Quasars, Blazars, and Active Galaxies

Hi all,

I’ve not been too well, and haven’t had a good chance to sit at the computer and write. So today I’m going to try to get back into the swing of things and tackle some high-energy monstrosities from outer space!

First of all, the structure of a galaxy

milky_way-e1566501200616.jpgTo first understand what we’ll be talking about, let’s go over the basic parts of a galaxy:

For spiral and lenticular galaxies, the outermost part of the galaxy is the disk. It is simply where most of the galaxy’s stars are.

Elliptical galaxies don’t have this feature.

As we get closer to the centre of mass, the galaxy bulges and no longer is a flat disk shape for spiral galaxies. Ellipticals are naturally this shape. The bulge is made of stars, like the disk, but they are more concentrated.

Here’s where the fun begins

In the centre of the bulge or elliptical galaxy is the galactic nucleus, home to a supermassive black hole. All galaxies have these, the milky way’s is Sagittarius A*.

Simulation of Sagittarius A*. ESO/EHT

In an Active Galaxy…

The nucleus isn’t just a supermassive black hole and lots of stars, but there is also an accretion disk. The black hole heats up material very close to it, and as a result the material emits a huge amount of radiation, which is exactly what we saw in M87.

M87. Credit: Event Horizon Telescope Collaboration

Relativistic jets can also be produced from the interaction of the accretion disk and black hole. These jets are simply two beams of ionised matter on each side of a galaxy, caused by the interaction of magnetic field of the black hole and the accretion disk. The mechanics behind them is not well understood (yet!)

Centaurus A and its jet streams. Image credit: X-ray: NASA/CXC/CfA/R.Kraft/Submillimeter: MPIfR/ESO/APEX/A.Weiss. Optical: ESO/WFI


These three elements are what make an active galactic nucleus (AGN). Interestingly, in an active galaxy, most energy output comes from the AGN, rather than the disk stars and interstellar medium.


An AGN’s luminosity and energy output can range. A quasar is the most luminous and extreme kind of AGN you can find in a galaxy. In fact, quasars are the most bright objects in the universe.

It is not an object on its own, but the nucleus of an active galaxy. When they were first discovered, they were thought to be just very bright stars because of how much they overpower the rest of the galaxy they belong to (they’re over 100x brighter!)

When their spectra were first inspected astronomers concluded that these bright objects were over 3 billion light years away and were too massive to be stars.

Being the most massive of cosmic objects, they are also very short-lived. The typical nucleus will stay active for 100 million to 1 billion years, judging by the rate of the galaxy’s matter falling onto the accretion disk.

3C 273 is a quasar in Virgo.

Credit: ESA/Hubble

This quasar is quite close to home (compared to other quasars), about 2.5 billion light years away.

Just from this photo, it looks just like a bright star. But take a good look a the galaxies dotted around the image. They are all so much closer to us than the quasar, but are nowhere near as bright!

3C 273 is actually the first quasar ever detected!


Here is the jet stream taken by Chandra.



Blazars are…quasars.

It’s all to do with perspective (as the Flat-Earthers say)

If you were to look at an AGN with the jet pointing directly at Earth, we would call that a blazar, not a quasar.

When the jet is pointing at us, the galactic nucleus looks much brighter than if it were pointed away from us like quasars, and we can see dramatic changes in brightness. It’s a bit like the difference in brightness of a torch when it’s shone in your eyes (very bright!) and when it’s shone sideways to you (a bit more manageable).

Markarian 421 is a very close blazar in Ursa Major, and can even be seen with large amateur telescopes!

Credit: Sloan Digital Sky Survey.

I know how easy it is to class blazars as a sub-category of quasars, but they are such helpful objects in the sky and should get some recognition of its own.

The most massive black hole discovered belongs to S5 0014+81, a blazar in Cepheus. This blazar is one of the most powerful objects ever discovered, with a luminosity over 300 trillion times the Sun’s.

The first time we ever used neutrino detectors for carrying out observations was when we discovered a blazar (its name still to be allocated). How cool is that? We detected neutrinos over 3 billion light years away!


This week’s astronomy lesson is complete. Let me know what you would like to see next week, and in the meantime, have fun!



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