How do we know how many galaxies are in the Universe?

"The human mind is not capable of grasping the Universe. We are like a little child entering a huge library. The walls are covered to the ceilings with books in many different tongues. The child knows that someone must have written these books. It does not know who or how. It does not understand the languages in which they are written. But the child notes a definite plan in the arrangement of the books---a mysterious order which it does not comprehend, but only dimly suspects."

-Albert Einstein

Earlier today, I had the pleasure of visiting a high school astronomy class via Skype, answering some very good questions from some very enthusiastic and curious students.

But there was one question that we ran out of time for.

Image credit: 2P2 Team, WFI, MPG/ESO 2.2-m Telescope, La Silla, ESO.

How do we know how many galaxies are in the Universe?
While that's a great topic for a blog post, I thought it would be an even better topic for an animated video! So for anyone's high school astronomy class, or anyone with five minutes to spare, hope you enjoy it! (And don't hesitate to full-screen it.)

For those of you curious about the original sources of these remarkable images and videos used in the creation of this, I couldn't resist sharing these amazing resources with you from around the world, in order of appearance.

Video of Milky Way rise, by William Castleman.

Image of the southern Milky Way, by A. Fujii.

Image of the Orion Nebula, by Ioannidis Panos.

Video of a flythrough from Earth to the Virgo Cluster, by R. Brent Tully.

Image of the Milky Way, by Derek Rowley.

Image of Hickson 68, wide-field, by Gimmi Ratto.

Image of NGC 7331 by Paul Mortfield and Dietmar Kupke/Flynn Haase/NOAO/AURA/NSF.

Image of Stephan's Quintet of Galaxies, by Adam Block/NOAO/AURA/NSF.

Image of the star field near M31, retrieved from

Deep image of distant galaxies with foreground stars, retrieved from

And all the other images and videos are derived from these two fabulous data sets:

Images courtesy of R. Williams (STScI), NASA and the Hubble Deep Field Team.

And perhaps most spectacularly...

Images and videos courtesy of NASA, ESA, S. Beckwith (STScI) and the HUDF Team.

And here we are, less than a century after learning that the Milky Way wasn't the only galaxy in the Universe, we now know that there are at least 100 billion of them, and possibly even more than that!

More like this

I know I can't fathom the universe & its billions of galaxies & the vastness of each galaxy & the billions of stars in each galaxy & the billions of planets accomanying these stars and the potential for life on these planets in all these galaxies. But it is the single MOST intriguing THOUGHT and reality for ANY human!!! I am fascinated & overwhelmed by the wonders in the KNOWN universe & the beyond. How do the space experts wrap their minds around the fact that given the VASTNESS of the universe....when we had the beginning of the universe (the big bang) from the size of a "pea" & billions of years later we have what we have now. What was before the universe....what did it expand into? In other words what has it REPLACED!! Science says nothing was before....but if it was nothing then how has nothing become so vast to "HOLD" the universe ...."something" had to be there first.

By Mike Jackson (not verified) on 15 May 2012 #permalink

Thank you Ethan.

Uhh, ditto.

...Do I win a lamest post ever award?

Nicely done.

By NewEnglandBob (not verified) on 13 Oct 2011 #permalink

Could you also estimate the count by looking at the count of 'nearby' galaxies, and scaling the volume to the volume of the universe?

I suspect it would have a different type of error bar.

By MobiusKlein (not verified) on 13 Oct 2011 #permalink

In the photo of the Orion Nebula, there's what looks like a sort of conical shape with a light in the middle. Is this an illusion, or is it really shaped like that?

Thanks!! Truly you capture the immensity and grandeur of space. Then you can extrapolate and are overwhelmed....

Do we stand any chance of ever seeing beyond the re-ionization period of the universe? Are there any ideas out there that could potentially allow us access to the 1st 400,000 yrs (aka the dark ages), or will that chapter in our 13.4 billion year history remain 'dark' & forever be locked away from our prying eyes?

I am confused:

a) If we are looking at galaxies from the early universe, then they should be from a much smaller universe. How can these be uniformly distributed from our viewpoint?

b) Since we are viewing very old light, how can we know how many galaxies there are NOW?

