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Quasars & Active Galaxies

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Library of Past Questions and Answers


The Question

Who discovered the quasar and when?

The Answer

The discovery of quasars was really spread over time. Quasar is a shortening of "quasi-stellar radio source", and they've also been called quasi-stellar objects or QSOs. In the late 50s, several radio sources were matched with very dim optical objects that looked like stars, but had strange spectra with a lot of ultraviolet excess. One of them, 3C273 had it's position very accurately measured by C. Hazard and co-workers, using lunar occultations. In 1962, M. Schmidt obtained a spectrum of this "star", which showed a redshift of 0.158. This is when QSO was coined, because this was a very distant object that was masquerading as a star, a quasi-stellar object.

This description is paraphrased from a book, "High Energy Astrophysics", by M.S. Longair.

Also, you can see

Thanks for your question!

Eric Christian and Maggie Masetti
for Ask an Astrophysicist

Question ID: 980316b

The Question

Is Earth affected in any way by quasars? And what exactly are quasars?

The Answer

The word 'quasar' is a contraction of 'quasi-stellar radio source'. The first quasars were discovered in the early 1960s, when astronomers measured their very strong radio emissions. Scientists were subsequently surprised to see faint blue star-like points of light, rather than galaxies, when they looked at the same parts of space with optical telescopes. When the quasar's light was analyzed, it was seen that patterns known from laboratory studies of atomic processes were present with very large redshifts (which means the objects are moving away from us at high velocity!).

We now know that, in fact, most quasars are 'radio quiet', i.e., they have very little radio wave emission, but the name quasar has been kept anyway. We also now know that many (perhaps all) quasars are small regions of intense activity within otherwise normal galaxies.

What is responsible for all the energy that quasars are seen to be producing - sometimes hundreds of times the energy from normal galaxies? The best explanation seems to be that quasars are super-massive black holes in the centers of galaxies. As material spirals into the black holes, a large part of the mass is converted to energy. It is this energy that we see.

Because of their great distances from us, quasars have no real effect on the Earth.

Question ID: 960905

The Question

Was the Milky Way a quasar in the distant past?

The Answer

The answer is that we aren't really sure. Our Galaxy does have a supermassive black hole at the center, which is what astronomers believe powers the enormous emission in quasars. However, whether or not it was ever a quasar is still up for debate. It's entirely possible that it was, but we don't have any proof one way or the other.

We hope this helps!

Barbara and Stefan
For the "Ask an Astrophysicist" team

Question ID: 040407a

The Question

I'm a student of physics from Canada, and I was wondering how I could find out about quasars on a very detailed level.

The Answer

Since you know about black holes, I assume you know that quasars are a subset of a class of galaxies called active galactic nuclei (AGN) that are probably powered by a supermassive black hole. If you haven't already, check out "Active galaxies" under "Advance High-Energy Astrophysics" at our Learning Center. If you are mostly interested in the physics of accretion onto a black hole, the standard text is "Accretion Processes in Astrophysics" by Frank, King and Raine.

On the other hand, if you are more interested in AGN in general, the basic textbook is "The Astrophysics of Gaseous nebulae and Active Galactic Nuclei" by Osterbrock (the emphasis here is on optical spectra but it contains a lot of the physics of photoionzation which is important in AGN).

Quasars are AGN that are very luminous and radio-bright, and we think that in general they are radio-bright because we are seeing synchrotron emission from a jet of relativistic particles coming from the AGN. In radio-quiet AGN, either the jet is not present or it is directed away from us (since the particles are relativistic, the emission is beamed along the direction of the jet). There are some nearby radio galaxies that may be low-luminosity descendents of quasars, that show radio jets and evidence of many high energy particles (see books below). Blazars are an extreme case of quasars where we think we are looking directly into the jet.

A couple of books and articles on Radio Galaxies and Jets are:

  • Chapter 13 of "Galactic and extragalactic Radio Astronomy", edited by G.L. Vershuur and K.I. Kellermann, 1988, Springer-Verlag.
  • "Beams and Jets in Astrophysics", by P.A. Hughes, c. 1991 Cambridge University Press
  • "Extragalactic Radio Jets" in Ann. Reviews of Astrophysics, 1984, 22:319-58 by A.H. Bridle and R.A. Perley


Andy Ptak and Jonathan Keohane
-- for Imagine the Universe!

Question ID: 970409a

The Question

How many quasars do you think there are in the universe?

