Astronomy: Pulsars, Quasars, Cosmic Masers

ASTRONOMY: PULSARS, QUASARS, COSMIC MASERS

Quasars Discovered

One of the major discoveries of the 1960s was the quasar, short for quasi-stellar object. The first identified quasar was 3C-48. The "3C" stands for Third Cambridge Catalog of Radio Sources, a catalogue used by radio astronomers. Allan R. Sandage of Mount Wilson and Mount Palomar Observatories in California reported it at the 107th meeting of the American Astronomical Society in New York in 1960. Employing two mobile ninety-foot parabolic radio antennae, Sandage used a technique called triangulation to locate the object. Thomas A. Matthaus had noted variable radio emissions from a small area and predicted a visible star there. Sandage found the object in the predicted location. But 3C-48 did not act much like a star. It was extremely hot (over one hundred thousand degrees Celsius). Sandage looked at the spectrum of the area (spectral lines tell astronomers what elements are present in and around a star). He found no hydrogen, which is present in essentially all stars. Instead of hydrogen, Sandage found 3C-48 contained calcium ions (charged calcium), helium, helium ions, and strange ions of oxygen. He suspected it could be a star surrounded by high-energy electrons in a magnetic field moving at the speed of light. This might explain how it gave off radio and light waves. Another possibility was that it was the remnant of a supernova.

What Are They?

It took some time to find that none of this was true and that 3C-48 was really a quasar. The Harvard Observatory had a collection of pictures of the region over many years. Harlan J. Smith and Dorrit Hoffleit of the Yale Observatory looked over twenty-five hundred plates in the Harvard collection taken from 1895 to 1952. In 1961 they reported they found no supernova detected by light astronomy in the region. They thought 3C-48 looked like some exploded object, though—maybe a much earlier supernova. In 1962 British astronomer Cyril Hazard located another "star" like 3C-48, named 3C-273. American Maarten Schmidt at Mount Palomar, who took photos and spectra of 3C-273, found that the normal spectrum lines were red-shifted, a condition caused by objects moving away from the observer at a high speed. Schmidt calculated 3C-273 was two billion light years away.

Quasars as Old as the Universe

Margaret Burbridge of the University of California, San Diego, studied these objects extensively. She concluded they were neither stellar objects nor galaxies, though they had some properties similar to those of stars. Thus they were quasi-stellar objects. Burbridge posited that quasars are galaxies passed through each other. Material falls to the center of a quasar where a black hole, a massive star that has collapsed on itself, is located. The black hole sucks everything around it into itself, even light. The galaxies passed through each other when the universe was young. Now the black holes are surrounded by light, and hydrogen is being sucked into them. The light can be seen by telescope, and the radio signals are energy from hydrogen racing at high speeds to the black hole, heating enough in the process to give off radio waves. Quasars, among the most distant objects visible from earth, have the energy level of one hundred or more large galaxies, resulting from gravitational collapse. They move very quickly away from earth, so the light we see today from quasars was created when the universe was young.

Masers Discovered

There was quite a bit of excitement when a maser (microwave amplification by stimulated emission of radiation) was found in outer space. Some thought it was aliens, but it was just a cosmic event. Masers explained a substance in space called mysterium. There is a region around stars where the heat causes hydrogen to ionize. Astronomers at MIT's Lincoln Laboratory figured out what caused the maser signal in one such area, the W3 area of the cosmos. In 1965 various astronomers at the University of California, Berkeley, found that W3 contained the usual elements plus a previously undetected radio-wave pattern they called mysterium. In 1966 the group from Lincoln Lab determined mysterium was an unusual form of hydroxyl ion (OH^-).

Identifying Mysterium

The chemical formula for water is H₂O. Putting a strong electrical current in water can cause it to separate. It could form hydrogen ions and hydroxyl ions: H₂O+ high energy = H⁺ + OH^-. The physicists and astronomers from MIT and Lincoln found mysterium was really a form of OH^- in an unusually excited state, emitting polarized radiation in a narrow band.

Solving a Mystery

The question was, How did the hydroxyl ions produce such a pattern? They could only be that narrow and polarized because they acted as a maser. But then what stimulated the hydroxyl ions to act as a maser? The Lincoln Lab group showed that the hydroxyl ions could be stimulated in this way by being close to a hot star. The heat would produce enough energy to give the hydroxyl ions maser activity.

Pulsars Discovered

Pulsars were first discovered in Britain in 1968. They were radio wave sources that had characteristic on-off cycles thought to be from neutron stars. Neutron stars rotate, and the "pulses" of the pulsar correspond to the rotation of the neutron star. The cycles resembled the pulse from the heart, so the name given them was pulsar.

American Astronomers See a Pulsar

American astronomers made a major contribution to pulsar study. Pulsars were heard but never seen until some University of Arizona astronomers, John Cocke, Michael Disney, and Donald Taylor, pointed a little thirty-six-inch Steward Observatory telescope at the Crab nebula, a supernova. The pulsar in the Crab nebula heard by radio astronomers had the fastest pulse frequency known. The star explosion that created the Crab nebula was witnessed on Earth in A.D. 1054. Records from all over Earth indicate the explosion caused a bright spot seen for six months, even during the day, though the Crab nebula is five thousand light-years from earth. When the University of Arizona astronomers pointed their light telescope at the Crab nebula, it showed a constant light. Using computer equipment, they checked the light intensity twelve thousand times a second. They found that the light peaked thirty times a second, too fast for human eyes to see, and they also found the exact frequency of the radio pulses from the same spot. Before long, two other American observatories confirmed these findings.

Sources:

"Celestial Maser?," Scientific American, 214 (January 1966): 48-49;

"First Look at a Pulsar," Time, 93 (7 February 1969): 57-58;

"First True Radio Star?," % and Telescope, 21 (March 1961): 148;

"Quasars Are 'Crazy,' "Science News Letter, 86 (15 August 1964): 106;

"Radio Source 3C-48," Sky & Telescope, 22 (September 1961): 131.