@ #8: I think rockyjoe's question "a" regarding our viewpoint from Earth might be related to a paradox that baffled me wonderfully for many years: How is it that our sightlines into space can diverge forever but also converge to a point at the Big Bang?

Robert Osserman's lovely little book, Poetry of the Universe, finally showed me the light, so to speak. Osserman uses a simple three-dimensional analogy that enabled even lil' ol' math-challenged me to (sorta) visualize the path of light in four-dimensional spacetime. If you were a two dimensional critter living on a polka-dot planet on the surface of a spherical universe, and if light in your universe always followed the shortest path, i.e. a great circle, then your sightlines would diverge in all directions before converging at the point opposite your position on the sphere. But in reality, we are 3-D folk living on the 3-D surface of a curved 4-D space-time universe (a hypersphere if the curvature is flat, yes?) Light travels along the shortest path in spacetime, so our sightlines back toward the light sources must diverge in all directions.

My head hurts from the effort of trying to wrap it around this concept. Ethan, could you please do a post on this sometime and help ease the pain?

A very beautiful and insightful video... and a John Fahey soundtrack. Wow.
Thank you.

Stan P. Thanks for the reply. I'm not math challenged, so I can understand the (Friedmann) equations, but the concepts are still confusing to me. I'm not sure I buy the spherical shell argument -- as I think our universe is supposed to be pretty flat. The best I can do is to think of our view of the universe as being limited by the "fog" of the blackbody radiation, or more critically by the age of the universe itself -- and that we are far enough inside the universe that anything we can see looks uniform.

It is marvelous and amazing what a few billion bucks and good scientific theorizing can bring to us. Yet the question is more profound than the "chicken and the egg" theory. for instance, how did we get a picture of the Milky Way which itself is thousands of light years in (chose from the following 1. length, 2. width, 3. Height, 4. "the ends of being and ideal grace".) By our own admission, our own spacecraft launched some forty years ago has only recently "left' our solar system.

Another similar question springs from a more recent picture of the universe which showed that 'it' was flat, just like the earth used to be. (I theorize that it was taken from the same theoretical camera which was used to take a theroretical picture of the Milky Way

So in conclusion let me say in paraphrasing Einstein's recently republished picture when there was some new evidence that there is a particle which travels faster than light, "I'm right, at least in theory!"

Compound suppositions X 10 nth produce a universe which is expanding faster than we will ever be able to see. I suggest that we start with a smaller problem, counting the grains of sand in the shifting Sahara. When we are finished we will know that a grain of sand is itself a galaxy (in theory at least)

By Kenneth L Rothey (not verified) on 15 Oct 2011 #permalink

For a schoolchild, the best possible answer is "we don't". Time learn about what knowledge is and what it isn't.

By Ian Kemmish (not verified) on 17 Oct 2011 #permalink

i'm feeling myself so little right now,we are infact nothing but a dust in the universe!!blessed God who is the greatest....

how selfish is mankind that thinks is the master of the universe!!we are nothing but a dust in the universe!blessed God who is the greatest...

@7 There are infra-red telescopes capable of lifting the veil to some extent. But we'll never be able to look back to the "moment of creation."

@8 From what I understand, there really is no moment of "NOW" holding the universe together in one simultaneous moment of time. Simultaneity (sp?) is one of the casualties of the Einstein Revolution. That said, by observing the relative velocities of distant galaxies, you might be able to figure out which ones merged and eventually formed elliptical galaxies, etc., so you could have a better idea of whether the overall number of galaxies approaching the present moment has grown or declined.

Beyond which, keep in mind that Ethan is only talking about numbers of galaxies in the OBSERVABLE universe. If the UNOBSERVABLE universe is as huge as some cosmologists suspect it is (the Observable Universe being comparable to a proton divided by the Observable Universe, in size, relative to the Unobservable Universe), then you could cover the surface of the earth with googleplex exponential zeros and still not have a large enough number.

By Jack Dawe (not verified) on 23 Oct 2011 #permalink

As computing power seems to double every few years so does the estimate of the number of galaxies in the universe. I predict that by the year 2045 that number will reach one trillion.

