The quasar is a distant high-energy active galactic nucleus. Quasars are extremely bright objects. They are also very distant and very old objects. The current explanation says that the quasar is a compact region in the center of a massive galaxy that surrounds a supermassive black hole in its core. Its size is 10 – 10,000 times the Schwarzschild radius of a black hole. The quasar is driven by an action disk around a black hole. They are usually located in the centers of active, young galaxies and are among the brightest, strongest and most energetic objects in the known universe.

The first identified them as sources of electromagnetic energy, including radio and visible spectrum, with a high redshift, which appeared as points, similar to stars, and not galaxy-like sources at all.
Quasars were much more common in the early universe.
The brightness of quasars is variable, varying from hours to months. This means that quasars generate and radiate their energy from a very small area, which means that a quasar with a period of several weeks cannot be larger than a few light weeks.
The energetic radiation from the quasar causes dark galaxies to glow, which helps astronomers understand the obscure early periods of galaxy formation.
Because quasars are extremely distant, clear, and seemingly small, they are useful reference points for determining distances. The International Celestial Reference System (ICRS) is based on hundreds of extragalactic radio sources, mostly quasars, spread across the sky.
Most quasars are invisible to small telescopes, but 3C 273 with an apparent magnitude of 12.9 is an exception. At a distance of 2.44 billion light-years, it is one of the most distant objects that can be directly observed with amateur equipment.

Quasars are thought to be driven by the accretion of material into a supermassive black hole in the core of a distant galaxy. They represent a radiant version of a general group of objects known as active galaxies. Since radiation cannot escape from the supermassive black hole in the center of the quasar, the escaping energy is actually generated outside the event horizon by gravitational forces and the unreal friction of the material falling into the black hole. The mass of black holes in the quasars was determined by measurements at 106 to 109 masses of the Sun.

We know more than 200,000 quasars, mostly thanks to the Sloan Digital Sky Survey. The spectrum of all observed quasars has a redshift from 0.056 to 7.085. According to Hubble’s law, these quasars are 600 million to 28.85 billion light-years away. Due to their enormous distance and limited speed of light, we see quasars and their surroundings as they looked in a very young universe.
A multiple quasar is a quasar that is subjected to gravitational lensing, resulting in a double, triple, or quadruple image of the same quasar. Since quasars are rare objects, the probability of three or more different quasars close to each other is very low. The first true triple quasar was discovered in 2007. In 2013, the second true triple quasar QQQ J1519 + 0627 was discovered.
When two quasars are so close to one position when viewed from Earth that it could be one quasar, but they can be distinguished using different telescopes. The Twin Quasar consists of two different quasars, not one that would be doubled by a gravitational lens.

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