# Colliding Galaxies
**URL:** https://boards.straightdope.com/t/colliding-galaxies/748504
**Category:** Factual Questions
**Created:** [March 10, 2016, 2:16am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504 "2016-03-10T02:16:28Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@whataboutbob](https://boards.straightdope.com/u/whataboutbob)
#### Post date: [March 10, 2016, 2:16am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/1 "2016-03-10T02:16:28Z")
I hope someone could understand this question. It’s incredible that we have pictures of colliding galaxies 100 MILLION LIGHT YEARS away. I guess this is actually a couple of questions. If we are seeing these pictures now we are apparently looking at the past. Now, as we are seeing the light now which is 100 million light years away how many earth years is that equal to? Also, if the collision had somehow caused a huge planet like object to crash into say, a star when would that be observed by hubble?
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### Author:  [@running\_coach](https://boards.straightdope.com/u/running_coach)
#### Post date: [March 10, 2016, 2:20am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/2 "2016-03-10T02:20:25Z")
Reported for forum change.
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### Author:  [@AHarris](https://boards.straightdope.com/u/AHarris)
#### Post date: [March 10, 2016, 2:21am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/3 "2016-03-10T02:21:16Z")
> [@whataboutbob](#):
>
> I hope someone could understand this question. It’s incredible that we have pictures of colliding galaxies 100 MILLION LIGHT YEARS away. I guess this is actually a couple of questions. If we are seeing these pictures now we are apparently looking at the past. Now, as we are seeing the light now which is 100 million light years away how many earth years is that equal to? Also, if the collision had somehow caused a huge planet like object to crash into say, a star when would that be observed by hubble?
A light year is a measurement of distance equal to the distance that light travels in a year.
If an object is 100 million light years from Earth, any images that we see of it are 100 million years old.
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [March 10, 2016, 3:16am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/4 "2016-03-10T03:16:29Z")
Moved from ATMB to GQ.
Colibri
General Questions Moderator
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [March 10, 2016, 3:32am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/5 "2016-03-10T03:32:23Z")
> [@](#):
>
> Also, if the collision had somehow caused a huge planet like object to crash into say, a star when would that be observed by hubble?
Such a small event wouldn’t be detectable by the Hubble. But if the Hubble is seeing the galaxy collision now, then a hypothetical more powerful scope that could see such a small event would also see it now, or however long after now it took for the galaxy collision to cause the event.
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### Author:  [@Jackknifed\_Juggernaut](https://boards.straightdope.com/u/Jackknifed_Juggernaut)
#### Post date: [March 10, 2016, 3:45am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/6 "2016-03-10T03:45:01Z")
Could 2 galaxies collide “head on”? If so, what circumstances could create such a collision? In general, aren’t galaxies moving away from each other?
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### Author:  [@Trinopus](https://boards.straightdope.com/u/Trinopus)
#### Post date: [March 10, 2016, 4:16am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/7 "2016-03-10T04:16:54Z")
Yes, in general, galaxies are getting farther apart, as space expands. But there are still local statistical exceptions, in the form of individual galaxies moving toward each other.
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### Author:  [@Ornery\_Bob](https://boards.straightdope.com/u/Ornery_Bob)
#### Post date: [March 10, 2016, 4:26am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/8 "2016-03-10T04:26:05Z")
When galaxies are said to collide, it just means their gravitational fields are closely interacting. It doesn’t mean stars are crashing into each other. Galaxies are mostly empty space and on top of that is the problem that about 80% of the Milky Way is dark matter. Visible stars are maybe 4%. In other words, when you try to collide with a galaxy, there is gravity to interact with, but not a whole lot of anything else.
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### Author:  [@dtilque](https://boards.straightdope.com/u/dtilque)
#### Post date: [March 10, 2016, 5:12am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/9 "2016-03-10T05:12:40Z")
> [@Trinopus](#):
>
> Yes, in general, galaxies are getting farther apart, as space expands. But there are still local statistical exceptions, in the form of individual galaxies moving toward each other.
Galaxy collisions are actually quite common. All of them are of galaxies that are part of the same cluster running into each other. Clusters are gravitationally bound and are, for the most part, not expanding with the general expansion of space.
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### Author:  [@Mijin](https://boards.straightdope.com/u/Mijin)
#### Post date: [March 10, 2016, 5:38am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/10 "2016-03-10T05:38:49Z")
> [@Trinopus](#):
>
> Yes, in general, galaxies are getting farther apart, as space expands. But there are still local statistical exceptions, in the form of individual galaxies moving toward each other.
Including, of course, our own galaxy and Andromeda, with a “collision” due to happen in [about 4 billion years](https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision).