If there is only one or two of some object in a trillion(s) does that make it less or more significant? What if there were 8/8 earths in our solar system instead of 1/8 earths, would that make earth more or less significant? What if there were just fish on those other earths, or what if they were full of billions and billions of other "earthlings". Would that make the people on earth #3, less or more significant? It's kind of creepy how we're all alone next to big gaseous planets on a tiny little planet....with no-where with air to fly to.......(and an asteroid coming in 800 years!).

Nicely done Ethan...the more this type of incredulous information on the vastness of the universe gets out to the general public, and in particular, to those that look to the stories of the Bible as to where it all started, the more they will convince themselves that it couldn't possible have been created in the 6,000 yrs that the Bible tells us, but those stories where created by our ancient, and superstitious ancestors, who were looking for some answers, and hope for what happens to us in the afterlife...why is it that so many of the most brilliant and respected people of all time were non-believers of any religion, not just Christianity...Einstien, Hemingway, Frank Lloyd Wright, Sigmund Freud, Susan B. Anthony, Issac Asimov etc...even some of our founding fathers were non believers...Thomas Jefferson and Ben Franklin, to name just a few. But I would never suggest to the faithful to give up their hope of meeting their loved ones in the afterlife...if they need to believe that to get through this life, then it would be cruel to try to take that faith and hope away...

By skipperblue1 (not verified) on 02 Feb 2012 #permalink

We look back in time and space and have ascertained that the the big bang occurred c13.7billion years ago. As light travels at a constant speed presumably we know how far that matter has traveled forward? ( which would give the size of the universe) or is the 13.7billion only relative to the time the light reaches earth? in which case I don't understand how we have any clue to how large the universe is.

Universe is Expanding 72 kilometers per second, in such a case the center point of the Big bang explosion can easily be measured if we do it as fast as the universe growth. It seems to be endless and God has no Origin and End.

By AG SRINIVASASIV (not verified) on 15 Nov 2012 #permalink

There is no centre. The expansion is universal.

God doens't exist. That precludes ending and origin both, so to an extent, you're correct.

I don't remember reading anything in the bible or quran about all of this! can someone correct me? where does god mention the creation of all these universes and worlds.. the billions of planets and stars.. and who knows how many 'worlds' and lives are there among all these galaxies... yet the so called god was obviously unaware of all of this vastness.. and only mentioned what Man could already see and know back then, i.e the sun, the moon, etc etc how many discoveries do we need so that religious people can finally start doubting and abandoning their religions?? People still believe that with ALL that endless space around our planet, 'god' still forgot about everything in order to help Mary conceive!!

as with all theological questions and as is written in our constitution, religion and intelligence, ie science, are to be treated as separate and equal (or something like that)

are to be treated as separate and equal (or something like that)

No, nothing like that.

They are orthogonal and unrelated.

Science is about the world we live in.

Religion is about fairy tales that make us think or behave in a certain way.

Science CAN be taught in US schools.

Religion CAN NOT be taught in US schools.

Science DOES NOT get tax breaks.

Religion DOES get tax breaks.

Just a few of the differences.

how many discoveries do we need so that religious people can finally start doubting and abandoning their religions??

It's happening quite quickly.

People are not requiring religion to tell them how things work, how to avoid disaster, how things happen.

Religion is becoming little more than a social club (Which was probably how it started out too).

This is why there are so many attempts to either ban science or get religion into schools again. Religion requires an extreme suspension of disbelief to get in to someone's head, and that requires an unformed mind.

zak and wow,

I think religion's primary purpose, at least for most religious people, really has nothing to do with explaining the world anymore. Science pretty much has that covered. The real purpose of religion is fear of death. Death is inevitable. Religion gives its believers the comfort of being able to believe that they won't really die, but will continue to live on after physical death. Science, based on rationality and evidence, will never be able to give this comfort, which is most likely based on nothing more than wishful thinking.

Many are "socially XXXian". Be it Jewish, RCC, Methodist, Mormon or Moonie.

