r/askscience • u/[deleted] • May 08 '19
Astronomy Do galaxies have clearly defined borders, or do they just kind of bleed into each other?
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u/jobyone May 08 '19
I guess strictly speaking they don't have "clearly defined borders." It's not like there's some force holding every start within a specific hard boundary. They're just all orbiting the same gravity well, so they hold together-ish, but the edges are fuzzy because a galaxy isn't a single solid thing.
The thing is though that for the most part galaxies are so staggeringly, unfathomably far away from each other that they don't remotely "bleed into each other."
Even in cases where galaxies are "colliding" there's basically zero collisions happening, because even within a galaxy the vast overwhelming majority of the space is empty space between stars.
I guess my point is that space is mostly, well, space.
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u/things_will_calm_up May 08 '19
The "collision" part of the collision is more about how different they look if and when they separate. The gravitational interactions can reshape them, or combine them into one.
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u/NotAPreppie May 08 '19
Imagine being on a planet orbiting a star that got flung out of its galaxy during a merger hundreds of millions (billions?) of years before... We think the Milkyway looks amazing edge-on but imagine seeing the disc side-on half the year.
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u/MasterOfComments May 08 '19
Half the year? You’d see it every night!
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u/BroderFelix May 08 '19
Depends. When you are on the side of the solar system that would put the sun in front of the collision, then you wouldn't be able to see it because of the sun outshining it. On the night side you would only see darkness because the galaxy would only appear on the other side.
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May 08 '19
Depends on how the solar system is oriented. If the galaxy is north/south then it will be visible every night, but only from one hemisphere. If its mostly along the plane of the ecliptic (the plane the planets orbit in) then it will only be visible half the year, when the galaxy is opposite the sun, so its in the sky during night. Constellations are frequently only visible in summer (like Scorpio) or winter (like Orion) for this reason.
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u/smashedhijack May 08 '19
There’s a post over in the elite dangerous forum where someone found an earth like planet way above the centre of the Milky Way. Living there would give this affect!
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u/pwasma_dwagon May 08 '19
Download Space Engine and look around the edges of a galaxy for a star with a planet. You can watch a full galaxy rising during night time. Its pretty amazing, and also terrifying. Space Engine made me understand lovercraftian horror in a way hahaha :(
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u/scatters May 08 '19
It'd be too dim to see properly, unfortunately. We can only see the Milky Way because it's edge on. Side on galaxies are only visible as more than a faint blur via long exposure photography and/or telescopes.
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u/juche May 08 '19 edited May 08 '19
Loooongtime astronomy nerd here.
I think there are a lot of planets like that.
In fact, many astronomers believe that the majority of all planets are 'rogue' planets like that, orbiting no star, just flying around loose in interstellar or even intergalactic space.
A planet in intergalactic space would have very dark skies all the time, I guess.
If there is some degree of seismic/volcanic activity, they could still sustain life as well.
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u/pretentiousRatt May 08 '19
He wasn’t thanking about a rogue planet he was saying a rogue solar system with a sun that just isn’t part of the main galaxy. It would still be fine and could potentially support life just the night sky would look very odd.
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u/tombombdotcom May 08 '19
I’ve thought about that too. We would just think it’s what the normal night sky looks like because it’s all we know. We would say “imagine if we were inside the galaxy and looking at it inside the plane” like we do now. It would be a great computer animation to show inside an IMAX planetarium type theater though. The Narrator would say “Imagine our night sky......looked like this” boom!
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u/blakkstar6 May 08 '19
I feel like that would be pretty boring compared to edge-on. The Milky Way is awesome because we are looking through tens of thousands of light years of astral phenomena condensed into a very small angular area in the sky. All that spread out across the full diameter of the disk... probably not so epic. Even the Sombrero Galaxy is way more interesting because we see it edge-on.
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u/Matathias May 08 '19 edited May 08 '19
People might be thinking of something like this, where you can see the entire structure of the galaxy in its magnificence without a telescope.
I have no idea if this is actually possible (the image is from Halo 3, a video game), but if it is, it seems a lot more fantastic than the milky strip that we see now.
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u/sl600rt May 08 '19
The Andromeda Galaxy is bigger than the moon in our sky. Yet is too faint to be seen unaided. https://waitbutwhy.com/2014/06/andromeda-brighter-youd-see.html
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u/phosix May 08 '19
Consider the Andromeda Galaxy takes up an area larger than the full moon in our night sky. Thing is, it's really faint so you need to have really dark skies to see it, and even then it's kinda fuzzy.
But, if you do get to see it, and the night is sufficiently dark to make out the spiral with the naked eye (or even look at it through a telescope), it's an amazing sight to behold!
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u/Eikdos May 09 '19
So when Andromeda collides with the Milky Way, most likely won't even be affected?
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u/McKarl May 09 '19
Orbits of already existing star systems will be effected and since clouds and nebulas will merge from both galaxies, star formation will increase
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u/ohgodspidersno May 08 '19
Could a star be peeled away from its galaxy by the gravity well of the other galaxy, or a specific body in the other galaxy?
