Blazars Vs. Quasars: The Most Extreme Objects In The Universe Explained

The universe is filled with uncountable wonders, from the comets and meteor showers of our own solar system to the magnificent supernovae that outshine their distant home galaxies. But as impressive as these phenomena are, they are literally outshined by the restless heart of a galaxy with an active galactic nucleus (AGN).

Most galaxies are held together by a supermassive black hole at their center. A typical black hole formed by a dying star will be about 10 times as massive as the sun, but the supermassive black holes residing in galactic nuclei can be millions of times more dense than the Sun. If there are any stars or gases too close to these black holes, they spiral into the black hole, heating up and emitting massive amounts of energy across the electromagnetic spectrum. It's this "feeding" behavior that distinguishes active galaxies from normal galaxies like ours.

Normally, these active galaxies can only be distinguished when looking at their spectral lines. Whereas normal galaxies have a spectral output equal to the sum of their stars, the nuclei of active galaxies shine brighter than the sum of their stars, sometimes outside of the visible spectrum. The most powerful of these AGNs are known as quasars and blazars and they are brighter than anything else in the universe.

Brightest of all the bizarre things floating around in space, quasars and blazars are born from the supermassive black holes at the center of galaxies. When there is a lot of matter falling into the black hole, it will form what's called an accretion disk, a terrifyingly awesome swirling cloud of gas orbiting the black hole at speeds that can be over 53 billion miles per hour. The friction produced by these fast-moving gas clouds causes it to heat up to millions of degrees, giving off enough light to outshine the surrounding galaxy.

Because quasars and blazars are so bright, it can be almost impossible to see their host galaxies. They were first spotted in the 1950s when the sky was first being surveyed with radio telescopes. At the time, they were just anomalous sources of radio emissions too faint to be seen with the naked eye. When they were observed with an optical telescope, they appeared to be faint, nebulous points of light, so they were given the name quasi-stellar radio objects, which was soon shortened to quasar.

[Featured image by ESO/L. Calçada via Flickr | Cropped and scaled | CC BY 4.0]

To understand the difference between quasars and blazars, it helps to understand the different tiers of active galactic nuclei (AGN) that lead up to a blazar. A galaxy is said to have an AGN if its supermassive black hole has an accretion disk. If this disc is luminous enough it is a quasar. The black holes at the heart of some quasars (around 10%) have powerful magnetic fields that funnel material from their accretion disk into jets that shoot out from the black hole's poles.

Usually, these galaxies are at an oblique angle from our perspective, but rarely, in the case of blazars, the jets from these quasars are pointed directly at the Earth. This makes blazars brighter than quasars, but it also means they are more rare. Over 1 million quasars have been discovered so far, but fewer than 3,000 of those are blazars.

So if blazars and quasars are so bright, how come you can't see them from Earth? The closest blazar to Earth is about 400 million light-years away and you would need a decent telescope to see it under perfect viewing conditions. The brightest known quasar (and the brightest object in the universe) is over 500 trillion times brighter than the sun but it's also 12 billion light-years away — and one of the most ancient objects in the universe — so you're going to need an expensive telescope to catch a glimpse.