The Answer

There are about 12,000 known quasars today. I'm sure that as our telescopes get better, that number will go up. As a guess, I would estimate a lot.

Here is some other information on quasars:

Hope this helps,
Mike Arida
for Ask an Astrophysicist

Question ID: 981222a

The Question

Has anyone calculated the high and low limits of mass in the probable number of quasars?

The Answer

For your first question, the mass of quasars seems to be included in the mass of galaxies, so that is not the missing mass.

For nearby galaxies, the mass is often estimated by measuring the rotation curve (for spiral galaxies) or the velocity dispersion (for elliptical galaxies). The higher the velocities, the greater the mass which must be enclosed. This is actually a straightforward application of Newtonian gravity (which is still a reasonable approximation far from the black hole). This method includes the mass of any single central black hole (or stellar mass black holes) lurking within.

Relating these masses to the luminosity of the galaxy, we develop a mass-luminosity relation for galaxies that is applied to estimate masses of galaxies too distance to obtain a rotation curve or velocity dispersion.

Considering that recent Hubble results suggest black holes may lie in the centers of many, if not all galaxies, then the mass of these black holes is already included in our mass estimates of the galaxies and therefore of the universe. Our current models of quasars suggest they are simply regular galaxies being viewed down a jet being ejected near the central black hole. The jets are probably formed by complex magneto-hydrodynamical interactions in the accretion disk with the spin of the black hole.

Anyway, as a result, the quasars and their mass are included in our mass estimates of the Universe. There have been some studies searching for possible populations of small black holes roaming around between galaxies but the observational constraints do not suggest these could be the source of the 'dark matter' or 'missing light', whichever you choose to call it.

Tom Bridgman and David Palmer
for Ask an Astrophysicist

Question ID: 970701a1

The Question

Most known "naked" quasars have no nebulosity. This poses an obvious threat to some prominent theories about quasars. Do you have any ideas about the lack of host galaxies?

The Answer

Thank you very much for the question regarding quasars. It certainly is a puzzle: why in some cases, we do not see the nebulosities that we would expect to see surrounding quasars in the context of the current hypothesis of quasars being nuclei of galaxies. Most astronomers believe that those "naked quasars" are simply nuclei of relatively faint galaxies, and we simply haven't detected them with the most sensitive currently available instruments such as the cameras onboard of the Hubble Space Telescope. This is an area of intense study, and no consensus has been reached as yet. Needless to say, the discovery of an alternative answer would have an enormous payoff: abolishing the current theory of nature of quasars would lead to fame (but perhaps not fortune, as astronomers generally are not paid very high salaries...). Of course it is possible that the central region of the host galaxy formed stars first, and the more remote regions have much lower star formation rate... (the nebulosities we see are due to stars).

Greg Madejski and Damian Audley
for Ask an Astrophysicist.

Question ID: 980216b

The Question

Do you truly believe that red-shifts can be utilized as a valid means of distance indication, and if so, on what grounds? I am encountering an increasing number of individuals who claim that large red-shifts do not occur in the spectra of quasars (though a few maintain that small red-shifts (z < 2x10^-3) could be present in the spectra)

The Answer

Regarding the redshift issue: in general, the galaxies with higher redshifts are fainter and subtend smaller angular sizes on the sky, so for galaxies, the redshift - distance relationship is reasonably well established. Regarding quasars, there is a rather substantial number of them that have redshifts determined from the absorption lines -- and these are similar to the redshifts measured from the emission lines normally present in quasars. These lines arise from atmospheres of stars in the host galaxy. Those often can be measured either when the quasar is faint (many quasars vary by quite a bit, in some cases by as much as a factor of 100!), or via a study of the spectrum with the central object masked off to reduce the "swamping" of the spectrum of the host galaxy by the intense light of the quasar. In some cases, we have absorption redshifts available up to z ~0.25. We also have a few cases of quasars residing behind intervening galaxies, and the gas in these intervening galaxies imprints an absorption redshift on the quasar spectrum. A well-known example is AO 0235+164; strictly speaking it is a BL Lac object, but this means that the emission lines are very weak, but still measurable at z = 0.94. The spectrum shows an absorption redshift at z=0.524, which is due to a clearly identifiable intervening galaxy. The quasar then must be behind the intervening galaxy!

It is important to note that the cosmological redshift of quasars is not believed by everyone, and in some cases, for good reasons. If you want to follow up on this debate, look for articles by Drs. H. Arp, or G. Burbidge.

