Cosmic Times

Quasars: Express Trains to the Netherworld

The primary message of this article is the idea that there are objects near the edge of the known Universe that appear to be speeding away from us due to the expansion of the Universe. Secondary messages include the use of Hubble's law to determine distances and the shear power of some of the objects in the Universe that we are able to see them given the huge distances involved.

Quasars were first observed with radio telescopes in the 1950s – they were seen in radio wavelengths with no corresponding visible object. When the first quasar was tied to an optical source (a source called 3C 48), its visible spectrum showed odd emission lines. Remember that different elements have signature emission lines, so it was unusual to see an object with unidentified lines.

In 1962, a second object, called 3C 273, was connected to an optical source. Astronomers subsequently made a detailed optical spectrum which showed unusual emission lines, similar to 3C 48. The unusual lines turned out to be hydrogen lines redshifted more than any other object had shown. The reason the lines looked unusual was not because they had not been seen before, but because they had not been seen in the observed waveband before.

One of the most important concepts in this article is how the "speed" of an object, or its redshift, is related to the distance of that object. In the 1929 edition of the Cosmic Times, we introduced this idea with Hubble's Law relating the redshift of an object to its distance. Quasars are an example of how this redshift-distance relationship was used in astronomy to determine the nature of an object. Using the large redshifts measured from the "unusual" spectra, astronomers determined that quasars lie at the edge of the known universe. The size of the Universe listed in the 1965 Cosmic Times is based on quasar distances.

After 1965, astronomers have identified quasars as a class of active galactic nuclei. A galaxy with an active nucleus is one that exhibits a high amount of emission from its nucleus, often so high that it outshines the galaxy itself. An active galactic nucleus is powered by a supermassive black hole (a black hole with the mass of a million, or more, suns). The black hole itself does not radiate, since once a black hole has swallowed something, even light, it can never return. However, as the black hole pulls in material, it forms a disk and the material experiences a lot of friction as it travels in that disk toward the central black hole. Hot matter can radiate a lot of energy.

Other resources

The following web pages have more detailed information:

A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC