PDA

View Full Version : How much empty space?



One Skunk Todd
2007-Sep-14, 08:24 PM
How much empty space is there in a galaxy like the Milky Way? If you could somehow line up stars and other objects so that they were just touching, how much of the rest of the matter in the universe (defined as stars and solid bodies) could you squeeze into the volume of our galaxy?

Saluki
2007-Sep-14, 09:00 PM
The short answer is lots.

The longer answer is that if you squeezed that much matter that close together, you would end up with some pretty interesting physics going on.

The very long answer (based on some very rough back of the envelope calculations) is that all the matter of the galaxy would easily fit within the volume of a small fraction of a cubic light year.

Since the Milky Way is roughly 100,000 light years by 10,000 light years, or about 1 million cubic light years in volume, you could fit many millions of galaxys in the space occupied by our galaxy.

Some assumptions I used:

1. Sol has a radius of 7x10^8 meters. I assumed this for an average volume. The actual average stellar volume is probably somewhat larger than this, but it would not make much difference given the roughness of this estimate.

2. There are roughly 100 billion stars in the Milky way. There are many estimates, but this seems to be in the ballpark of a consensus.

3. Density magically does not increase as you pack all this matter so close together.

4. The volume taken up by planets, asteroids, moons, comets, etc is negligible compared to the volume of the stars.

Cougar
2007-Sep-14, 09:21 PM
How much empty space is there in a galaxy like the Milky Way? If you could somehow line up stars and other objects so that they were just touching, how much of the rest of the matter in the universe (defined as stars and solid bodies) could you squeeze into the volume of our galaxy?
I don't quite understand your question, and probably wouldn't know the answer anyway. But I do know that when you compare the diameter of even a large star to the (average) distance to its closest neighbor within a galaxy, the star's diameter is next to negligible! That's why they say that when two galaxies collide, the odds of any stars actually colliding are about a billion to one. Not very likely!

astromark
2007-Sep-15, 04:35 AM
From all this, the point being that space is mostly empty. The distances between stars and galaxies is so great the most of the known universe could fit between Andromeda and the milky way..., but, you would need to get right out of the way. That amount of mater all being so cuddly would become an enormous gravity well and an extremely violent place. The dynamic effect of that would be the mother of all black holes.... or the biggest bang you can amagine... where have I seen this idea befor....?

EDG
2007-Sep-15, 05:54 AM
1. Sol has a radius of 7x10^8 meters. I assumed this for an average volume. The actual average stellar volume is probably somewhat larger than this, but it would not make much difference given the roughness of this estimate.

I think the average stellar volume should be smaller actually. The sun is a G V star, which is not an average star - about 70% of stars are M V, which have a radii between 0.3 and 0.4 Sols. There's also more K V stars than G V stars... so a better estimate for the average radius of a star is more like 0.5-0.6 Solar radii.

If we call it 0.6 radii, then the average radius is about 417,750 m, so the average volume is about 3e26 cubic metres. So if you crammed all 100 billion stars together (multiply that by 1e11), you get 3e37 cubic metres. Take the cube root and you get a volume that is about 3.12e12 metres on each side.

That works out as a cube of about 21 AU per side! Though that's gotta be way inaccurate, since a red supergiant star like Antares or Betelgeuse has a radius of about 3-5 AU on its own. So really I guess I'm assuming that all 100 billion stars are main sequence stars (which they're not, and the ones that aren't are going to push up that average a lot).

Still, assuming I've not made a stupid mathematical error anywhere, it seems that 100 billion stars are equivalent to a pretty tiny volume when you get rid of the space between them. Even if we're conservative and double it to about 40 AU, that's still a cube that's only about the size of our solar system! (I often hear that quasars have a galaxy's worth of energy output crammed into a volume the size of a solar system, so this kinda makes that sound reasonable!!).

A lightyear is about 63,000 AU, so a cubic lightyear is about 2.5e14 cubic AU. If our galaxy can be crammed into a cube 40x40x40 AU then that's a volume of 64,000 cubic AU. So that means we can fit about 4 billion of those cubes into one cubic lightyear - and our galaxy is generally considered to be a pretty hefty one, so your average galaxy probably can be crammed into a smaller volume.