Greg Madejski and Damian Audley
for Ask an Astrophysicist.

Question ID: 980216c

The Question

How long is a Quasar's life and what happens when it dies?

The Answer

The question you asked about quasar lifetimes is an excellent one, but we only know this on the basis of theoretical arguments that are consistent with observational data; all quasars ever discovered (the first discovery was about 35 years ago) are still "there," so this is the only truly undisputed observational measurement of their lifetime. Let me describe what we know about quasars, and from this, I will give you some arguments for their lifetimes.

In general, quasars are relatively bright point sources; we believe that they are centers, or "nuclei" of galaxies. They show large redshifts, meaning that they are moving away from us at large velocities. In the currently accepted scenario that the universe is expanding such that the velocity of expansion is roughly proportional to the distance from us to an object, this means that quasars are very distant objects, located often 1/2 way to the edge of the visible Universe. Two points are important here:

First, since quasars are relatively bright, yet very distant, so intrinsically, they must be extremely luminous - perhaps a thousand times more luminous than all stars in a galaxy put together. Yet this tremendous power has to arise in a region that is comparable in size to the Solar system, and we know this from the fact that their brightness varies on a relatively short time scales. Since no object can be larger than the distance light can travel over a time during which the object changes its brightness by, say, a factor of two - this implies that their tremendous light output arises in a relatively small volume. We believe that the best scenario is that quasars are powered by an infall of matter onto a very massive black hole, having a mass as large as a million to 100 million Suns.

However, there is only so much matter per unit time that a black hole can "swallow" - this is typically, for a 1 - 10 million solar mass black hole, about one solar mass per year. So, to the first order, lifetime of a quasar has to be at least one to ten million years.

Second point has to do with the number density of quasars as a function of their distance. At large distances - say, half-way to the edge of the Universe - there are many more quasars than we see at our local neighborhood. However, since light travels with finite speed, we observe these objects when there were about 1/2 as old as we are now. What happened to those quasars? Let's for the moment take the age of the Universe (meaning the time elapsed from the Big bang) to be 12 billion years. At the distance corresponding to the time of roughly 9 billion years ago, we already see many galaxies and quasars, and we infer that the masses of their black holes are about 1-10 million solar masses. What happened to them? To account for those quasars of the past, we should have a lot of rather anonymous local galaxies harboring very massive but inactive black holes in their center.

It turns out that recent theoretical work implies that for a given number or grams of matter falling onto a black hole per second, the efficiency of conversion of the gravitational energy to radiation (light) drops as the mass of a black hole increases, and eventually, the quasar gets fainter and fainter even with the same rate of mass accretion. So, this can account for local quasars being dim. In fact, very recent work on the basis of the Hubble Space Telescope clearly indicates that there are many local galaxies (some researchers think as many as 50% of all galaxies!) have "quiet" black holes in their centers. This implies that the quasar phase is probably shorter than a few billion years.

This gives you a general idea: we don't know, but today's best estimates are that the quasar - meaning a luminous phase of an accreting supermassive black hole - probably lasts for a time span of 10 million to a few billion years.

Greg Madejski for Ask an Astrophysicist

Question ID: 980301a

The Question

Why is it that active galaxies today are substantially less luminous than high redshift quasars (their presumed progenitor)?

The Answer

This is a good question, and one which is at the frontier of current research. So there is no definitive answer. It is possible that the apparent shortage of high luminosity nearby agns is an artifact of incomplete observations, and that more sensitive searches could reveal more low luminosity objects at high redshift. More likely is that the high luminosity phase is relatively short-lived, and that many low redshift galaxies harbor dormant AGN. The AGN phenomena may be regulated by the supply of gas from the galaxy to a massive black hole at the center, and a relatively uncommon and violent event (such as a collision between galaxies or a burst of star formation) is required in order to provide sufficient fuel.

Unlike most other objects massive black holes can't be destroyed, and at some level should be detectable through their gravitational influence on the stars in their host galaxies. As telescope technology improves these searches are becoming more sensitive, and more evidence for massive compact objects in otherwise inactive galaxies is emerging.

I hope this helps.

Tim Kallman
for Ask an Astrophysicist

Question ID: 981112a

The Question

I am somewhat acquainted with astronomy and relativity. My question is this: quasar 3C273 has a jet moving at 9 times light speed as seen from earth. This has been explained away by science by saying that the angle of the jet towards earth is just small enough to actually only give an illusion of faster than light speeds relative to earth.