Would the star eventually re-merge with one of the galaxies or could it be sent off into the depths of space all by itself?
If this happened to our solar system, what would our experience be, and what would the night sky eventually look like?
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u/Petersaber May 08 '19 edited May 08 '19
Could a star be peeled away from its galaxy by the gravity well of the other galaxy, or a specific body in the other galaxy?
With sufficient force? Yes. But it'd be the galaxy as a whole, not a specific body. If you're close enough to be affected by specific bodies, then you're well within the gravity well of the galaxy.
Would the star eventually re-merge with one of the galaxies or could it be sent off into the depths of space all by itself?
If it was torn away by another galaxy, it'd eventually merge into that galaxy. It's not easy to gain escape velocity required to be flung into the extragalactic void. I think it's possible, but most stars would just enter a Halley's comet-like orbit (except extend much further) and get closer to the new center with each orbital cycle.
If this happened to our solar system, what would our experience be, and what would the night sky eventually look like?
Our sky would greatly change. Constellations and all that. But within our solar system... not much would happen. And the process would take millions of years.
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u/Wrobot_rock May 08 '19
Could a star be peeled away from its galaxy by the gravity well of the other galaxy, or a specific body in the other galaxy?
It's more likely one of the stars in the other galaxy will redirect the orbit to a trajectory that circles the new galaxy rather than the new galaxy's gravity well being the main factor
Would the star eventually re-merge with one of the galaxies or could it be sent off into the depths of space all by itself?
Both are possible
If this happened to our solar system, what would our experience be, and what would the night sky eventually look like?
The stars we see are all from the milky way, so they would appear to move away as we travel further from the galaxies colliding. The galaxies we can see will move through the night sky as well, but probably less noticeably since they are much further away
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May 08 '19 edited May 08 '19
Only if the galaxies were close enough for a merger event. For example, the Milky Way and Andromeda galaxies will merge in about 4.5 billion years. That's about the same age of the Earth and Sun currently. In general, galaxies are way too far away for them to effect each other.
The answer to the second question, is it could be either one. It's a chaotic system.
The answer to the third question, is that the timescales are so immense that it would be irrelevant in comparison with the lifespan of any observer. The night sky would chance so slowly over millions of years that we really wouldn't notice anything unusual. If we ended up outside of a galaxy, we would just have a dark black night sky, apart from the other planets and the moon, but again, the timescales are so immense that we wouldn't notice anything strange about that.
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u/Alis451 May 08 '19
yes, yes
yes, yes
we would be dead. any force strong enough to pull our solar system away would probably have a large effect on our planet/orbit individually. otherwise pretty much the same as it does now with some changes in constellations. The Andromeda galaxy is pretty large in our night sky, just too dim to see it properly, and the Milky Way dominates the sky but most of the time we can't see its form due to light pollution so our sky just mostly looks like random(not really) points of light.20
u/houstoncouchguy May 08 '19
It’s worth noting that, without considering the expansion of the universe, the gravity wells that hold galaxies together extend all the way across the universe.
If you had nothing but 2 golf balls in the whole universe, and put one on each opposite side of the universe, the golf balls would eventually pull each other toward a collision. The whole universe would be dead by then, but the gravitational connection does somewhat exist.
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u/XorMalice May 08 '19
Isn't this an open question? Gravity at such a vast distance and tiny mass might prove a special case, right?
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May 09 '19
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u/Wermine May 09 '19
The space between the two golf balls would be expanding
But if the starting conditions are "If you had nothing but 2 golf balls in the whole universe", there wouldn't be dark matter nor dark energy and universe wouldn't be expanding?
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u/amazondrone May 09 '19
If you had nothing but 2 golf balls in the whole universe
The whole universe would be dead by then
If there were nought but two golf balls in the universe, the universe would already be dead wouldn't it?
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u/Theroarx May 08 '19
Could the stars and planets on the edge of these galaxies ever get far enough away that they are able to escape its gravatational pull and just start flying through space?
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u/jswhitten May 08 '19
They would have to gain escape velocity somehow. A close encounter with another star might do it.
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u/mikecsiy May 08 '19
I would say that when galaxies collide there is a lot of "colliding" going on, just not in the form of stars. You frequently end up with a lot of regions of heavy star formation where massive galactic clouds of H2 interact.
Think of the collisions like any sort of matter colliding. Much like stars in our example the nuclei of atoms don't literally collide, but the forces associated with them cause interactions that ripple through a much larger area.
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u/PM_M3_ST34M_K3YS May 08 '19
However, the large gas envelope they recently found aground the milky way is about a million light years across and is probably already mingling with the gas envelope around Andromeda... So technically speaking, for our two galaxy system, they are bleeding into each other.
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u/IAmBroom May 09 '19
Strictly speaking they certainly have well-defined borders, if not clear ones.
