Maarten Schmidt recognized that the previously mysterious “quasi-stellar objects” must be very luminous, very distant cosmological objects.

Between 1960 and 1963, radio astronomy Radio astronomy;quasars was faced with a major puzzle: Several remarkable sources of radio waves had been identified in the sky that seemed to have no normal visible counterpart. Whereas most previously studied radio sources were either peculiar galaxies or nearby gas clouds, this new class of source seemed to have no such identity. The objects were different from normal radio sources in that they were found to have small angular sizes, only a few arc seconds or less across. Known by their numbers in the massive Third Cambridge Catalog of Radio Sources (to which astronomers refer as the 3C catalog), the best studied of these were 3C 48, 3C 286, and 3C 196. Diameters were measured for these objects in 1960 at the giant Jodrell Bank radio telescope in England by Cyril Hazard and his colleagues, who found them to be surprisingly small. Quasars
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[kw]Schmidt Identifies Quasars (1963)
[kw]Quasars, Schmidt Identifies (1963)
Quasars
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[c]Astronomy;1963: Schmidt Identifies Quasars[07460]
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Schmidt, Maarten
Sandage, Allan
Matthews, Thomas A.
Hazard, Cyril

Intrigued by the peculiarities of these objects, Allan Rex Sandage took photographs of the sky at their positions in September, 1960, using the Palomar Observatory’s Palomar Observatory 508-centimeter telescope, then the largest in the world. Sandage and Thomas A. Matthews, an expert at identifying radio sources, studied the photographs but found nothing that resembled a normal radio galaxy or gas cloud. They noticed, however, that the photograph of the area at the position of 3C 48 included a star with a peculiar feature: A faint wisp of light seemed to be pointing at it—“as if,” Sandage excitedly exclaimed, “God’s finger were pointing to the true radio source.” At the next opportunity, in October of that year, Sandage obtained a spectrum of this star and measured its colors. (A spectrum is a picture of the light of a star that has been spread out into all its different colors, so that the effects of absorption and emission by gases of various composition can be seen and studied. Astronomers can use stellar spectra to deduce the compositions and physical conditions of stars.)

Sandage’s spectrum of 3C 48 was extremely puzzling. As he explained to the members of the American Astronomical Society at their December, 1960, meeting, the spectrum resembled nothing that had been seen before. Instead of a bright continuum of light of different colors, with various dark lines, the spectrum of 3C 48 had a weak continuum with broad, fuzzy lines superimposed. The most puzzling feature, however, was that none of these lines corresponded with any known elements seen in stars; they were completely unidentifiable.

It was a mystery as to what this strange object could be. Sandage showed that its colors were somewhat similar to some classes of unusual stars, including white dwarf stars (which are collapsed stars near the end of their lives), novas (which are double stars that include one member that is a white dwarf), and irregular variable stars called U Geminorum stars, after the type example in the constellation Gemini. Unfortunately, none of these objects had a spectrum anything similar to that of 3C 48, and astronomers remained unconvinced that the puzzle could be solved by invoking some strange sort of collapsed star. Nevertheless, the colors did indicate that whatever it might be, it was definitely a very hot object, with a temperature on the order of a hundred thousand degrees.

Maarten Schmidt.

(California Institute of Technology)

The distance to 3C 48 (and to the other similar “quasi-stellar objects,” which in 1964 astrophysicist Hong-Yee Chiu Chiu, Hong-Yee shortened to “quasars”) could not be measured, but there was a method to estimate it roughly. T. D. Kinman Kinman, T. D. of the Kitt Peak National Observatory and others around the world showed that the quasars varied in brightness by considerable amounts on a short time scale, often changing in brightness in only a day or so. This could be interpreted to mean that the object, whether stellar or not, must be no larger than one light-day, or else the light travel time would smear out the variations, making them undetectable from Earth. Thus, they were deduced to be small objects, not much bigger than the solar system and therefore, most likely, stars. This led to distance estimates for 3C 48 of, at most, a few hundred light-years, well inside our galaxy and relatively near the Sun.

A breakthrough occurred in 1962, when Hazard and his collaborators used the Parkes radio telescope Parkes radio telescope in Australia to make a high-precision measurement of the position of the quasar 3C 273. As seen from Parkes, the Moon happened to pass directly over the position of this object, and therefore a careful measurement of the time of its disappearance and later reappearance gave a very accurate measurement of its location, as the precise position of the moving Moon was known. When these radio astronomers compared the measured position with optical photographs of that part of the sky (in the constellation Virgo), they found that the smaller of the two components that were detected corresponded exactly with the position of a fairly bright star. The apparent brightness of this “star,” the brightest quasar in the sky, was approximately six hundred times fainter than the faintest star visible without a telescope. Other quasars were later found that are ten thousand times fainter yet.