However there must be a counter-jet and I have been told by an astronomer from Flagstaff, AZ that this is true. The existence of a counter-jet showing the same relative speeds would surely obviate the 'small angle' explanation and would then prove that faster-than-light velocities do exist in the universe relative to earth. This, then would indicate that relativity is wrong in its assertion that FTL relative to earth cannot be. Could you please tell me where I'm wrong. It is a major stumbling block in my acceptance of relativity. Thank you very much and could you, if possible, indicate who is answering if I should have a follow-up question. Again, thank you.

The Answer

I don't know offhand whether the counterjet for 3C273 has been seen.

implies that it hasn't. It would be expected to be dimmer (since radiation tends to be beamed in the direction of motion) and not show apparent superluminal motion.

You are right that IF a superluminal counterjet were seen, that would mean that something is very wrong with our understanding of the system or of relativity.

I do know that the the 'microquasar' in our galaxy 1915+105, shows both jets, and the one pointed towards us is apparently-superluminal while the one pointed away is not.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

Question ID: 981228a

Other Active Galaxies

The Question

I have absolutely no experience in astrophysics but a friend of mine did a a thesis on agn. She could not explain it to me because she is Polish and I am an American. so my question comes in two parts. What is AGN exactly? What implications does it have for understanding how the universe works?

The Answer

An AGN is short for "Active Galactic Nucleus." Some galaxies have nuclei (centers) that are 'active', meaning they emit large amounts of radiation (radio, optical, X-rays, gamma-rays, particle jets, etc.), and/or are highly variable. (For example, a galactic nucleus starting at 30 billion times as bright as the Sun, then growing to 45 billion times as bright as the Sun in just half an hour).

Since they vary so rapidly, the important region must be small, no larger than the inner solar system (since the time over which something can vary is limited to the time it takes light to get from one side to the other). Since they are so bright, that small region must have unbelievable energies in it.

Gigantic black holes, billions of times as massive as the Sun, swallowing stars and gas clouds, are the only reasonable theories that seem to fit the data.

Do a search on our website for 'active' to find more information

David Palmer
for Ask an Astrophysicist

Question ID: 980608a

The Question

I am a high school student and I am interested in this subject. Why do some galaxies have an active nucleus and others don`t?

The Answer

According to current theories, a galactic nucleus is active if it has a large black hole which is consuming large quantities of matter.

If a nucleus doesn't have a large black hole, then it is not an agn. If it has a black hole, but no stars etc. are falling into it, then it is also not an AGN. If all of the stars that had been in orbits that get very near the black hole have already been eaten, then the remaining stars are safe, so that nucleus is no longer active.

David Palmer
for Ask an Astrophysicist

Question ID: 980124a

The Question

I don't know very much about astronomy but I have to do I paper on it for school. I was very confused at your info on Blazars. I would like a simple answer to: What are blazers? Thanks!

The Answer

This is a good question, and it might be good to divide it up into 2 parts: first, what does a blazar look like to astronomers, and second, what is actually going on in a blazar? It turns out that astronomers themselves can't even answer the first question with certainty. I would say that in order to be called a blazar, an object must have the following characteristics:

1) It must appear point-like on the sky, i.e. not appear fuzzy like a galaxy or a nebula. Some blazars have nebulae around them, but most of the light comes from a point source.

2) Their spectra appear to be smooth (i.e. no strong absorption lines that a star might have) and flatter than a star. These 2 properties by themselves would make them a quasar.

3) Their visible light is often partially polarized.

4) Their output in all wavelength bands varies more rapidly and by a larger amount than a quasar.

Now, what is going on? As the website says, possibly a jet of material coming from near a black hole, with the gas in the jet moving very nearly the speed of light and coming nearly straight toward us.

I hope this helps,

Tim Kallman
for the Ask an Astrophysicist team

Question ID: 980116b

The Question

Your page on active galaxies says that quasars are agns viewed down the jet. How does this reconcile with images of 3C273 which clearly show the jet shooting off to the side?

The Answer

I'm not sure what page you are referring to specifically. Generally, blazars are the quasars that are viewed jet on, while quasars are very bright and compact AGN. So a quasar such as 3C273 which has a visible jet is not a blazar.

It is important to remember that in the case of AGN (and to a certain degree astronomy in general), naming was often done based on observed qualities rather than an understanding of the physical nature of things (which was only understood later) so some naming schemes aren't perfect or particularly clear.

John and Mike
for "Ask an Astrophysicist"

Question ID: 100706a

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