All the matter bound by that galaxies gravity is part of it. No other matter is.
An analogy would be the object 'Oumuamua, which recently shot through our solar system, boomeranged once around dear old Sol, and shot off into the Void again. It was never part of our Solar System. By contrast, particles ranging from a flea's fart to near-planetoid in size orbit the sun in our Oort Cloud; every one of them is part of our solar system, though they may be much farther than Pluto, and orbit slower than ice ages on Earth.
Colliding galaxies have obviously confused boundaries between them, just like two beers spilled (NOOO!) on the same bar floor.
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u/jobyone May 09 '19
I think our disagreement comes from a philosophical difference. In practice it would be very difficult, nigh impossible, to draw a good map of that well-defined border of a galaxy, even though it exists.
We'd necessarily be making concessions to our own sensing capabilities, a few judgement calls about what size of matter we should concern ourselves with, stuff like that.
Sure, you could theoretically draw a hard shell in space and say "this is the border," but in practice it's fuzzy with noise and incomplete data and stuff.
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u/IAmBroom May 09 '19
Yes, you're talking about practicality, and I'm talking about exact definitions.
Frankly, the OP is asking a false dilemma: they either have borders, or they bleed into each other. Most of the universe is outside of galaxies, by either of our definitions, and so a third possibility is that they have ill-defined borders but don't overlap (except in the case of collisions).
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u/malcontented May 08 '19
Are there stars between galaxies? Such that you can’t say which galaxy a star belongs to?
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u/jobyone May 09 '19
Apparently about half the stars in the universe are floating around in the vast intergalactic nothings.
https://www.space.com/27682-rogue-stars-between-galaxies.html
I don't like this fact, because it means about half the life in the universe is presumably also trapped in the vast intergalactic nothing, where even getting to another star is unfathomably difficult.
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u/RikenVorkovin May 09 '19
Makes me think though. If some of those rogue stars are big enough. When they supernova wouldn't the resulting gases possibly be enough to form a few more stars? Then possibly self propagate a galaxy eventually? Or does this not work because matter is finite so the amount of gas from that supernova wouldn't make more matter.
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May 09 '19
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u/IAmBroom May 09 '19
To be fair, it's only 4 years at Warp 1... :D
OK, to be accurate: a top speed of 0.1 c could get to the nearest star in a few decades - within a typical Earthbound human lifetime (radiation would be a huge problem for the travellers).
Now, unfortunately it's pretty much a given there's no intelligent life on Proxima Centauri, nor on its neighbors Alpha Centauri and Alpha Centauri B. Probably no life at all. Probably no "Goldilocks" planets orbiting (planets with surface temperatures where water exists as a liquid).
So, practically speaking... yeah.
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u/sparkyroosta May 09 '19
Until we get FTL working... the Eastern and Western hemispheres were months away from each other until we got planes.
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u/Dubanx May 08 '19 edited May 08 '19
he thing is though that for the most part galaxies are so staggeringly, unfathomably far away from each other that they don't remotely "bleed into each other."
Eh, most big galaxies have smaller satellite galaxies orbiting them. For example, the Milky Way has 59 such satellite galaxies (that we know of). I believe the big galaxies also tend to cluster together into local groups that aren't too far away. For example, our closest major neighbor, Andromeda, appears larger than the moon in the night sky.
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u/hilburn May 08 '19
Andromeda is (approximately, soft borders and all that) 220,000 light years across, and is a bit more than 2.5 million light years away.
It's like looking at a pair of tennis balls 75cm (2.5ft) apart - yes the edges of the balls are fuzzy, but they are clearly distinct from one another.
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u/GoSox2525 May 08 '19
Note, though, that the Milky Way and Andromeda occupy a group-sized system; the largest objects in the universe are galaxy clusters which are several orders of magnitude larger than the local group, and the galaxy population is much more dense. See this APOD
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u/Hold_onto_yer_butts May 08 '19
Ah, yes, the image in every single textbook with the heading Fig 12a, Gravitational Lensing.
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u/Dubanx May 08 '19
It's like looking at a pair of tennis balls 75cm (2.5ft) apart - yes the edges of the balls are fuzzy, but they are clearly distinct from one another.
Relative to its diameter, it's still closer to us than the moon, though. While it's true that Major galaxies are distinct, there isn't nearly as much empty space between them as his post suggests. Even our own solar system has (relatively) less empty space between bodies than galaxies have within the local cluster. All I'm saying is that they're not THAT far away from us. Especially when you go into the satellite galaxies which do somewhat blur into our own.
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u/Mytzlplykk May 08 '19
there isn't nearly as much empty space between them as his post suggests.
If his math is correct, he didn’t suggest anything really. It’s hard numbers. Your description is a bit more fuzzy:
All I'm saying is that they're not THAT far away from us.
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u/BluScr33n May 08 '19
But Andromeda is still 2.5 Million light years. Many times larger than the size of either galaxy.
