Quasars (simple version)

Quasars are extreme cosmic objects, conceiving the classification of active galactic nuclei. One of the most prominent factors of quasars is their brightness. A quasar itself can produce more brightness than 1 million galaxies (it is worth noting the fact that galaxies have, on average, 100 billion stars). Although extremely powerful, a quasar has a volume similar to that of the solar system. But after all, what is a quasar? Quasars are "hungry" supermassive black holes, fed by a diet based on stars and dust. As these blackholes consume matter, they create an accretion disk (already discussed in detail in an article published in the website), resulting in an immense release of energy, as evidenced in the presence of relativistic jets.

The terms "quasar", "blazar" and "radio-galaxy", although different, refer essentially to the same object, however, the terms are used according to the angle of observation that we take in reference to the mentioned object. For the sake of simplicity, the term “quasar” will be used throughout the article to refer to the cosmic object of study.

The fact that most quasars are more than 10 billion light years away from Earth combined with the blinding brilliance of the quasar, makes optical telescopes ineffective for studying such objects. In astronomy, the study of quasars is carried out mainly by observations made by radio telescopes, given the large emission of radio waves produced by these objects. The discovery of the first quasar, made by the astronomer Allan Sandage, occurred in 1960. At first the quasars were thought to be blue stars, however, the high radio emission discarded this possibility. The name of quasi-stellar radio source was thus conferred, which later became more compact in the term "quasar". The discovery of quasars brought evidence about the existence of black holes, a fact that graced Hawking (due to the lack of evidence to prove the existance of black holes at the time), resulting in the flourishing of several theoretical works.

The brightness mechanism of the quasars is due to the accretion disks. Such disks are circular agglomerates of matter/dust that orbit a central body, in this case a black hole. The extremely hot gas spirals towards the black hole, converting its gravitational potential energy into thermal energy and electromagnetic radiation, irradiated on the disk's surface. The relativistic jets observed in quasars (and also in pulsars) are jets of matter, along the poles of the central body, which eject extremely energized particles at speeds close to that of light. Furthermore, it is theorized that relativistic jets arise from interactions of magnetic fields in accretion disks.

The formation of accretion discs occurs when the black hole, mainly through tidal forces (secondary gravitational effect), "shatters" the star, thus forming a disc, which is responsible for the emission of electromagnetic radiation.

What is the reason for the lack of nearby quasars? What is the reason why such light sources “turn off”? There are two main types of quasar “deactivation mechanisms”. The first is the result of black holes' "hunger". By consuming all the nearby matter and radiating all the energy, the black hole begins to “starve” and, as it no longer has an accretion disk, it stops shining. The other mechanism is the product of tidal forces. The larger the black hole (more massive and greater event horizon), the weaker the tidal forces will be. To better illustrate, consider Newtonian gravitation. In it, the gravitational force decreases with the square of the object's radius and, therefore, when dealing with a larger black hole in extension, we will be dealing with a less violent gravitational attraction, which, indirectly, influences the action of the tidal forces. Therefore, if the black hole becomes too large, it is possible that the tidal forces become insufficient to "destroy" a star, making it impossible for an accretion disk to form, causing energy not to be released (quasar is not formed). In the case described, it is likely that the star will simply be consumed "in one bite" by the black hole.

Quasars are yet another extreme object in our Universe and still hide many elementary mysteries. Due to the enormous distance, they are subject to study difficulties, as in the case of the gravitational lens (consequence of general relativity), however, with the incredible speed of the evolution and adaptation of science, perhaps one day we can better understand the true luminaries of Cosmos.

Reference material:

http://astro.if.ufrgs.br/galax/quasar.htm http://www.if.ufrgs.br/oei/cgu/cmna/cmna.htm https://www.youtube.com/watch?v=Y52nJ0crFJc

Death by Black Hole, Astrophysics for people in a hurry, Origins (Neil deGrasse Tyson); 50 Astronomy Ideas You Really Need to Know (Giles Sparrow); Do átomo ao buraco negro (Schwarza)

Photo 1: Artistic representation of a quasar

Photo2: X-ray image of quasar PKS 1127-145, which is approximately 10 billion light-years from Earth. It is also possible to observe the presence of a huge X-ray jet.

Photo 3: Model of a quasar showing the possible angles of observation of the object.