top of page


Sorry, but it doesn't look like we could find a post that matches your search results...

Return the blog page below.

Quasars - are they black holes, stars, or something else ?

A quick run-down on the name...

The term, “Quasar” is short for “Quasi-stellar radio source”, since upon its first discovery in the early 1960s astronomers reported that the signal looked like it was coming from a star, but seemed to emit radio waves (which is uncommon for a star).

In fact, the name means exactly that - “star-like radio source".

Very creative as always…

A quasar however, is far more frightening than a star...

Since then, we have discovered that these mysterious objects are not stars, but supermassive/ultra-massive blackholes (we’re talking the biggest of the biggest black holes with masses of billions times that of the sun) that are surrounded by enough gas to make them shine incredibly brightly.

They’ll often outshine their entire own and neighbouring galaxies.

Think about that for a second….

Out of trillions of stars, this one blackhole will outshine them all… that’s the kind of luminosity that we’re talking about.

It seems kind of contradictory to call these black holes luminous, since they’re...well... black?

But it’s important to remember that it’s not the black hole itself that’s emitting this vast quantity of radiation, rather the surrounding gas and dust that’s being superheated due to a combined effort of tidal forces and friction.


To discuss more about the radiation that’s emitted from quasars, it’s interesting to note that despite the name "quasi-stellar radio source", we now understand that only about 10% of quasars actually emit substantial amounts of radio-waves.

However, they were first discovered using radio-wave instruments, so upon discovery, it was only certain that they emitted radio waves.

Other more common types include visible light, but can go right down through to x-ray and gamma.

You might now be wondering why there's a variation, but unfortunately that’s not something that’s very well understood among astronomers at this moment in time. Maybe check back here in a few years!

Images taken of quasars present in host galaxies. Notice how they outshine their entire galaxy that's full of trillions of stars... Image Credit: NASA

Images taken of quasars by NASA

As you can imagine, the conditions that are required for a supermassive blackhole to become a quasar are incredibly tight and, given this, they’re not all that common of a sighting these days.

Notice how I said “these days” ?

Well, that’s because it’s a generally accepted idea that in the past when the universe was young and only hundreds of millions of years old, quasars would have been far more apparent, as the energy density in the universe would've been a lot higher; black holes would have had more of an opportunity to collect sufficient amounts of gas and dust to become quasars.

A future that's not so bright...

The universe is expanding which means that, over time, the energy density is become lower and because of this, it’s unlikely that enough quasars will continue to form to allow us to study them for adequate lengths of time.

Continuing on from this, it’s interesting to note that every quasar that we have ever detected has thought to be millions if not billions of light years away.

In fact, quasars seem to only be found in the most distant galaxies from us, with the light coming from them experiencing remarkably high redshifts of around 0.3 (most galaxies that we can see are below 0.2).

This is a significant finding for astronomers, as this means that coupled with our understanding of the expanding universe, the detection of quasars allows us to probe the universe for some of the earliest and hence oldest galaxies in the universe, essentially allowing us to see farther back in time.

As mentioned previously, we would expect the formation of quasars to decrease over time as the energy density of the universe gradually decreases, which agrees with experimental observations that quasars exist int he furthest known galaxies.

Due to the limitations of the speed of light, the further the galaxy from us, the further back in time we're able to see.

If that concept sounds confusing, then perhaps check out this article here.

In fact, the nearest known quasar is Markarian 231 which comes in at about 600 million light-years from Earth.

To put that into perspective, the nearest star system to us the alpha-centauri system, which is about 4.3 light years away...

It’s therefore thought that perhaps our own supermassive blackhole, “Sagittarius A*” may have once been a quasar, but over time consumed enough of its surrounding gas and dust to slowly fade away, leaving the less luminous supermassive blackhole that we see today.

Quasars are so distant that making out any kind of structural feature to them is very difficult.

A blackhole is tiny compared to its accretion disk, which is the collection of gas and dust that swirl around the black hole travelling up to speeds of around 17,000km per second!

For a quasar, the accretion disk is typically around a few light days in diameter which is next to nothing compared to the average galaxy of around 100,000 light years, yet still produces enough radiation to outshine the billions of stars.

If the features of a black hole interest you, then feel free to check out my post here where I cover everything that you could possibly want to know about them.

I think we’ll end the discussion on quasars here as we’ve covered the fundamentals as well as some other interesting features.

As I’ve mentioned throughout, we really don’t know all that much about them as they’re so far away and seem to be in such little numbers that studying them for any significant amount of time has become apparently difficult...

For now, just remember that they're supermassive/ultra-massive blackholes that are consuming enough gas and dust to produce jets of intense radiation!

Thanks for reading!

Receive daily free information about the universe by subscribing to our newsletter below!

bottom of page