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May 09 '19
About 10 times the distance of Andromeda itself. Only one order of magnitude, not too far really, speaking on cosmic scales
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May 08 '19
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u/jswhitten May 08 '19 edited May 08 '19
On that scale a kilometer is a billion light years. Driving for 15 minutes would put you outside the observable universe.
If the pile of rice is the Milky Way, then Andromeda would be another pile of rice 2.5 meters away on the other end of the table.
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u/CaneVandas May 08 '19
Technically, he said 4 hours away.... he didn't define how fast you were traveling.
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u/1ifemare May 08 '19
Technically, if it took you 4h to walk 2.5m, to any outside observer, you'd be standing still.
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u/Sharlinator May 08 '19
Yep, galaxies are surprisingly close to each other compared to the distances between stars. If we get the Milky Way colonized, the next step is going to be fairly easy :)
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u/pbmadman May 08 '19
Umm no. The Milky Way is 105,000 light years in diameter. Andromeda is 2.5 million light years away. The closest star is what, 4.6 light years away.
This is like saying that Australia isn’t all that far from me here in Nc when I haven’t even left my bed today and will never in my lifetime make it out of my apartment much less to a neighbors.
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u/jswhitten May 08 '19
Well, there's a speed limit. Hopping from one star to the next at .1 c is feasible, and you can colonize the entire galaxy with a lot of < 10 light year hops that would take less than a century each. It will never take less than 2.5 million years to reach Andromeda, and realistically it'll take more like 25 million years.
Unless of course you waited a few billion years until Andromeda is colliding with the Milky Way.
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u/pbmadman May 08 '19 edited May 08 '19
Edit: I slightly goofed, I did all the math in meters but used 1012 instead of 15 for my light year conversion. So the sizes of the galaxies and the distances between them is actually 1000x as big as I figured. Or, you could use something 1/1000 the size of rice. Maybe a grain of salt for the stars instead.
So here we go, a grain of sushi rice, vaguely spherical is about 2.5 mm in diameter. Our sun is 1.4 million km in diameter, a roughly average sized star or at least for my analogy it is since it’s the star we are most familiar with. The Milky Way Galaxy is 105,000 light years across. A light year is 9.461x1012
So we have: (0.0025/1400000000)1050009.461e12
Giving us as an answer the diameter of the Milky Way at the “grain of rice as a star” scale. 1,773,937.5 meters.
Andromeda is 2,500,000 light years away. Using the same scale it is 42.2 million meters away.
So. Take 250 billion grains of rice and spread them out over Alaska. Go 1/10 the way to the moon and do the same thing.
Can you not see how being technically precise isn’t really all that helpful here? Can you not see how using comprehendible numbers made sense and actually might help OP vaguely understand?
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u/pelican_chorus May 08 '19 edited May 08 '19
Wait, you're really defending your original post by showing how the "handful of rice" ends up needing to be 1700 km wide?
In your first post you said "drop a handful of rice on your kitchen table. That’s a galaxy"
In your second you suggested that the scale you were using was "grain of rice as a star," when clearly that doesn't match at all, as you yourself have shown.
Instead, if we take your first statement at face value, where a "handful of rice" is a galaxy, we don't care how many grains of rice there are. Instead, the diameter of a "handful or rice" is about 10 cm. So our scale is 0.1m = 1e21m, or 1:1e22.
The distance to Andromeda is 2.5e22m, which would be 2.5 meters away at this scale.
More importantly, the distance is 25x the diameter of the milky way galaxy. That is the important figure, and holds whether the galaxy is a handful of rice, a frisbee, a city... whatever you want. And by that measurement, the statement "drop a handful of rice on your kitchen table... Now drive 4 hours away and do the same thing on a strangers table. That’s 2 galaxies" is clearly not correct. That's wrong by many, many orders of magnitude.
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May 08 '19
Are there any stray grains of rice along the road between the two?
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u/amazondrone May 09 '19 edited May 09 '19
Yes, particularly if we allow rice to represent dark matter as well as regular matter.
Edit: Huh, and someone else posted this link further down: https://www.space.com/27682-rogue-stars-between-galaxies.html
As many as half of all stars in the universe lie in the vast gulfs of space between galaxies, an unexpected discovery made in a new study using NASA rockets.
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u/LittleJohnnyNations May 08 '19
That's completely incorrect. The size of the Milky Way galaxy is a little over 100,000 light years across and the distance to Andromeda is around 2.5 million light years. If a grain of rice is around 1 cm long then Andromeda is less than 25 cm away. Plus there are plenty of other smaller galaxies closer to us then Andromeda.
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u/n3xas May 09 '19
You read that wrong, the Galaxy is many rice grains and bigger than 25 cm
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u/LittleJohnnyNations May 09 '19
Same still applies. The other galaxy would be in the same room. The pile of rice grains will only be as far part as 25 times their size.