If they were REALLY worried about the afterlife, they'd be a lot more assiduous about keeping on the Good Word's good side.

Except they don't.

They will ignore the teachings if doing so means they live in THIS life more comfortably.

Look at the Xtians who decry "Welfare Scroungers" despite JC's story documenting his insistence that if there is someone who needs it, you must give them the shirt of your back, and you will be rewarded with life ever after.

Or those getting up in public and proclaiming their holy richteousness, despite the word being that these people who pray loudly in public are not His followers, but that those who Pray in private are heard by Him Who Is Everywhere and will be rewarded.

It's truly amazing how people can believe it all started from the Big Bang with a molecule ,but can't conceive the idea of god,at least believing in god there is hope..there is a grand design,to believe that trillions of stars ,and billions of galaxies were created from a single atom is far more ludicrous than believing in god...

@ Dan

"It’s truly amazing how people can believe it all started from the Big Bang with a molecule"
- well, maybe you believe that. No one here believes it all started from a molecule. lolz...

"but can’t conceive the idea of god"
- can conceive the idea. Choose to trust the data and evidence

"at least believing in god there is hope"
- substitute god with any other object and apply faith.. results are the same. So, indulge me, and worship your toothbrush for a week and see what happens. Confess to it, pray to it, adore it.. and see what happens. I guarantee you will find your toothbrush just as powerful as god.

" is far more ludicrous than believing in god…"
- yeah.. and a concept of an omnipotent god needing humans as a sort of an aquarium to look at when bored is truly sane...

By Sinisa Lazarek (not verified) on 17 Feb 2014 #permalink


"at least believing in god there is hope"
Actually, this sentence of yours makes me sad. Sad because you see no hope in life without god. Without that father figure you believe your existance to be futile. I feel very sorry for you. You do need organized religion in order to tell you how to live your life. But not all do. Homo sapiens is not for everyone. To look at the world with your own eyes and realize you are all there is, and ever will be. No god, no nothing, only you and what you make of it. And be happy, because it's the ultimate sandbox.

By Sinisa Lazarek (not verified) on 17 Feb 2014 #permalink

Demon Haunted World by Carl Sagan.

The absolute guide to logical thinking.

Not things being so because we wish it.

Should be required reading, sadly, so many
adults cannot wrap their mind around the truth.

Science has theory after theory fail. Grasping at latest to "prove" self-creation. Logic knows an unknown can't mysteriously appear without scientific explanation. WHAT IS THE ORIGIN OF THAT DENSE OBJECT???

Ah, so letting evidence change your mind is a weakness, only firm and unshakeable belief is allowed.

ISIS are ABSOLUTELY CERTAIN that God wants them to create a caliphate and convert all heathen non-believers to their Islam.

I guess you applaud that sort of unshakeable faith, hmm?


Yo mamma.

Regardless of the pro god anti god arguments,i for one remain convinced that "somewhere" among the billions of known galaxys,there are other "intelligent beings" of indeterminate shape/size/makeup...or whatever else we can immagine,probably thinking exactly what "us earthlings" are thinking,namely are we alone,or are there countless numbers of other civilisations....out there?????

"Regardless of the pro god anti god arguments"

Well what does that have to do with aliens?

Bugger all?


So why "Regardless" and why bring it up in the first place? It's as odd as saying "Regardless of the pro and anti god arguments I think bacon sandwiches are really tasty if done well."

It doesn't really have a point in being said.

(there aren't any anti god arguments: you can't be anti something exists, only pro reality, and the pro is basically "I believe I'm right, despite all reality being against me" which really isn't any argument for a god at all, only the ego and limitations of the one making the argument)

Just noticed this:

"In the photo of the Orion Nebula, there’s what looks like a sort of conical shape with a light in the middle. Is this an illusion, or is it really shaped like that?"

Pity I missed this. Too late to tell them that this is an illusion caused by a dark cloud of dust interrupting the light from the excited gas of the newborn stars in the Orion Nebula (which is actually a lot bigger than the constellation, though most of that not in it) causing that dark funnel, known as "the Fish's Mouth where I live". And the bright dots are the bright new stars we can start to see, known as the Trapezium. A small telescope can separate them into their four components. A bigger one can resolve them into five or more.

