Quasars (complete 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. If the observation occurs in a way that the accretion disk can be seen clearly, the term “quasar” is used. If the observation results in an unclear vision aof the central region of the object due, for example, to the presence of an opaque dust ring, the term “radio-galaxy” is used. Finally, if the observation results in a view pointed downwards, along the relativistic jet expelled by the poles of the black hole, the term used is “blazar”. 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. Later, in the year 1963, astronomer Maarten Schmidt realized that the spectral lines of the first detected quasar (3C 273) were, in fact, the famous hydrogen emission lines, however, under an intense redshift, the result of Doppler effect, which indicated that the quasar was moving away from Earth at 1/6 the speed of light, result of the universe's expansion. Observations from other quasars provided similar data. 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. Now that we know the essentials about accretion disks, we can proceed to the matter of their formation process. In the most common cases, the formation of accretion discs starts from stars unlucky enough to have their paths affected by a cosmic devourer. In the case in which “wandering stars” pass in the vicinity of a black hole, the tidal forces first cause the star to stretch and then, due to differential rotation, there is shear, thus transforming the star into a super hot gas disk ( accretion disk). The "tidal force" is the difference in the gravitational force "felt" along the object (example: an astronaut falling with his feet pointed towards a considerably small black hole will undergo the process of "spaghettification", due to the tidal force, since his feet will feel a greater gravitational attraction than the rest of the body, thus causing a “stretch”). More formally, the tidal force depends on the gradient of the gravitational field. The “differential rotation” occurs in rotating objects (liquid or gaseous), with sections with different angular velocities. The "shear" is, in a simplified way, the deformation suffered by a body due to the action of forces which cause a displacement in different planes in order to generate a “tendency to divide the body”. In order to remain “healthy”, emitting such an amount of energy, the black hole of a quasar must maintain a diet consistent in the consumption of several stars per year. If there is no more material available to quench quasar “hunger”, the emission of energy is stopped over time. Today it is known that most galaxies have a supermassive black hole in their galactic nucleus, as in the case of the Sagittarius A * in the Milky Way, however, the occurrence of quasars is observed at distances of more than 10 billion light-years from Earth (which makes optical telescopes practically useless). In the future, in approximately 4.5 billion years, when the Milky Way collides with the neighboring galaxy Andromeda, it is quite possible that with the increased matter arrangement, the supermassive black hole of our galaxy (which is not considered an active nucleus because it is inactive) will “wake up”, thus constituting a quasar.

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.

In the night sky, we can easily see that the stars “blink”/"sparkle", this is due to the fact that the luminous rays of a star (point light source), when crossing the various layers of Earth's atmosphere, suffer refraction, which diverts the path of light each time it is refracted, thus generating the “blinking of the stars”. In early 2017, a group of Australian researchers noticed, through the use of radio telescopes, that the quasar PKS 1322-110 had a variable brightness, which decreased and increased again within hours. The researchers noted the presence of a hot, bright star near the quasar and questioned whether that star was responsible for that variation in brightness. Afterwards, two other quasars, which also blinked, were observed and these were also accompanied by hot and bright stars. In more careful observations, astronomers noticed that in the three stars observed there was the presence of hot gas filaments, which led them to believe that the filaments were the responsibles for the blinking of the quasar light (light, when passing through the matter of the filament of the gas, suffers a deviation that generates the effect of brightness variation). It should be noted that the sparkling effect is not observed in isolated quasars (which reaffirms the explanation given by the scientists). Plasma, commonly referred to as the "fourth state of matter", is a state in which the particles are so energized that the behavior they exhibit differs from the "conventional" behavior of other states. Plasma is closer to the behavior of gases, due to the freedom of movement of the particles, however, it promoves electrical conductivity and adheres to magnetic fields that pass through it. Quasars produce a plasma of matter from the accretion disk, which releases ultraviolet radiation that ionizes the atoms space outwards (thus forming plasma). 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.