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u/nicksneiderfilm May 09 '19
So how are the “borders” defined? Why doesn’t each galaxy extend until it meets another? Why did we decide to just leave the empty space as empty space instead of organizing that space in relation to the closest galaxy?
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u/pbmadman May 09 '19
So in the infancy of the universe everything was in the same place and it started to spread out. Matter attracted other matter and it clumped together. The bigger clumps attracted more matter and the universe kept growing. At some point the distance between the clumps got too large for them to attract each other.
Imagine a bunch of magnets all stuck together. They aren’t naturally going to spread out evenly. As the pile is ripped apart they will clump together still until the clumps get so far apart they don’t attract each other enough.
As far as our organization and defining goes I can only guess. But it seems reasonable. Just how a city generally doesn’t include the empty space around it until you reach the next city I suppose there wasn’t much point in making the lines around galaxies any bigger than needed?
Also galaxies are moving in relation to each other so dividing up the empty space between them probably isn’t useful for anyone.
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May 08 '19
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u/Void__Pointer May 09 '19
TOTAL EXONERATION. Also you can't see the report or talk to the guy that wrote it.
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u/unic0de000 May 08 '19
I mean, a planet can also crash into another planet without any of their atomic nuclei ever touching. When we're talking about 'contact' between galaxies we probably don't mean contact between their component stars.
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u/TrumpetSC2 May 09 '19
This is a little bit wrong. Their visible mass is seperated by large amounts of space, but many darm matter models show a much more continuous distribution of dark matter halos. This means whiles the dark matter never really stops between galaxies. Of course we still dont really know what dark matter is so it could be a different distribution than we have simulated.
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u/jobyone May 09 '19
This is true.
Presumably when people talk about a "galaxy," though, they're talking about the normal matter that we can actually see and interact with (not to mention we know with a great deal of certainty it actually exists). Most people aren't talking about that plus the theoretical weird stuff we can't see that is maybe why the galaxies are where they are.
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u/TheFirsh May 08 '19
How was there enough time for them to drift so far apart?
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u/jobyone May 08 '19
There just ... was? At one point everything was closer together, but currently it is less close together.
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u/Fuibo2k May 09 '19
So what you're saying is that the stars in the galaxies are kinda like atoms. There is so much space in between them yet with so many of then they seem to form a cohesive thing.
(I'm obviously not saying stars are atoms [or are they] just saying it's an interesting similarity)
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u/zjanda May 09 '19
This is true, and the reason they’re able to have such low densities is thought to be because of dark matter which is, correct me if I’m wrong, but invisible, untouchable matter. Dark matter really only has gravity to show for itself, and A LOT of it; so much as to actually bend light to a considerable degree. I’m likely mis-stating something in some way but this is the general gist of why galaxies are, for lack of a better term, incredibly un-dense clumps of celestial bodies.
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u/jackofslayers May 08 '19
The fact that stars do not really collide when Galaxies collide was the most mind blowing thing I learned in Astronomy. It makes sense but it is still crazy to think about.
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u/AndyCalling May 09 '19
Summed up rather well by Mr Douglas Adams:
Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.
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u/d1rron May 08 '19
Aren't Andromeda and our galaxy technically already merging at their very "edges"? I know that's not representative of your typical galaxy, but a neat coincidence.
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u/Adwokat_Diabla May 08 '19
https://www.zooniverse.org/projects/zookeeper/galaxy-zoo/classify
This is a neat crowd-sourced project where you look at pictures of galaxies and answer some basic questions about what you see (Human eyes are better than computers for seeing cool things that an astronomer might be interested in). Some galaxies are merging with others and some simply are not.
Here are 3 that are merging:
Another 2 that are merging:
https://dailyzooniverse.files.wordpress.com/2016/09/56f3de535925d90042033d7f.jpeg
Here's a bunch of "galaxies of the week" where you can look at galaxies and what they're doing:
https://daily.zooniverse.org/tag/amazing-galaxy-of-the-week/
I like this one:
edit: To answer your question, yes and no. There are billions of galaxies and we've only really scratched the surface. Some merge with others and some don't. Some are spiral-shaped, and others look like perfect spheres. There's lots of variety.
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u/Pas__ May 09 '19
How do they know they are merging? Can't they be simply in front of each other separated by large distances (but having different sizes, so their apparent sizes are similar)?
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u/Adwokat_Diabla May 09 '19 edited May 09 '19
In general they have weird asymmetrical shapes formed by their merging so it becomes relatively easy to spot.
