eight hours, and then a brighter object that I might observe for one or two hours. And that was the whole night—two plates. Two objects. Slow work."
3C 273's two radio sources proved to be a relatively bright star and a very faint jet of gaseous material. And by "relatively bright," Schmidt means "dim." Since ancient times, astronomers have ranked stars by their magnitude, with first magnitude being the brightest: Sirius, Arcturus, Vega, Antares, and the like. The faintest stars visible to the naked eye are sixth magnitude. This star was magnitude 13, but it far outshone the radio galaxies he was used to photographing. "The typical objects I worked on were six or seven magnitudes fainter. No wonder I didn't know how long to expose it," he says. His first attempt, on December 27, was so overexposed as to be useless, but he got it right the second time two nights later.
When this plate was developed, it showed four nice, fat lines that, once again, didn't match anything. The mystery remained for about six weeks, until Schmidt was invited by Parkes Observatory director John Bolton (who had overseen the construction of Caltech's Owens Valley facility a few years earlier) to submit a paper to Nature to go with the one that Hazard and company were preparing on 3C 273's position.
And so it was that after lunch on Monday, February 5, 1963, Schmidt was sitting in his office trying once again to make sense of his results. He popped the plate into the viewer, and it suddenly dawned on him that three of his lines, plus one in the infrared that Associate Professor of Astronomy J. Beverley Oke had found using the 100-inch telescope on Mount Wilson, formed a series whose spacing and intensity decreased uniformly from red to blue.
The Balmer series, the best-known set of lines in hydrogen's emission spectrum, does exactly the same thing, but there was one small problem with applying that explanation to 3C 273: the brightest line in the Balmer series, known as H-alpha, is red. As in visible red, not infrared. The wavelength of Oke's line was too far into the infrared by a factor 15.8 percent. However, the brightest, reddest line on Schmidt's plate was in the correct spot for H-beta, the second line in the Balmer series—if its wavelength were also 15.8 percent shorter; H-beta is normally cyan in color. A redshift of 15.8 percent is equivalent to a distance of about three billion light-years in the currently accepted scale of the universe. That's billion. With a 'B.' Our galaxy, the Milky Way, is a mere 120,000 or so light-years in diameter. And Andromeda, our nearest galactic neighbor of any consequence, is only about 2.5 million light-years away. By great good luck, the spectrum of 3C 273 was redshifted by a small enough amount that the Balmer spectrum was still recognizable, but yet redshifted enough to place it really, really far beyond our galaxy.
In his excitement, Schmidt began pacing the hallway, where he buttonholed Professor of Astrophysics Jesse Greenstein. Greenstein had been puzzling over a similar object, 3C 48, whose position had been worked out in 1960 by Matthews and Allan Sandage at the Carnegie Observatories up the street from Caltech. They had found a 16th-magnitude variable blue star, whose spectra, taken by Greenstein and Professor of Astronomy Guido Münch, had the usual assortment of unidentifiable emission lines. Greenstein pulled out the unpublished paper he was working on, and, as Schmidt recalled in his oral history, "in about five or seven minutes, we found a redshift of thirty-seven percent. . . They mutually confirmed each other."
The hubbub attracted Oke, and for the rest of the afternoon the three astronomers tried to come up with an alternative explanation—some weirdly ionized states of relatively rare elements, perhaps—that would allow these "stars" to remain comfortably in our own galaxy. When six o'clock rolled around and no other solution had presented itself, they decided to call it a day. But rather than heading home as usual, "we all trooped with Jesse to his house," Schmidt continued. "And Naomi [Greenstein, Jesse's wife] was immensely surprised, because we all wanted a drink. I came home late that night, and I think I said to my wife, 'Something terrible happened at the office.' It's not necessarily the right expression, but that's what I said."
The "terrible" part was that if 3C 273 really was three billion light-years away, it had to be shining 40 times more brightly than the brightest galaxies. Explaining how this could happen would have to wait until 1969, when a former postdoc of Schmidt's, Donald Lynden-Bell at the University of Cambridge, showed that material swirling around a black hole at a galaxy's center could radiate such staggering amounts of energy before being sucked down the drain.
On March 16, 1963, Nature published four articles back-to-back. The first, by the Parkes people, described the radio observations of 3C 273. The second, by Schmidt, announced the redshift. The third was by Oke about his infrared observations, and the final one, by Greenstein and Matthews, presented the corroborating redshift of 3C 48.
In his article, which was all of two-thirds of a page long, Schmidt noted that it was possible that 3C 273 could be a star in our own galaxy, but that "it would be extremely difficult, if not impossible, to account for" its peculiar spectrum, and that "the explanation in terms of an extragalactic origin seems most direct and least objectionable."
The papers referred to 3C 273 and 3C 48 as "star-like objects," for lack of a better term. Over the next few months, as more of them were discovered and it became abundantly clear that they were not stars, they began to be referred to as "quasi-stellar radio sources," or QSRs. The term "quasar" was coined by Hong-yee Chiu of NASA's Goddard Institute for Space Studies in a May 1964 article for Physics Today.
In 1965 Schmidt published a paper on five quasars, one of which had a redshift of 2.01, placing it halfway across the visible universe. And since distance and age are the same in astronomy, these very, very far-off objects give us an inkling of what the very, very young universe was like. Ever since the '60s, quasar studies have helped us map the universe, figure out why it is as it is today, and even work out how it all began. "The night I discovered the redshift, it was a fantastic prospect," says Schmidt. "We could now easily get to very large redshifts, because these darn things are so bright."