When this quite positive identification was announced, Maarten Schmidt of the California Institute of Technology decided to use the Palomar telescope to obtain a photograph and a spectrum of the “star.” The photograph showed a bright stellar object with a faint, wispy structure to one side, “pointing” toward the other object, much like that found by Sandage next to 3C 48. The spectrum looked much like that of 3C 48, but with the broad emission lines in entirely different places. This remarkable fact might have confounded the situation even more, but Schmidt had a brilliant insight as he examined the spectrum. He realized that the lines would make sense if they were, in fact, normal lines of common elements that had been “redshifted” Redshifting to longer wavelengths than usual. (A source of light that is moving rapidly away from an observer will have all its light shifted in wavelength to redder, longer wavelengths, by an amount that depends on the velocity.) If he identified four of the lines as normal lines because of hydrogen gas—the most common element in the universe—then he found that the corresponding velocity of the object away from Earth must be about 48,000 kilometers per second. Under the assumption (later confirmed) that this high velocity is merely the cosmic velocity of expansion of the universe, it was possible to measure reliably the distance to 3C 273, which was nearly 2 billion light-years.

Schmidt’s discovery was announced to the scientific world early in 1963, a date that marks the beginning of the solution to the puzzle of the quasi-stellar sources. Soon after Schmidt unscrambled the puzzle of 3C 273, Matthews and Jesse Leonard Greenstein Greenstein, Jesse Leonard identified the lines in Sandage’s spectrum of 3C 48 as the same familiar hydrogen lines, but redshifted much farther, indicating a velocity of recession of about 109,000 kilometers per second. Other quasars were rapidly identified, some with even greater redshifts.

Quasars were seen at last to be objects with astounding properties. Large numbers of astronomers, led from the beginning by the galaxy expert Margaret Burbidge, Burbidge, Margaret explored the intriguing properties of the many quasars that were discovered—more than 100,000 as of 2005. It is now known that quasars are manifestations of conditions in the relatively early stages of the universe, when gravitational interactions and collisions between galaxies were much more common than they are now. Quasars resulted when two galaxies interacted and material fell into the center of one of them, collapsing to a massive black hole Black holes that caused a brilliant outburst of light from its environs, brighter than 1 trillion suns. Although these events occurred billions of years ago, the distances that light had to travel to reach today’s Earth are so vast that we are only now seeing these events. Therefore, quasars reveal information about conditions in the early universe, when it was smaller and when galaxies were closer together.

Schmidt’s discovery of the nature of the quasars as cosmological objects began a long and sometimes frustrating campaign on the part of many of the world’s astronomers to understand what quasars can tell astronomers about the universe. It took nearly two decades to gain enough understanding to see that they are the brilliantly overluminous centers of normal galaxies and that the mechanism that explains their nearly incredible amounts of energy must be gravitational collapse; no other physical mechanism could be found that could do it. Astronomers came to understand quasars as the nuclei of galaxies that have close companion galaxies, with the adjacent galaxy losing material to the quasar galaxy. This material, mostly hydrogen gas, has fallen into the very center of the object, where it has collapsed to form a very massive black hole. The black hole is not seen; however, the newly captured gas that is falling toward it heats up to extreme temperatures and emits huge amounts of light as it streams its way into the black hole. It is now believed that most galaxies, even the Milky Way, may have gone through such an “active phase” in their evolution, and that the black hole originally at the center eventually became quiescent when there was no more surrounding matter to fall into the accretion disk.

Quasars are perhaps most significant for what their properties are able to tell cosmologists about the properties of the early universe. For example, quasars have revealed that galaxy collisions and interactions were far more common billions of years ago than they are now. The spacing between galaxies was closer then and scientists now know—basing their information on quasar densities at various redshifts—about the early stages of galaxies and how their physical properties were different at that time from their physical properties now. Astronomers continue to discover older and older quasars, as measured by redshifts greater than 6 in several cases. Quasars
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• Burbidge, Geoffrey, and Margaret Burbidge. Quasi-Stellar Objects. San Francisco: W. H. Freeman, 1967. Published five years after Schmidt’s discovery, this comprehensive book outlines the early history and first results of systematic quasar studies. Some of the text is quite technical, but the descriptive and historical parts are relatively readable.
• Hodge, Paul. Galaxies. Cambridge, Mass.: Harvard University Press, 1986. A book for the general reader, this volume covers the physics of quasars as well as the history of their discovery.
• Kembhavi, Ajit K., and Jayant V. Narlikar. Quasars and Active Galactic Nuclei: An Introduction. New York: Cambridge University Press, 1999. A textbook for advanced undergraduates and graduate students that addresses theoretical models and observations of quasars and active galactic nuclei (AGNs). Covers quasar surveys, continuum radiation, time variability, relativistic beaming, accretion disks, jet sidedness, gravitational lensing, and unification; emphasizes radio, X-ray, and gamma-ray observations.
• Shipman, Harry L. Black Holes, Quasars, and the Universe. Boston: Houghton Mifflin, 1980. Designed as a supplement to standard astronomy textbooks of the time, when quasars were only beginning to be investigated, this standard, though dated, book remains available and still of interest for budding astronomers interested in a layperson’s introduction.
• Taschek, Karen. Death Stars, Weird Galaxies, and a Quasar-Spangled Universe: The Discoveries of the Very Large Array Telescope. Albuquerque: University of New Mexico Press, 2006. Designed for high school students and general readers, this review of the discoveries of the Very Large Array (VLA) radio telescope includes current information on quasars as revealed by radio astronomy.
• Weedman, Daniel W. Quasar Astrophysics. Cambridge England: Cambridge University Press, 1986. Weedman’s book is fairly technical. A reader with a familiarity with basic astronomical terms and techniques will find that it is a thorough and insightful source of the results about quasars and their physics.

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