A guide to quasars, the powerful black hole objects

What are these powerful yet fleeting objects, and when was the first quasar discovered?

By Patrick Moore

Published: Thursday, 06 April 2023 at 12:00 am

When I began taking a real interest in astronomy, the types of objects in the sky seemed fairly straightforward.

We had the Sun and the planets of the Solar System, we had the stars which are suns, we had groups of stars, clusters of stars, nebulae and galaxies.

Today there are terms which were totally unknown in my younger days. No one had ever heard of things like pulsars, black holes or – quasars.

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A Hubble Space Telescope image of a pair of quasars that existed when the universe was just 3 billion years old. Dual quasar J0749+2255 is embedded inside a pair of colliding galaxies. Credit: NASA, ESA, Yu-Ching Chen (UIUC), Hsiang-Chih Hwang (IAS), Nadia Zakamska (JHU), Yue Shen (UIUC)

For a long time quasars were indeed mysterious; nobody knew just what they were. (They were known initially as ‘Quasi-stellar Objects’.)

They were found by radio astronomy – and even radio astronomy is comparatively new; it began in the 1930s, more or less by accident.

Now, of course, it is a vital part of astronomical research. In those early days there were radio telescopes that could pick up discrete radio sources in the sky, but could not define their positions very accurately.

Then, as time went by, radio telescopes were improved and could pick up very small radio sources.

It was known at an early stage that bright stars such as Canopus were not powerful radio sources; the strong emissions came from objects such as supernova remnants, notably the Crab Nebula.

Actually the first quasar to be identified is known as 3C 273, in Virgo (3C indicates the Third Cambridge Catalogue of radio sources, published in 1962.)

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An image of quasar 3C 273, as seen by the Hubble Space Telescope. Credit: ESA/Hubble & NASA

It was known to be a strong radio emitter but identifying 3C 273 with a visual object was very difficult.

Then, on 5 August 1962, radio astronomers in Australia working with the Parkes telescope followed an occultation of the radio source by the Moon, and were able to pinpoint its position very accurately.

It seemed to be identified with a faint bluish star, bright enough to be seen with a small telescope. But what was it?

In 1963, Maarten Schmidt at Palomar studied the spectrum of the object, and was totally baffled.

3C 273 was not a star at all, but something much more exciting.

There were spectral lines, but they did not seem to fit with known substances, and for some time the lines remained unidentified, but eventually the problem was solved.

The lines were due to hydrogen, highly redshifted. Now, this was significant.

According to the famous Doppler Effect, an object moving away from us will have all its spectral lines shifted to the red, and the greater the shift the greater the speed of the recession.