The old galaxy zoo project faq:
http://zoo1.galaxyzoo.org/faq.aspx
Q. How do I tell the difference between a merger and a projection effect?
A. If the galaxies are truly interacting then the should show signs of disruption; long, extended arms are typical, and the presence of more than one nucleus is also a good sign.
http://zoo1.galaxyzoo.org/images/faqs/587742629059362935.jpg
If they are merely overlapping or close to each other, and show no signs of disturbance, then they are probably just a chance projection and not a real merger.
http://zoo1.galaxyzoo.org/images/faqs/587736941981466667.jpg
Also, some galaxies are so big that they can cause something called a gravitational lens, whereby a huge galaxy in front of other smaller galaxies (From our perspective) can cause the light to "bend" on its journey to us:
https://www.zooniverse.org/projects/aprajita/space-warps-hsc/talk/1745/629162
edit: Sometimes it's hard to tell. :)
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u/Braelind May 08 '19
Galaxies may have fuzzy borders because the borders end where there stop being stars. They do not bleed into each other though, because there's an unimaginably large distance between galaxies.
The universe isn't full of galaxies. It's a vast empty nothingness, with the odd galaxy here and there.
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u/eukaryote_machine May 08 '19
Excuse me, the Universe told me that it actually likes to be referred to as a vast empty somethingness.
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u/MoltresRising May 09 '19
You should certainly check out Neil Degrass Tyson's book "Astrophysics for People in a Hurry." All of this and more is covered, especially when talling about galaxy clusters and dwarf galaxies.
No defined borders, but they can tell a lot based on gravity and other aspects as to which element of overlapping galaxies originally belonged where.
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u/XorMalice May 08 '19
While there are galaxies that arguably are blended together, there's also galaxies with vast distances between them- as in, the distance from one galaxy to the other is several times the radius of the first galaxy.
The ones you can argue are blended are really more in some amazingly slow version of merging or something.
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u/Gregrox May 08 '19
Galaxies are usually so far apart that each galaxy is clearly distinct from another. But each galaxy sort of smoothly blends into intergalactic space. Surrounding each galaxy's disk or elliptical core is a much larger region of dark matter, globular clusters, and sparse stars called the halo. And just like planetary atmospheres, there's no definite end of that halo.
Sometimes galaxies collide though. During those collisions, galaxies can be blended and mixed together without unambiguously being a single galaxy.
But sometimes galaxies have very close satellites. For example, the small and large magellanic clouds orbit close enough to the halo of the milky way that there could be some ambiguity as to where the edge of the milky way ends and the magellanic galaxies start. Same is possibly true for other dwarf galaxies orbiting near the milky way.
You can think of it like cities and suburbs. Sometimes cities blend together into one major metropolitan area, sometimes the city centers are separate but the suburban area blends together, and sometimes cities are so far apart that the suburbs are completely separate and the cities are separated by vast rural areas where there's almost nothing. Important note of course is in this analogy you have to ignore the existence of city limits! And also you have to let cities collide sometimes.
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u/VirtualMachine0 May 09 '19
A lot of responses here say "galaxies are really far apart." Not really. Space between galaxies is often on the scale of 10 times the width of a galaxy.
By proportion, the Earth and Moon are much farther apart.
There are tons of stars in between galaxies, and tons of galaxies in the "voids" you might have heard of between the large structures if the cosmos.
The density is the thing. Intergalactic stars make up about half the population of stars, but are spread out in something like 1000 times the volume, so they're about 1/1000th as dense.
There's definitely random variation, too, so this is all a Fermi Approximation. There will be some places where a galaxy might have a really sharp edge, too.
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u/sillysoftware May 08 '19 edited May 08 '19
I remember reading a scientific article about this before. It suggested that for independent systems that don't interact with each other (not even sharing distant starlight), they do not share space time (I think it's called spacetime locality). The maximum distance the first photons traveled would be a hard boundary on the edge of the galaxies space time. And the black hole in its center would be another hard boundary.
So given a mass of galaxies close enough together that they are visible only to each other, the boundary would look like a giant ball of light filled with seeds (black holes). However as there is no space time beyond the inner and outer limits, it might be fair to say the boundary is curved and has no length.
TLDR: yes they do have hard boundaries, which have no length.
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u/meowgrrr May 09 '19
It might help you to think about or look into where the "end" of our atmosphere is and where "outer space" begins. Basically, there isn't really a border, the atmosphere just gets thinner and thinner and thinner, etc etc etc. You could look into the Kármán line, which is an arbitrary height some people use as the "end" of the atmosphere, but no matter what metric you use, you are sort of arbitrarily defining it.
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u/YoungAnachronism May 09 '19
The problem with the question, is that no answer can fully encompass all the possibilities a universe this size presents.
Our neighboring galaxy, Andromeda, has a clearly defined shape to it, and you can say that it is clear that it is a structure made up of enormous numbers of stars and star systems, dust and so on bound together by the gravitational attraction between each part thereof. However, the precise, to the millimeter point at which material at the periphery is not a part of that galaxy, but instead just part of the wider, non-distinct distribution of matter in the universe, is well nigh impossible to pin down. The answer, for Andromeda certainly, is therefore yes and no. It has a limited span, but the exact and precise location of its edge is nearly impossible to define.
