Science In The Pub Adelaide


Quasar or quasi-stellar object is a very bright active galactic nucleus that has a supermassive black hole with a mass that could be millions or billions more than that of the sun inside it and surrounded by an accretion disk that is gaseous.  When the gas in the disk falls into the black hole, electromagnetic radiation occurs, and this can be observed through the electromagnetic spectrum. Quasars radiate enormous power, and the luminosities of the powerful quasars surpass that of a galaxy like the Milky Way more than a thousand times.

The term was derived from a quasi-stellar radio source which was how these space objects were discovered in the 1950s. At first, they appeared in photographic images as a faint star-like point of light. However, high-resolution images have shown they are much more than that. They are usually found in the centre of galaxies.

It was Maarten Schmidt who recognised the emission lines pattern in the brightest quasar known, the 3C 273. He discovered that the emission could be identified as originating from hydrogen atoms.

Further observation of quasars shows that the brightness varies tremendously on timescales, even ones that do not exceed a few days. This meant that a quasar is only a few light days across in total size. Thus, if the quasar is this small, the radiation pressure within it must be so strong for it to be that luminous. In fact, the only means through which a quasar can prevent itself from blowing up due to its radiation is if it is massive. This led astronomers to wonder how an object that is no bigger than a solar system has a mass of about one million stars, yet it outshines a galaxy that has over a hundred billion stars by more than hundred times.

The answer to this question came from Igor Novikov, Yakov Zel’dovich, and Edwin Salpeter. It is due to the accretion by gravity into the massive black holes. Not all astronomers were satisfied by this explanation for the combination of small sizes and high luminosities, and they suggested alternative explanations in which the quasars did not have to be at the large distances that their redshifts imply. All these alternative interpretations were later discredited. A final answer to this controversy came in the 1980s with Todd Boroson and John Beverly Oke showing that the halos around some quasars are starlights from the galaxy that hosts the quasar.

It was later discovered that it is not all quasars that have strong radio emission, and only about 10% of quasars will qualify as radio-loud.  This led to the classification of quasars into radio-loud and radio-quiet. The discovery of quasar has helped connect the field of astronomy and physics in some ways, for instance, the gravitational lens effect which was predicted by Einstein’s theory of relativity was first observed in a double quasar.