Think that Wow miss,s the point i was trying to make,namely,as were all interested in the same subject,dos,ent logic dictate that there simply has to be alien life "out there" someplace?..Carl Sagan said something along those lines,and subsequently other scientists have also said the same sort of thing,..its for sure that one day,we will know for sure.............

"Think that Wow miss,s the point i was trying to make"

And that would be because you didn't make that point.

Your VERY FIST SENTENCE had fuck all to do with aliens.

It merely had you blathering on about god. If you really were talking without regard to god, why the hell point it you?

I feel you missed MY point *entirely*. And unlike you, I didn't mix multiple ones on.

If your point was about aliens, then why didn't you just make that point? If your point was nothing about god, why did you mention it? And lastly, if you only had one point, why did you make two?

@ WOW #30
if this character is ' everywhere ', he must be better than ol' St. Nick. It would seem 'he' can travel faster than light, too. Man, what a feat !!

Wow the Heathen's ignorance on display AGAIN:
Vince is referencing God spoken about in this thread as a simple polite digressive departing to a topic he prefers.

By Ragtag Media (not verified) on 28 May 2015 #permalink

Wow the Heathen’s ignorance on display AGAIN

Nah, ignorance is believing crap like your mythology.

You can depart a topic *by not entering the topic at all*.

So it's entirely INCONSISTENT to enter a topic just so you can depart.

And it still doesn't absolve Vince of not being able to count past one. Or expecting others not to be able to.

PJ, did you mean a different comment?

But it's indicative of the lack of thought. If "He" is everywhere, then "He" doesn't move. He's there already.

It's also odd that nowadays it's accepted that there is no other god (tm) but that it's a "He".

Of course, ORIGINALLY, it was well known that "He" was a he because *he* had a wife. She was excised mostly, and most interpretations of the ancient myths nowadays only sideways refer to her (or, indeed, any other god, originally it was accepted that there were MANY gods, and this was just the god of the Hebrews, coeval with all the other gods of the pantheon of the time).

Yes, that's right, the sexism was a later invention of Christianity.

Also gives the lie to nutbag's "ignorance" claim...

Those who know most about the actual bible are least likely to believe it. Even those who are most religious: priests and "sophisticated theists" have the most knowledge of the bible and the least belief in it.

That wasn't derogating your comment, PJ, the ignorance of the religious that take the myth seriously.

C,Mon Wow,lighten up!...point 1,there is no "god"..period!..point 2,..With an estimated 100 billion known galaxeys,..dont YOU think there has to be "other life"???..point.3..using profanity sure dosent help YOUR argument now does it,....was,t aiming to make an electronic slanging match,..just trying to make a point,or ok,if you so wish..two points,...THREE if you include this blog!

Since this god thing is supposed to be everywhere, and, we are supposed to be made in his (its) image, the thought processes must then be the same. Imagine the brain of it spread all over the universe - it would take a long time to make any decision. Since it does not exist, it does not matter. ;<)

"C,Mon Wow,lighten up!…point 1,there is no “god”..period!."

Then why did you bring up "regardless"????

And why is it MY problem you couldn't count to two, which would be fixed by my "lightening up"? Indeed, why would it do anything?

How about instead of me lightening up, you look at what you did and go "Yeah, I guess I didn't have to add that in, since I really didn't WANT to put my penneth in."? Doesn't mean you have to admit to ME (or anyone else) you made a mistake. All that it means is you've learned that if you really don't want to make a point on a subject, don't make a point on that subject.

You know, learn.

It's rather amusing watching you pagan arsonist run around putting out God fires you start.

By Ragtag Media (not verified) on 29 May 2015 #permalink

Ah, so your imagination removes all you godbotherers starting this shit, then, right?

See post #15

Yours is typical fundie moron lying.

Why you think it could possibly work is yet another sign of the mental crippling that religion confers to humans.

Non Sequitur you idiot dolt. Why not just say "See Emperor Constantine" and avoid the fact that Vince lit an imaginary match you created in your own head to satisfy your own miniscule self aggrandizing purpose.

