Active Galactic Nuclei
At a glance…
Quasars, short for quasi-stellar radio source(s), are some of the brightest objects in the universe, with luminosities about 100-10000 times that of entire galaxies! While, blazars are also radio-sources, again with huge luminosities, that emit plasma (ionized matter) towards the earth as well. And when there is something that bright, obviously a question arises: Where does that much light come from? First, we must know about black holes.
Every star has a lifetime. When massive stars undergo a supernova (explosion due to gravitational collapse after depletion of fuel for fusion reaction), they shed of most of their outer envelope. However, the core (mostly iron, as iron-56 has the third greatest binding energy) is pulled further in to become a neutron star (all nucleons fuse so that only neutrons remain). However, in case the mass of the core was enormous, the escape velocity at or above it’s surface exceeds even the speed of light! Then, that is called a black hole. Just like stars, black holes vary in size (mass) and they keep feeding on a HUGE amount of surrounding matter, including gas, planets, stars, (and even other black holes) which start flowing, in a spiral path rather than in a straight path, into the black hole, as shown below. This region around the black hole, which contains the matter starting to flow into the black hole, is called accretion disk. AGNs (Active Galactic Nuclei), which are centres of galaxies with very high luminosities, are nothing but black holes with accretion disks.
This is where quasars come into the picture. Supermassive black holes (SMBHs) with super-bright accretion disks are called quasars. These quasars are about a few light-years wide in diameter. And almost all of them are AGNs. Despite being so small in size (relative to galaxies, not stars!), they outshine everything else. Infact, a quasar tens of light years away can outshine our sun! So, we come back to the question, with an addition: If nothing, not even light, can escape a black hole, where does that much light come from? Firstly, by ‘nothing can escape a black hole’, we mean nothing can escape it after crossing a particular surface where the escape velocity is c, called the event horizon. So, light can come from beyond the event horizon. When the matter is being pulled into the quasar (or any AGN), the particles in the accretion disk move at extremely high velocities, and acceleration. And, of course, just like humans in a crowd, these particles do not move in a line, but keep hustling into each other. The high amount of friction produced, is enough to convert much of the mass around the black hole into light energy, much more than that produced by fusion reactions, thus giving them luminosity greater than anything else. Most of the quasars also throw out bipolar jets of ionized matter, which are discussed later.
Coming on to blazars (combination of BL Lac objects and quasars), they too are AGNs, but one of their jets is directed towards the Earth, making it possible to detect their emissions. Hence, blazars are defined relative to their orientation. Quasars can be blazars if any jet is directed towards the Earth. Now…, what are jets?
Jets, more precisely astrophysical jets, are streams of plasma thrown out from the axis of an accretion disc. These can reach millions of light years and their particles move at near light speed. Surprised, aren’t you?!?! How can a high gravity object, which is supposed to pull in, and hence decelerate matter, emit high speed jets? This is believed to be the result of a complex magnetic field that directs a plasma beam away from the centre of the disc, along the axis.
Perhaps, some day, you may find this out for sure and post it in your own blog!
- Hila Science