Things get even more complicated when you realise that galaxies like Andromeda, and our own galaxy, will potentially collide with one another in the future, and have collided with other galaxies before. This blurs the line significantly, because at the time of their meeting, there will be much chaos. The outer edges of both structures will begin bleeding through and into one another long before the cores come into proximity with one another, and this process will take a very long time indeed. For the period where this is in progress, the shape of both galaxies will warp and change. When the cores, that is the densely packed material comprising the centre of those galaxies come into close proximity, there is a chance that one of the super massive black holes which feature at the centres of those galaxies, may be kicked out, consuming material and warping the shape of the evolving galaxy still further on its way out. On the other hand, the black holes may merge, which has its own interesting way of effecting the resulting shape of the structure.
But during the whole event, there will be material kicked out of both galaxies, stars booted from their systems, planets too, some of them will wander the cosmos absent surrounding structures for great stretches of time, becoming rogue planets, rogue stars. Its also worth pointing out that last I checked, the process is not likely to happen as a result of one pass. The two galaxies will collide, move slightly away from one another, then back toward one another, as the attraction between the two black holes bleeds their momentum off, then overcomes it completely. This may happen several more times before any core collision really begins.
The edges of the structure will change shape constantly throughout this period and in very pronounced ways, with material being thrown out, attracted in...
There are some galaxies that are very fuzzy indeed in appearance, and its obvious even to the casual observer, that their shape and therefore their border is diffuse. So again, it is true that galaxies do have a defined border, that being the furthest point from its centre, that the core can still exert a gravitational force on material in the structure. But there is no way to know where that point is for any given galaxy, to a precise, down to the millimeter fashion, with the technology and knowledge available today.
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u/MadVikingGod May 09 '19
Let's say you roll out a big canvas on a gym floor, then you were to load up a paint brush with a ton of paint and fling it all over the room. Just for good measure your friends were to do the same.
You could think of galaxies like the drops of paint. What galaxy any bit of paint is part of basically comes down to "well it's closest to this splotch".
While in our night sky galaxies might look next to each other we can't really see depth, so there can be millions of light-years between each galaxy. They only really get close to others when they collide. And simulations of two galaxies passing through each other tearing themselves apart are usually really cool.
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u/aartadventure May 09 '19
One day our galaxy (the Milky Way) will collide with Andromeda. But thanks to vast distances between solar systems, you basically wouldn't even notice anything. (unless you were outside the galaxies and watching them collide, in which case, can I also ride in your awesome spaceship?)
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u/extropia May 08 '19
Related question: I've read often that it's hypothesized that dark matter orbits galaxies in a larger disc around the visible matter. Do we have any estimate how far out, and pertaining to op's question, would it even come remotely close to the dark matter from another galaxy?
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u/GoSox2525 May 08 '19 edited May 08 '19
Dark matter doesn't only exist at some far orbital around a galaxy-- rather, each galaxy is embedded in a dark matter clump. Indeed, the dark matter is generally most dense at the same regions where the baryonic matter is most dense (at the centers of galaxies).
And, yes, virtually all galaxies are posited to be "connected" by dark matter structures (often called the "cosmic web" or "large scale structure"). This web consists of density peaks in which galaxy clusters form, where those peaks are connected to one another by matter "bridges" known as "filaments" and "walls".
Have a look at this Illustris Simulation video which shows the evolution of the web in dark matter density, temperature, and metallicity. At the very end, it shows where actual galaxies might be and what we would be able to see with a telescope.
Here's another one from AREPO showing galaxy discs emergent in the large-scale gas distribution.
Join us in r/cosmology if you like thinking about extra-galactic scales :)
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u/panckage May 09 '19
Very small add: there are a few galaxies that have been found to be devoid of dark matter http://blogs.discovermagazine.com/d-brief/2019/04/01/galaxy-without-dark-matter/
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u/GoSox2525 May 09 '19
Indeed, these are interesting objects; they still do reside in the web, but are thought to maybe undergo some kind of local stripping event during flyby of high density regions. But, no one really knows.
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u/womplord1 May 09 '19
Isn’t this all hypothetical?
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u/GoSox2525 May 09 '19
No, it's not. Everything I said, and the videos I linked, describe the best current model of cosmology. So, it is theoretical, but that doesn't mean hypothetical or in any sense imaginary-- the parameters of the theory are grounded in decades of observational support, and the result of our simulations looks very much like the real universe in many ways.
Whether dark matter or dark energy exists at all is perhaps an open question, but if we are right that it does, then we should be able to describe it via theory as best we can and see what the model predicts via simulation, and largely it works. That's exactly how meteorology or biology or geology or any science works, too.
In any case, we do know for a fact that the cosmic web really is the true matter distribution of the universe. We know for a fact that the universe is homogenous and isotropic on scales larger than the web variations. We are very confident that expansion is real. In short, those videos I linked to should be viewed with confidence, and aren't the product of an individual's animation work, but rather the implementation of the current model, described by thousands of papers by thousands of cosmologists over the past few decades, run on supercomputers.
Did this answer your question or nay?