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [March 10, 2016, 12:35pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/11 "2016-03-10T12:35:58Z")
Well, maybe. We know that our two galaxies are getting closer together, but we do not know that they’re going to collide, and there’s circumstantial evidence that we’re significantly more likely to miss each other. Think of a car coming the other direction down the road: You’re getting closer together, but as long as you both stay in your own lanes, you’ll be fine.
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### Author:  [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)
#### Post date: [March 10, 2016, 1:05pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/12 "2016-03-10T13:05:34Z")
I remember as a kid in the 1960s reading an article in some science publication about galaxies colliding, and it showed a picture of what it looked like. It had the caption [something to the effect of]:
“Despite the collision, the stars are actually so far apart they simply pass through each other.”
I think we know now that while that is literally almost certainly true, there are still casualties. In the titanic clash of wills between two galactic gravity wells, that some stars are going to be seriously displaced, and probably with merging clouds of dust some others might be born.
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### Author:  [@MikeS](https://boards.straightdope.com/u/MikeS)
#### Post date: [March 10, 2016, 1:29pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/13 "2016-03-10T13:29:27Z")
> [@Earl\_Snake-Hips\_Tucker](#):
>
> I remember as a kid in the 1960s reading an article in some science publication about galaxies colliding, and it showed a picture of what it looked like. It had the caption [something to the effect of]:
>
> “Despite the collision, the stars are actually so far apart they simply pass through each other.”
>
> I think we know now that while that is literally almost certainly true, there are still casualties. In the titanic clash of wills between two galactic gravity wells, that some stars are going to be seriously displaced, and probably with merging clouds of dust some others might be born.
I hazily recall hearing an estimate that if two spiral galaxies (like the Milky Way) collided, there would be a few direct collisions between stars — but no more than you could count on both hands.
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### Author:  [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)
#### Post date: [March 10, 2016, 2:25pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/14 "2016-03-10T14:25:40Z")
There used to be a “tidal theory” of solar system formation, which proposed that a solar system formed after the rare near collision of two stars. This was once widely held, but is now long since discredited for reasons of rarity of near collisions and for orbital mechanical reasons.
This theory was in vogue (I suppose) when E.E. Doc Smith wrote his 'Lensman" series, where an epic battle for civilization takes place in two adjacent galaxies, both populated with many planets (as is only proper in space opera). In one of the books, his characters proposed that the galaxies had collided in the past, causing numerous near collisions and planet-forming events. They went on to muse about the consequences of alternate dimensions colliding, but gave up 'cuz it hurt their heads.
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### Author:  [@md2000](https://boards.straightdope.com/u/md2000)
#### Post date: [March 10, 2016, 6:56pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/15 "2016-03-10T18:56:30Z")
Pluto is just over 5 light-hours from the sun. From there, I think the sun looks like a small bright star. So imagine a star passing by half a light year away, or even 1/4LY. It would just be another star. Gravity, like light, falls off as the square of distance - so it would have to be a pretty close near miss to significantly disrupt the solar system.
Alpha Centauri A and B are stars like our sun, yet they are only very bright stars despite being only 4LY away.
I suspect inside a colliding galaxy, unless we went through a tight cluster or near the center, we would simply see a somewhat more dense nighttime sky, maybe dual milky ways…
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### Author:  [@Andy\_L](https://boards.straightdope.com/u/Andy_L)
#### Post date: [March 10, 2016, 7:13pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/16 "2016-03-10T19:13:25Z")
> [@Blue\_Blistering\_Barnacle](#):
>
> There used to be a “tidal theory” of solar system formation, which proposed that a solar system formed after the rare near collision of two stars. This was once widely held, but is now long since discredited for reasons of rarity of near collisions and for orbital mechanical reasons.
>
> This theory was in vogue (I suppose) when E.E. Doc Smith wrote his 'Lensman" series, where an epic battle for civilization takes place in two adjacent galaxies, both populated with many planets (as is only proper in space opera). In one of the books, his characters proposed that the galaxies had collided in the past, causing numerous near collisions and planet-forming events. They went on to muse about the consequences of alternate dimensions colliding, but gave up 'cuz it hurt their heads.
In the opening of “Triplanetary” (one of the Lensman books) there’s this quote: “Two thousand million or so years ago two galaxies were colliding; or, rather, were passing through each other. A couple of hundreds of millions of years either way do not matter, since at least that much time was required for the inter-passage. At about that same time—within the same plus-or-minus ten percent margin of error, it is believed—practically all of the suns of both those galaxies became possessed of planets.”
> [@md2000](#):
>
> Pluto is just over 5 light-hours from the sun. From there, I think the sun looks like a small bright star.