By Ragtag Media (not verified) on 29 May 2015 #permalink

No, it's completely relevant, retard.

Your claim "we" started it is complete bunk.

Either you knew or you didn't care (or you knew and didn't care).

But you heard of non-sequitur and therefore wanted to use it. Find out what it means before you try again.

Nope, an athy chasing another athy over a false assumption.
Your fault not mine... Nice try though.

By Ragtag Media (not verified) on 29 May 2015 #permalink

Nope, you were wrong.

If you accepted mistakes, this wouldn't be a problem, but you don't.

You flail instead. It isn't working.

It was started by a fellow godbotherer.

As usual.

Vince: "Regardless of the pro god anti god arguments,i for one remain convinced that “somewhere” among the billions of known galaxys,there are other “intelligent beings”"

wowzer: "Well what does that have to do with aliens?."

Vince: "Think that Wow miss,s the point"

Why, Because wowzer next said: "Your VERY FIST SENTENCE had fuck all to do with aliens."

WOWzer is now a lying sack of shit, because he already knew what Vince meant.
Right there you from the start you willfully FALSELY misrepresented what Vince was talking about (FIRE..FIRE..).
So you could in-turn pounce to snuff out your own false assumption thus stroking your own delusion of grandeur.

By Ragtag Media (not verified) on 30 May 2015 #permalink

Rag: "you pagan arsonist run around putting out God fires you start."

Rag later: "I totally didn't say you started it!"

Because a fellow godbotherer was the cause.

Seems like wow is getting mighty steamed up about every comment,anyone makes,let alone me!
..You really have to chill out wow-zer,...regroup your thought process,quit insulting anyone who dares to have an opinion contrary to yours,and just getta life!

Zippo from wow??...gues he musta been swallowed up by a black hole?? endeth the lesson!

Yes, vinnie, i don't have to answer you.

I guess that you lost the case and therefore you're looking for a sop to your conscience.

And no, I still haven't bothered reading your two posts there apart form the opener on the above, which was merely a whinge.

OK, so I read them.

Now what? There's bugger all in there. Your claim I should "chill" is nothing other than passive-aggressive BS, merely stated to pretend that I'm illogical and emotion-driven therefore you have nothing to answer for, since you can "safely" dump all your problems on me.

In what way was that supposed to work?

Does it change your errors on iota? Nope.

Does it make me less correct? Nope.

All it did was make you feel better. Big fucking whoop.


You state (to paraphrase) that with all the vast number of galaxies and presumably planets available, that there just has to be life somewhere out there. Well, the answer is no, there doesn't just have to be life out there. What is the probability of life developing on a given planet? We have no idea. We can speculate and take some guesses, but we don't really know the full mechanism of how life arose on earth, so all we can do is make guesses. It is logically possible that some special, unique condition existed on earth that allowed life to develop.

Consider an analogous argument: There are an infinite number of natural numbers. Doesn't there just have to be another even prime number other than 2? Of course not! The property of being an even prime is a unique property. Perhaps there is some property of earth that is similarly unique.

Personally, I believe that there is life out there. However, that's just a personal feeling, and I have no evidence to back it up. Without that evidence, then, there's no basis for claiming that it's not possible for the universe other than earth to be empty of life.

Hi Sean.T,...Yep,agree your final apargraph,because,like you,its just a personal point of view,..both of us may be waaaay off base,or perhaps not?...not something thats has a definitive answer,but a good post ner the less by you....think you may be a kindred spirit?

With regard to wow-ser,..really cant be bothered with you any more kid,.anyone who has to emphasise a comment by profanity,which you obviously think a coll thing to do,is quite frankly,a waste of space,..but not to worry,junior school will be back in bizz after the summer,so apply now,and they might just let you join.......but im not holding my breath,..and nor is anyone else i suspect.

Stop spitting on the bible when you know not what your talking about. In the beggining, earth was already here, space was already here, who knows how long before God decided to take form on the earth. The sun and the moon were created after land was formed. But the earth was here already. It was just a planet with water in it. No mountains, nothing. So it took 6 days to form earth, but earth is much older than that.