Solving the Riddle of Quasars

Gamma-ray bursts are not the only cosmic mystery that astronomers have attempted to solve by invoking the bizarre properties and effects of black holes. In 1963, an extremely strange object was discovered in the constellation of Virgo, the virgin. Dubbed 3C 273, the object is situated at the enormous distance of 2 billion light-years from our solar system. (This means that it takes light and other radiation emitted by the object 2 billion years to reach us!) Astronomers noted that 3C 273 gives off both visible light and strong jets of radio waves and other kinds of electromagnetic radiation. What made the object seem so unusual was that this outpouring energy is hundreds of times greater than that of the entire Milky Way galaxy, which is made up of billions of stars; yet 3C 273 appeared to be extremely small the size of a single star. Thus, the bizarre object and others like it discovered in the years that followed looked something like stars. But they were obviously not ordinary stars. So astronomers called them quasi-stellar (“starlike”) objects, or quasars for short. By 2003 some thirteen thousand quasars had been found; and experts estimate that as many as ninety thousand more will be discovered in the next couple of decades.

At first, astronomers had no credible idea of what could be causing quasars to shine so brightly. Nor did they think to connect them with black holes, which in the early 1960s were still viewed as fascinating but mainly hypothetical constructs. As time went on, however, careful observations of quasars revealed certain exotic characteristics increasingly associated with superdense bodies. In the words of University of Alabama astronomer William C. Keel:

Despite shining far brighter than ordinary galaxies, quasars change brightness on short timescales. Indeed, their X-ray output can vary in minutes. . . . This shows that most of the radiation must come from tiny regions, maybe no more than light-hours across, or roughly the orbit diameter of Uranus or Neptune. Along with their small size, quasars must have central engines with gravitational fields strong enough to hold onto gas that’s moving at thousands of kilometers per second. Also, the gas must be exposed to high temperatures or energetic radiation to create the observed . . . [amount of] X-ray emission. Finally, the extent and speed of the radio-emitting jets show that the source has a directional memory [i.e., always emits the jets in the same direction] and can eject material so close to the speed of light that it escapes in spectacular fashion. So what kind of an object can do all this? The most reasonable explanation is an enormous black hole.

A quasar-producing black hole gives off a tremendous outburst of energy in much the same way that an ordinary stellar black hole reveals itself by an X-ray signature. In the latter situation, matter from the accretion disk that is about to cross the event horizon is annihilated, releasing powerful bursts of X rays. Astronomer Mark A. Garlick describes the similar process involved in quasar formation:

Most astronomers are now convinced that a vast accretion disk of gas and dust, the gravitationally shredded remains of countless stars and nebulae [cosmic gas clouds], surrounds the black hole in a quasar. As the gas in the disk spirals toward the black hole’s deadly maw, the material becomes so compressed and heated that it generates the truly enormous quantities of light that make quasars conspicuous across billions of light-years.

What, then, makes a quasar-producing black hole different from an ordinary stellar black hole? Keel and Garlick hint at the answer in their use of the phrases “enormous black hole” and “vast accretion disk.” Only a black hole that has acquired a gigantic mass equal to that of millions or even billions of stars could produce the spectacular celestial fireworks called quasars. The notion that such objects could actually exist was eye-opening enough for scientists. But even more intriguing was the fact that quasars are always located in the centers of galaxies. From this realization, it was only a minor leap to the startling suggestion that gigantic black holes might exist in the hearts of all galaxies.

Twinkle, Twinkle Quasi-Star

For a number of years scientists viewed quasars as weird and mysterious objects that did not seem to belong in the “normal” universe. This air of mystery was captured perfectly in 1964 by renowned Russian born American physicist George Gamow (1904–1968) in a short poem titled “Quasar” (which has since that time been reproduced in hundreds of books and articles about black holes, quasars, and other cosmic oddities).

Twinkle, twinkle, quasi-star

Biggest puzzle from afar

How unlike the other ones

Brighter than a billion suns

Twinkle, twinkle, quasi-star

How I wonder what you are.

This bright quasar with a long X-ray jet lies 10 billion light-years away.