Among the most distant objects we know of, quasars are celestial bodies shining so bright that they eclipse the ancient galaxies that host them. Quasars have fascinated astronomers since their discovery more than half a century ago, bringing physics and astronomy closer and playing a crucial role in our understanding of the early universe.

The discovery of quasars dates back to the 1950s. With the rise of radio astronomy, the earliest radio surveys of the sky began to show unusual objects with properties that defied any known explanation. These objects emitted enormous amounts of radiation in several frequencies, and in best cases could be traced back to point-like objects, like distant stars.

The first photographs of such objects, however, revealed they weren’t stars as they were embedded in faint, fuzzy halos, leading astronomers to label them as “quasi-stellar radio sources” (in short quasars), a name that best underlined their mysterious nature.

Thought to inhabit the centre of active galaxies, quasars are in fact extremely bright Active Galactic Nucleus (“AGN”) – compact regions at the core of a galaxy with much-higher-than-normal luminosity. They are among the most luminous, powerful, and energetic objects known in the universe, outshining all of the stars in the galaxies that host them.

Most quasars are believed to be powered by supermassive black holes with mass ranging from millions to tens of billions of solar masses. When gas particles fall toward the black hole’s event horizon, they heat up under the effect of friction and extreme gravity, thus emitting vast amounts of energy in the form of electromagnetic radiation.

Read more about Black Holes: Black Holes – The dark side of gravity.

While today it is widely believed that every large galaxy hosts a supermassive black hole at its core, quasars are not as common: they are usually found in very distant galaxies with large gas supplies, particularly in strongly interacting or merging galaxies. This is because younger, nearby galaxies, including our own, don’t have that much gas available.

However, some astronomers believe that a quasar could form when the Andromeda Galaxy collides with our own Milky Way in approximately 4.5 billion years.

Quasars are often categorised as “radio-loud” or “radio-quiet”. Indeed, not all quasars produce strong radio emission, with only around 10% of those we have identified so far estimated to be “radio-loud”. Some of them even change their luminosity on timescales as short as a few days, suggesting the total size of the quasar cannot be more than a few light-days across, with an extreme power density.

The visible properties of a quasar can vary depending on the mass of the black hole, the rate of gas accretion and its orientation, or the extent of occultation by dust-laden gas clouds within the accretion disc. Some quasars also emit radio jets from their central region; these are beams of plasma propelled out at nearly the speed of light, which can be observed at X-ray and radio wavelengths.

Latest surveys have identified more than a million quasars confirming they were far more common in the distant past, approximately 10 billion years ago. Unfortunately, due to their extreme distance and their small size, in most cases it’s impossible to resolve all the structure within a quasar, even with the most powerful telescopes. At least with the current technology.

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Interesting facts about quasars

• The first quasars to be discovered in the late 1950s as radio sources in all-sky radio surveys were the quasars 3C 48 and 3C 273.

• The closest quasar to Earth (that we know of) is Markarian 231, located approx. 600 million light-years away in the constellation Ursa Major.

• To date, 3C 273 in the constellation of Virgo is the brightest quasar in the sky. With an apparent magnitude of 12.9, it is one of the few quasars that can be seen with small telescopes.

• In 1979 the gravitational lens effect predicted by Albert Einstein’s general theory of relativity was confirmed observationally for the first time with images of the double quasar 0957+561.

• A study published in February 2021 showed that there are more quasars in one direction (towards Hydra) than in the opposite direction, indicating that we may be moving in that direction.