What is a quasar?


The word quasar stands for quasi-stellar radio source. Quasars got that name because they looked starlike when astronomers first began to notice them in the late 1950s and early 60s. But quasars aren’t stars. They’re now known as young galaxies, located at vast distances from us, with their numbers increasing towards the edge of the visible universe. How can they be so far away and yet still visible? The answer is that quasars are extremely bright, up to 1,000 times brighter than our Milky Way galaxy. We know, therefore, that they’re highly active, emitting staggering amounts of radiation across the entire electromagnetic spectrum.

Because they’re far away, we’re seeing these objects as they were when our universe was young. The oldest quasar, currently, is J0313-1806. Its distance has been measured as 13.03 billion light-years, and therefore we see it as it was just 670 million years after the Big Bang.

What was happening in our universe at that time to make quasars so astoundingly bright?

Astronomers now believe that quasars are the extremely luminous centers of galaxies in their infancy. After decades of intense study, we have another term for these objects: a quasar is a type of active galactic nucleus, or AGN. There are actually many different types of AGNs, each with their own tale to tell. The intense radiation released by an AGN is thought to be powered by a supermassive black hole at its center. The radiation is emitted when material in the accretion disk surrounding the black hole is superheated to millions of degrees by the intense friction generated by the particles of dust, gas and other matter in the disk colliding countless times with each other.

The inward spiral of matter in a supermassive black hole’s accretion disk – that is, at the center of a quasar – is the result of particles colliding and bouncing against each other and losing momentum. That material came from the enormous clouds of gas, mainly consisting of molecular hydrogen, which filled the universe in the era shortly after the Big Bang.

Thus, positioned as they were in the early universe, quasars had a vast supply of matter to feed on.

As matter in a quasar/black hole’s accretion disk heats up, it generates radio waves, X-rays, ultraviolet and visible light. The quasar becomes so bright that it’s able to outshine entire galaxies. But remember … quasars are very far away. They’re so far from us that we only observe the active nucleus, or core, of the galaxy in which they reside. We see nothing of the galaxy apart from its bright center. It’s like seeing a distant car headlight at night: you have no idea of which type of car you are looking at, as everything apart from the headlight is in darkness.

On the other hand, there are galaxies which are not classed as quasars but that still have bright, active centers where we can see the rest of the galaxy. An example of this type of AGN is called a Seyfert galaxy after the late astronomer Carl Keenan Seyfert, who was the first to identify them.