Still... if we assume that there are 500 billion galaxies in the universe (generally considered to be a high estimate, I think), then we should be able cram the entire universe into a volume of about 125 cubic lightyears. Which is pretty tiny on the grand scale...

EvilEye
2007-Sep-15, 05:35 PM
I think their question was more simple than that.

Imagine the entire galaxy was spread over your dinner table, and you just pushed all of the "stuff" together into one clump... How much empty table would you have?

EDG
2007-Sep-15, 05:49 PM
That's basically what I just calculated. Though the volume of the milky way is actually about 3e14 cubic lightyears (treating it as a cylinder of radius 100,000 ly and height 10,000 ly) not one million cubic lightyears as stated earlier.

Even if you assumed that the entire galaxy could be crammed into a cube that is 1 ly on each side (which is vastly larger than the 40x40x40 AU cube that I calculated before), you'd still be able to fit the entire compacted universe into it easily.

Or to use your dinner table analogy, the compacted version would probably be sub-microscopic on that scale.

m1omg
2007-Sep-15, 07:03 PM
How big a cube full of at water at room temperature (density 998.2071 kg/m3),air at room temperature (1.204 kg/m3),solar plasma (average density 1409 kg/m3),martian air (i guess around 0.009 kg/m3), venusian air (guess 150 kg/m3) and neutron star degenerate matter (2 × 10 to 17th power kg/m³) would be if it holds a mass of entire observable universe (3×10 to 52th power kg)?Is there some online calculator?

astromark
2007-Sep-16, 07:32 PM
OMG... the numbers are coming to get me...
Galaxies are not round. They are mostly dinner plat style disks, ( with exceptions noted ).
The space between stellar objects is enormous by comparison to the area the objects them selves ocupies...
The gaps between galaxies is even bigger...
On that dinning table top that has been used here to demonstrate this scale. Try to picture it as a 3d cube. Place a Small egg on the table. Some small bird like a finch would do.
The whole of the known matter of the universe could fit inside that egg...No trouble. From the far end of the table ( universe ) you might just miss seeing it as the expansion rate could hide it from you.. Think big. No, think bigger...
Now tell me why we are talking about this?

EDG
2007-Sep-16, 10:19 PM
Now tell me why we are talking about this?

Because someone asked? :wall:

I've noticed that on threads posing this sort of question you consistently post saying that "you can't do this" or "this doesn't make sense" or "why are we discussing this" - it really seems to me that you just do not get the concept of "hypothetical questions (http://en.wikipedia.org/wiki/Hypothetical_question)" or "thought experiments (http://en.wikipedia.org/wiki/Thought_experiment)".

Case in point here: the OP is not asking what would happen if you crammed everything as close to eachother as possible - he's just asking how small a volume you could hypothetically pack it all into. So comments about how it's not possible because everything would collapse, or whether there's any point in discussing it because it can't happen are entirely beside the point and not helpful.

JohnBStone
2007-Sep-16, 10:50 PM
How much empty space is there in a galaxy like the Milky Way?
For practical purposes all of it. The mass occupies such a miniscule volume of space.

If you could somehow line up stars and other objects so that they were just touching, how much of the rest of the matter in the universe (defined as stars and solid bodies) could you squeeze into the volume of our galaxy?
For practical purposes all of it. Though dark matter might throw a spanner in the works. And that much mass close together would be enough to close the universe and make it teeny too.

Nereid
2007-Sep-17, 02:44 AM
How much empty space is there in a galaxy like the Milky Way? If you could somehow line up stars and other objects so that they were just touching, how much of the rest of the matter in the universe (defined as stars and solid bodies) could you squeeze into the volume of our galaxy?Here's a spanner that you may wish to consider: the estimated mass of (ionised) hydrogen (plus ~25% of He, and trivial amounts of other elements) in the diffuse medium between galaxies in rich clusters is ~an OOM (order of magnitude) greater than the mass of stars (and gas, and dust, and SMBH, and ...) in the galaxies in those clusters.