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u/goodtogo_joe May 08 '19
The mass of the dark matter halo of a galaxy can be estimated using a variety of methods, including how the stars move within the galaxy, and how nearby satellite galaxies orbit the Milky Way. Once you have the total mass of the dark matter, the radial extent can be hypothesized using a simple equation called the Virial Theorem (although there is no real hard boundary, just like in a galaxy). This assumes that the kinetic energy of the dark matter balances the gravitational energy such that you get a bound, spherical object. The radius of the dark matter of the Milky Way calculated through this method is approximately 200 kpc.
The distance from the Milky Way to Andromeda is 700kpc. If you assume Andromeda has a similar DM halo of 200kpc, then they are still somewhat distinct, though much less so than the galaxies themselves (radii of ~15kpc). In galaxy clusters (of which M87 is a member), the galaxies are close enough such that their dark matter halos have merged together into one super halo, which can be up to 1000x the mass of the dark matter halo around our Milky Way!
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u/pompomhusky May 09 '19
Another interesting question could be,
if two galaxies are revolving around each other, would they be considered one or two?
What if some of the stars from one "bled" past the boundary of another? (merging galaxies)
In essence, at what point exactly do Milkyway and Andromeda turn into Milkomeda?
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u/Briyaaaaan May 09 '19
No, and No ...well only sometimes. Galaxies edges look distinct from a distance but there is a disperse outer layer like our oort cloud on the solar system that is mostly debris. Usually there is vast mostly empty space in between. There are so many galaxies though that we have an unending list of interesting examples of clusters and merging/post merge galaxies, but the norm is they have plenty of room between them. The stars in a galaxy themselves are so spread out they would very very rarely collide even when they 'crash' through each other.
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u/unknoahble May 08 '19
Galaxies are mostly dark matter, and the dark halo always has a radius much larger than the baryonic matter. In general, however, galaxies are far enough away from each other that they only interact gravitationally, but even that is rare. Most galaxies are indeed surrounded by a "boundary" of "empty" space, which is always increasing due to accelerating expansion. If you want to get philosophical, nothing has clearly defined borders!
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May 08 '19
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u/GregoleX2 May 08 '19
There are in fact a decent number of intergalactic stars. So small that at that scale you don’t see them but they are there
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u/sandanx May 09 '19
Imagine being a civilization on one of the planets orbiting those. Think we are lonely? Imagine knowing that you'll probably never get the chance of exploring any other place in the universe.
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u/pddle May 09 '19
On the other, do we really have much of a shot of ever visiting a neighbouring solar system?
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u/wadss May 09 '19
that totally depends on how you define "edge" to be. and in different fields of astronomy uses different definitions of what an edge is. a common method is to define a scale factor for the over density of a body relative to the critical density of the universe, e.g. the end of a cluster is the radius at which the density of the cluster is 200 times the critical density of the universe, and the "core" of a cluster is the radius at which the density is 2500 times.
similar things happen for even the earth. for example where you say the earths atmosphere ends is also reliant on your definition of "end".
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u/I_Got_Back_Pain May 08 '19
I was going to compare it to the borders in international waters, yeah they exist but it's not like you can clearly define them
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u/CorporealLifeForm May 09 '19
They don't "bleed into each other" unless they're colliding (which takes so ling that for now they might as well be considered weird double galaxies) Stars get less common around the edges which could be seen as a kind of "bleeding" but are massive distances apart so it's not really bleeding into anything. Stars in between are extremely rare.
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May 09 '19
you can do an experiment to see it work first hand.
Get to water pumps, that are strong enough to cause a vortex in a pool of water, start them semi far apart, you now have two "galaxies".
Next move them together, slowly, and see what happens.
(this is overall very similar, but very much different, but can show the gist.)
(use dyes for even more amusement)
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u/omegacluster May 08 '19
No, like our atmosphere just bleeds out into space and we therefore kinda share an atmosphere with other planets, the exosphere, galaxies are not well-defined bodies, although you can broadly trace a circle around them and tell what's in and what's out, there might always be leftover stuff that you failed to include.
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May 08 '19
People shed skin cells everywhere we go, and they float in the air. So by that logic, we share a body with every other person on the planet.
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u/EZ-PEAS May 08 '19
A galaxy is loosely defined to be the collection of objects all orbiting around the same galactic center. The distribution of "stuff" in a galaxy is generally dense towards the center, and as you move away from the gravitational center you encounter things less and less often, but there is no final boundary after which nothing can orbit. As an analogy, consider how our solar system has the sun and the planets, but we also have the oort cloud and comets and many things that orbit our sun very distantly. And then there are even extra-solar objects that transit through our solar system but are traveling too fast to be caught by our gravitational pull.
Most of the time galaxies are extremely far apart, so the question of what belongs to what galaxy is not an issue. But, galaxies do collide on occasion, and in this circumstance our notion of a galactic border is not well defined. Here the question of what orbits what is continuously in flux.