By the end of the 1950s, radio astronomy had given a new way to look to the sky. In addition to imaging celestial objects with light, surveys of the sky could use radio emissions from space, showing up previously unseen features. Radio waves were found to come from the sun, the stars, and the center of the Milky Way, but there were also mysterious invisible radio sources. In 1963, Maarten Schmidt, a Dutch astronomer working with the Hale Telescope at Palomar Observatory, California, managed to catch a glimpse of the light from one of these objects. When he looked at its redshift, he discovered something startling. The object was 2.5 billion light-years away, which meant that it was unimaginably bright. Its absolute magnitude was –26.7 (the lower the figure, the brighter the object). The object in Schmidt’s eyepiece was 4 trillion times brighter than the sun (magnitude +4.83)—brighter than the whole of the Milky Way put together.[show_more more=”Continue_Reading…”]
Schmidt named the body a quasi-stellar radio source, which was later shortened to quasar. Before Schmidt, the object had been known as 3C 273. The 3C referred to the 3rd Cambridge Catalogue of Radio Sources (produced by the Radio Astronomy Group) and 273 because it was the 273rd object to be located in that survey. 3C 273 had been spotted in 1959, although the first quasar to be identified (or what would be later termed a quasar) was 3C 48, which had been found shortly before.
Improving radio astronomy
Radio astronomy had started in the 1930s after the accidental discovery of cosmic radio sources by Karl Jansky. Interrupted by World War II, and helped somewhat by the development of radar technology, surveys using radio telescopes did not start in earnest until 1950. Early surveys were hindered by the low frequency of 81.5 MHz (megahertz—or million cycles per second) used by early radio receivers. At that frequency, it was difficult to pinpoint the location of signals with a low flux density. (Flux density is a measure of the strength of a signal, and is measured in watts per square meter per hertz, simplified as the unit jansky [Jy].)
In 1955, the Radio Astronomy Group at Cambridge University began a survey using a radio interferometer, which picked up signals at 159 MHz. This was better at resolving faint radio sources, and led to the discovery of the first two quasars
The light from both objects was invisible to the optical telescopes available to the Cambridge researchers at the time. However, their measurements of the flux density told them that these radio sources were very compact.
In 1962, 3C 273 was occulted, or covered, several times by the moon. By watching for the reappearance of the radio source from behind the lunar disk, astronomers were able to get a very precise location of the source. Maarten Schmidt used those measurements to take a look at it through the Hale Telescope, then the largest optical telescope in the world. He found 3C 273 to be the brightest object yet known. He published his findings in Nature in March 1963, and in the same issue, two other astronomers, Jesse Greenstein and Thomas Matthews, presented data on the redshift of 3C 48, which showed that the object was moving away at one third of the speed of light, making it the fastest-moving object yet discovered.
By the early 1970s, hundreds of quasars had been identified. Many were even more distant than 3C 48 and 3C 273; today, most of the quasars that have been found are located about 12 billion light-years away. In addition, quasars are mostly brighter than the first observations suggested, with luminosities up to 100 times that of the Milky Way.
“Understanding [of quasars] has not developed very much in 50 years. You only see a point source; you don’t see its structure. It’s a difficult thing to get hold of.” Maarten Schmidt
The debate now began as to what these things actually were. One suggestion was that the enormous redshifts seen in quasars were not the result of the expansion of space, but were the result of the light crawling out of a large gravity well. Such a well would be created by a truly monstrous star, with a gravitational field close to that of a black hole. However, calculations showed that such a star could never be stable.
Another proposal was that a quasar was the opening of a white hole. A white hole is the opposite of a black hole. This idea was proposed in 1964, and white holes remain entirely hypothetical. They are generally ignored as a theory today, but in the 1960s and ’70s, black holes were also unobserved phenomena, so the concept of white holes carried more weight. The idea is based on a complex interpretation of the Einstein field equations of general relativity, which proposes that a black hole that exists in the future would link to a white hole that exists in the past. A white hole is, therefore, a region of space where light and matter can leave but cannot enter. This would match the focused streams of radiation and matter that were being observed firing out of quasars. The question remained over where all that energy came from. The answer offered was that it has come through a wormhole, or Einstein– Rosen bridge, a theoretical feature of spacetime that connects the future to the past.
Currently, the only event that is accepted as anything like a white hole is the Big Bang itself, and some theories suggest that the material entering black holes may emerge in another universe as “small bang” events. Nevertheless, as the understanding of black holes grew, the white-hole explanation of quasars faded away.
“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.” George Gamow
Supermassive black hole
Quasars are too luminous and energetic to be using nuclear fusion, the process that powers stars, to produce their energy. However, theoretical work on black holes showed that a region of material, known as the accretion disk, would form around an event horizon. Since this material was steadily pulled into the black hole, it would heat up to millions of degrees. A supermassive black hole, with a mass billions of times greater than the sun, would produce an accretion disk that matched the output observed in quasars.
The accretion disk theory also matched up with the beams of plasma, known as relativistic jets, that blasted out in opposite directions from some quasars. These are caused by the spin of the black hole, which creates a magnetic field and focuses matter and radiation into two streams. Superheated plasma blasts out at close to the speed of light from each stream.
Today’s understanding of quasars began to crystallize in the 1980s. The accepted view is that a quasar is a supermassive black hole—or perhaps two —at the heart of a galaxy, that is eating up the stellar material. A galaxy that behaves like this is said to have an active nucleus, and it appears that quasars are just one manifestation of these so-called active galaxies.
An active galaxy is detected as a quasar when the relativistic jets are angled to Earth’s line of sight. Therefore, the object is detected chiefly from its radio emissions. If the jets are perpendicular to Earth’s line of sight, then they can never really be detected, and instead Earth sees a radio galaxy—a galaxy that is pumping out a loud radio source. If the relativistic jets are directed right at Earth, an excellent view is gained of the active nucleus in an object, known as a blazar.
Most quasars are ancient objects, and Earth sees their activity from when the universe was young. Unlike in other active galaxies, the brilliance of a quasar’s nucleus makes it hard to discern much of the galaxy around it. It is thought that young galaxies always have active nuclei, and that once there is no material left for their black hole to swallow they become quieter places, like the Milky Way today. However, galactic collisions, in which one galaxy merges with another, can activate the nucleus again. It is likely that the Milky Way, which is on track to collide with Andromeda in 4 billion years, is destined to become a quasar itself one day.
Born in Groningen, the Netherlands, Maarten Schmidt went to his home city’s university and studied alongside Jan Oort. Schmidt earned his doctorate, before emigrating to the US and taking up a post at Caltech’s Palomar Observatory. He became a leading expert on star formation, encapsulated by the Schmidt law, which relates the density of interstellar gas clouds to the rate of star formation inside them. Schmidt also became one of the chief investigators of quasars. After a conference on the subject in 1964, Schmidt and the other leading figures in the field, including William Fowler and Subrahmanyan Chandrasekhar, were leaving on the same plane, which experienced a dicey takeoff. Fowler is said to have quipped: “If this plane crashes, at least we’ll get a new start on this quasar problem.” Schmidt went on to occupy several eminent roles in astronomical institutions.[/show_more]