So, whatever number you work out, for the Milky Way, you might consider increasing by ~10 (~2.1, as the side of a cube), to account for all this non-star, non-empty space (assuming a random ~cubic kpc of a rich cluster can be considered 'empty space').

EDG
2007-Sep-17, 03:46 AM
Here's a spanner that you may wish to consider: the estimated mass of (ionised) hydrogen (plus ~25% of He, and trivial amounts of other elements) in the diffuse medium between galaxies in rich clusters is ~an OOM (order of magnitude) greater than the mass of stars (and gas, and dust, and SMBH, and ...) in the galaxies in those clusters.

So, whatever number you work out, for the Milky Way, you might consider increasing by ~10 (~2.1, as the side of a cube), to account for all this non-star, non-empty space (assuming a random ~cubic kpc of a rich cluster can be considered 'empty space').

I don't think that's necessarily true - it's not the mass that's important here, it's the volume. While we're compacting everything, we can compact all those individual scattered atoms of H and He and other stuff floating around in space and probably not even notice an increase in the volume of our cube ('course I'm basically saying that we turn all that gas into a solid, since we're packing the molecules and atoms right next to eachother)...

Saluki
2007-Sep-17, 05:07 AM
I just realized that I failed miserably in calculating the volume of the galaxy. I forgot to turn the diameter into a circular area. I ended up with a side-wise cross-sectional area rather than a volume.

Regardless, everything else was accurate within the limits of the computation. The bottom line is that the matter would scrunch up into a very small area. Compared to the size of the galaxy.

One Skunk Todd
2007-Sep-17, 02:16 PM
The bottom line is that the matter would scrunch up into a very small area. Compared to the size of the galaxy.

Which is exactly what I was wondering and didn't know where to begin. Thanks for all the helpful answers.

ozark1
2007-Sep-17, 02:56 PM
I think that there is one aspect of this question that hasn“t been explored. At one stage of our universe, the entire visible universe fitted into a volume less than the volume of one atomic nucleus. That is - all the matter currently filling a space in excess of 100 billion light years in diameter, could fit into the volume of a proton and yes, it did happen

Not a hypothetical question at all...

Cougar
2007-Sep-17, 02:57 PM
The space between stellar objects is enormous by comparison to the area the objects them selves ocupies...
The gaps between galaxies is even bigger...
Not when compared to the size of the galaxies.

mugaliens
2007-Sep-21, 02:14 PM
It's pretty thin, actually, something like, on the average of about 3 hydrogen molecules per cubic foot, given all known observations of the known universe.

Naturally most of that matter clumped together long ago...

But all told, it's some pretty thin stuff.

tdigi
2007-Sep-30, 12:24 PM
This is interesting. I read about this on "Howstuffworks.com" and this is what they had to say:
----------------------------

If you were to move all of the matter in the universe into one corner, how much space would it take up?

It's hard to answer this question exactly because there are some unknowns. But if you are willing to accept three assumptions, we can come up with an answer that sounds reasonable...

The first question is, "How big is the universe?" No one knows, but this Question of the Day assumes that the universe is a cube that is 30 billion light years on each side. That means that the whole universe contains about 2.7E+31 cubic light years.

The next question is, "How much matter does the universe contain?" The mass of the universe is a source of debate right now because there is no easy way to put the universe on a scale. This NASA page and this " Extension, Age and Mass of the Universe" article discuss different techniques that scientists use to estimate the mass. The latter article also includes an estimate of about 1.6E+60 kilograms for the mass of the universe. Other estimates give other numbers, but all are in that ballpark.

The next question is, "What density do you want to assume the mass will have once you push all of it into one corner?" Now, if you were really to do this -- if you actually did move all of the mass of the universe into one corner -- it would condense instantly into a black hole and potentially ignite another big bang. But let's say that you could keep it from doing that, and you were somehow able to keep all of the mass evenly distributed at the density of the sun. According to "Magnitudes of Physics", the density of the sun is about 1,410 kilograms per cubic meter. (For comparison, the density of water is 1,000 kilograms per cubic meter.)

