Active galaxies

Active Galaxies have a very active nucleus. They are sometimes referred to as AGNs (Active Galactic Nucleus). All galaxies host a black hole at their centre and most of them are dormant. Which is to say that the material in these galaxies is in a stable orbit around the black hole. Astronomers explain that in active galaxies however, material is still falling inwards, creating an intense blast of radiation. Such galaxies emit an exceptional amount of energy over a wide range of wavelengths from radio waves to x-rays. The AGN is the compact, highly luminous core of the active galaxy. It is powered by the accretion of additional gas and dust spiralling into a supermassive black hole.

The intense magnetic field that surrounds the black hole snatches some of this material and ejects it as two narrow beams at the poles. These jets shine with radio-wavelength radiation due to the synchrotron emission mechanism. This is where electrons spiral (and hence change velocity) through a magnetic field, reaching close to the speed of light. It is the brightest artificial source of X-rays.

These types include Seyfert galaxies, radio galaxies, quasars and blazars. Though in structure they are pretty well the same they are named differently to reflect the way we seem them. And the brightest is a trillion times brighter than the Sun.

Seyfert and radio galaxies are scattered throughout the universe. But quasars and blazars are billions of light years away, suggesting they are a feature of a young universe and that they have since evolved to other forms.

Seyfert Galaxies

Seyfert Galaxies account for about 10% of all galaxies in the universe. It covers any of a class of galaxies known to have an active nucleus. Such galaxies are named after the American astronomer Carl K Seyfert (1911 – 1960) who in 1943 announced his discovery of a number of spiral galaxies with particularly bright, starlike points of light emanating from their nuclei. The light is produced by gas clouds orbiting the central region at high speed. They are classed as the weakest form of the AGN type.

Quasar 3C 273 is the brightest quasar as seen from Earth, 2.5 billion light years away. Credit: ESA/Hubble & NASA

In 1960 scientists found some mysterious bright objects far out in space. They looked like tiny stars but gave out far more energy. Their spectra in visible light suggested broad and bright emission lines far more powerful than those of the Seyfert galaxies. But initially their spectra did not make sense. American astronomer Allan Sandage (1926 – 2010) called them quasi-stellar radio sources (quasars), so-called because they emit radio frequencies similar to a radio star. But a quasar is not a star.

Then in 1963 Dutch astronomer Maarten Schmidt (1929 – ) worked out that they were in fact hydrogen emission lines. The difficulty in realising this was because the spectra was very heavily red-shifted. This was caused by the Doppler effect of the object moving away from Earth at around 50 000 kilometres an hour.

A quasar is much smaller than a galaxy but shines a thousand times more brightly, even compared to a galaxy that is 100 000 times larger. Quasars produce a hundred to a thousand times more energy than our own Milky Way.

Quasars are only found at extreme distances, at some 10 billion light years away, relatively close to the time of the Big Bang. In fact, in 2017 astronomers identified an 800 million solar mass quasar which existed when the universe was only 690 million years old! Aside from the Big Bang a quasar is the most violent explosion we know of in the universe. More than 200 000 quasars have now been catalogued.

Quasars form as a disc from the effects of supermassive black holes at the hyperactive heart of a galaxy. They are extremely luminous thousands of times greater even than an entire galaxy. As the quasar disc is pulled towards the black hole, energy is released in the form of electromagnetic radiation. The power radiated by quasars is enormous. Some observers simplify the description of a quasar by saying it is essentially light emitted during the early formation of a black hole. Others describe it as a black hole with fuel.

It is estimated there must be at least a million quasars throughout the universe, that most formed around 12 billion years ago and resulted from the collision of galaxies to form supermassive binary black holes.

Astronomers have yet fully to understand the nature of quasars. In some cases their luminosity is variable at nearly every wavelength known. All give off x-rays, ultra-violet radiation, radio frequencies, infra-red radiation and gamma rays and, of course, light on timescales ranging from a few days to several decades. A good example is 3C 273 (pictured above) lying within the border of the Virgo Constellation, and the one that Maarten Schmidt had studied to come up with his theory. It was the first quasar to be identified.

Quasars are the intensely powerful centres of distant, active galaxies, powered by a huge disc of particles surrounding a supermassive black hole. As material from this disk falls inwards, some quasars, including 3C 273, have been observed to fire off super-fast jets into the surrounding space. In this picture, one of these jets appears as a cloudy streak, measuring some 200 000 light-years in length.

The term, Quasar, is also applied to quasi-stellar objects (QSOs), which are not strong radio emitters.


A blazar is the most violently variable type of active galaxy. It has a higher density than other AGNs and can produce much more powerful bursts (gamma rays usually) than a typical quasar.

Blazars are divided into two subtypes: the BL Lacertae objects (BL Lac) and the Optically Violent Variable (OVV) quasars. There are also a few intermediate blazars, which have the properties of both the BL Lac and the OVV.

These extragalactic objects were discovered around 1972, and the name, blazar, was coined in 1978 by astronomer Edward Spiegel (1931 – 2020).

A blazar is the core of an active galaxy, which is the most variable type of quasar. We see it as a jet of ionised particles streaming towards Earth in line of sight at almost the speed of light. This is because the pole of the galactic nuclei from which the jet shoots out is pointing straight towards Earth. The jet makes blazars appear much brighter than if they were pointed in a direction away from Earth.

As material in the galactic disc falls toward the black hole, its gravitational energy is converted to light, making the centres of these galaxies shine very brightly.

A BL lacertae object has no detectable absorption or emission lines in its spectrum but which is believed to be similar to a quasar. The name derives from an object in the Constellation Lacerta which itself was at first thought to be a variable star.

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.

Last updated: Monday, 22 March 2021