If you are willing to accept these three assumptions, then:

1.6E+60 kilograms / 1,410 kilograms per cubic meter =
1.1E+57 cubic meters of matter in the universe

A cubic light year contains about 1E+48 cubic meters. So all of the matter in the universe would fit into about 1 billion cubic light years, or a cube that's approximately 1,000 light years on each side. That means that only about 0.0000000000000000000042 percent of the universe contains any matter. The universe is a pretty empty place!

http://science.howstuffworks.com/question221.htm

Kaptain K
2007-Sep-30, 05:50 PM
I think that there is one aspect of this question that hasn“t been explored. At one stage of our universe, the entire visible universe fitted into a volume less than the volume of one atomic nucleus. That is - all the matter currently filling a space in excess of 100 billion light years in diameter, could fit into the volume of a proton and yes, it did happen

Not a hypothetical question at all...
Not exactly! At the "beginning" the universe was mostly energy. IIRC, at about one second, the total mass of the matter in the universe was about one gram!

astromark
2007-Sep-30, 06:39 PM
How much empty space is there in a galaxy like the Milky Way? If you could somehow line up stars and other objects so that they were just touching, how much of the rest of the matter in the universe (defined as stars and solid bodies) could you squeeze into the volume of our galaxy?

Thats the OP.. So my answer is as right as any. Its got to be guess work doesn't it. If we do not start fiddling with the densities of objects the answer is still the same regardless. We can not be so precise as to argue about the actual size of space can we? Using the dinning table as our imaginary space. The small birds egg would paint the picture we all see.
Just imagine how heavy that egg would be. wow.

John Mendenhall
2007-Oct-02, 03:14 PM
That means that only about 0.0000000000000000000042 percent of the universe contains any matter. The universe is a pretty empty place!



Next time I'm caught in traffic I'll think about this.

The_Radiation_Specialist
2007-Oct-03, 10:33 PM
That means that only about 0.0000000000000000000042 percent of the universe contains any matter. The universe is a pretty empty place!


42!

But wouldn't this be arbitrary? I mean we could also squeeze it all to a very small point, such as what happened in the Big Bang.

So any number could be, right?

Jens
2007-Oct-05, 04:13 AM
Not when compared to the size of the galaxies.

I'm curious. Are the gaps between the galaxies approximately the same as the gaps between the stars, when calculating based on the size of the objects?

Svemir
2007-Oct-05, 10:39 AM
It's a highly relevant question. It's actually a strating point of any cosmology that assumes finite universe with a beginning.
So, if at the time of recombination (300 000 - 400 000 years after Bing Bang's ->0 time) you have estimated mass of universe in ? volume.
What is the gravity strength at that time (or what is the strength of the Dark Energy in other words, if we assume that DA is responsible for expansion at that time, too)?
It's just first assertion, of course some answers can be found on the Internet.

Cougar
2007-Oct-05, 03:07 PM
I'm curious. Are the gaps between the galaxies approximately the same as the gaps between the stars, when calculating based on the size of the objects?
No. The gaps between the stars when compared to the average size of stars are MUCH bigger than the gaps between galaxies when compared to the size of galaxies. Of course, there is a huge amount of space between galaxies, but galaxies are quite large, too. As an example, the Milky Way is around 100,000 lightyears across. Andromeda is about 2,500,000 lightyears (http://www.astronomy.com/asy/default.aspx?c=a&id=3640) distant. That's just 25 Milky Ways laid end-to-end. The diameter of the Sun is probably measured in light-seconds, but the nearest star is, what, 4 or 5 light years away? Without doing the math, I can tell ya the ratio there is much more than 25! Or even 42, for that matter. :p

trinitree88
2007-Oct-05, 04:34 PM
Not when compared to the size of the galaxies.

Cougar. Though the Milky Way at ~ 100,000 light-years across is only ~ 170,000 light from the Large Magellanic Cloud, I was under the impression that air at room temperature....where the molecules are ~ 1000 molecular diameters apart....was a good universal model for average separation of galaxies....~ 1000 galactic diameters apart (our Local Group notwithstanding)...No? Pete