Actually from Pluto the Sun would be very bright - brighter than the full moon, and far brighter than any other object in the sky. You wouldn’t mistake it for an ordinary star - it would stlll be too bright to look at directly for any length of time.
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### Author:  [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)
#### Post date: [March 10, 2016, 8:34pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/17 "2016-03-10T20:34:13Z")
[quote=“Andy\_L, post:16, topic:748504”]
In the opening of “Triplanetary” (one of the Lensman books) there’s this quote: “Two thousand million or so years ago two galaxies were colliding; or, rather, were passing through each other. A couple of hundreds of millions of years either way do not matter, since at least that much time was required for the inter-passage. At about that same time—within the same plus-or-minus ten percent margin of error, it is believed—practically all of the suns of both those galaxies became possessed of planets.”
QUOTE]
I forgot about that part. I only had access to the last books of the series when I was a kid. It took me forever to find “Triplanetary”, and I did not feel well rewarded so I remember little of it.
“Spacehounds of the IPC”- that’s another story. One of my favorite books when I was young. I’m not saying it was better or worse, just that I liked it.
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### Author:  [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)
#### Post date: [March 10, 2016, 9:51pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/18 "2016-03-10T21:51:30Z")
> [@md2000](#):
>
> Pluto is just over 5 light-hours from the sun. From there, I think the sun looks like a small bright star. So imagine a star passing by half a light year away, or even 1/4LY. It would just be another star. Gravity, like light, falls off as the square of distance - so it would have to be a pretty close near miss to significantly disrupt the solar system.
>
> Alpha Centauri A and B are stars like our sun, yet they are only very bright stars despite being only 4LY away.
>
> I suspect inside a colliding galaxy, unless we went through a tight cluster or near the center, we would simply see a somewhat more dense nighttime sky, maybe dual milky ways…
I don’t know what it looks like from the inside, but in this NASA simulation it looks pretty awful from the outside.
[http://www.bing.com/videos/search?q=simulation+colliding+galaxies&&view=detail&mid=A531D0AD68BD3AE21CF9A531D0AD68BD3AE21CF9&rvsmid=A531D0AD68BD3AE21CF9A531D0AD68BD3AE21CF9&FORM=VDMCNL&fsscr=0](http://www.bing.com/videos/search?q=simulation+colliding+galaxies&&view=detail&mid=A531D0AD68BD3AE21CF9A531D0AD68BD3AE21CF9&rvsmid=A531D0AD68BD3AE21CF9A531D0AD68BD3AE21CF9&FORM=VDMCNL&fsscr=0)
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### Author:  [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)
#### Post date: [March 10, 2016, 11:50pm UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/19 "2016-03-10T23:50:13Z")
> [@Blue\_Blistering\_Barnacle](#):
>
> This theory was in vogue (I suppose) when E.E. Doc Smith wrote his 'Lensman" series, where an epic battle for civilization takes place in two adjacent galaxies, both populated with many planets (as is only proper in space opera). In one of the books, his characters proposed that the galaxies had collided in the past, causing numerous near collisions and planet-forming events. They went on to muse about the consequences of alternate dimensions colliding, but gave up 'cuz it hurt their heads.
But is that actually possible?
My impression was that if 2 galaxies collided, only 1 survived. It was augmented by pieces of the other that it had acquired, and some fragments of either might be flung loose to float off orphan-like in their own directions, but there wouldn’t be 2 separate galaxies remaining.
Is that a valid understanding?
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### Author:  [@dtilque](https://boards.straightdope.com/u/dtilque)
#### Post date: [March 11, 2016, 12:08am UTC](https://boards.straightdope.com/t/colliding-galaxies/748504/20 "2016-03-11T00:08:52Z")
> [@t-bonham@scc.net](#):
>
> But is that actually possible?
>
> My impression was that if 2 galaxies collided, only 1 survived. It was augmented by pieces of the other that it had acquired, and some fragments of either might be flung loose to float off orphan-like in their own directions, but there wouldn’t be 2 separate galaxies remaining.
>
> Is that a valid understanding?
That’s more or less correct. When two large spirals collide, the usual result is a giant elliptical. They won’t pass through each other because 1) unlike stars, gas and dust clouds will collide and 2) tidal effects. The result of (1) is that there will be a big surge of star formation during the collision. The result of (2) is variable, but the most picturesque is tidal tails, which happen early on and can even happen with just a close approach. Examples:
[Antennae Galaxies](https://en.wikipedia.org/wiki/Antennae_Galaxies)
[Mice Galaxies](https://en.wikipedia.org/wiki/Mice_Galaxies)
[Tadpole Galaxy](https://en.wikipedia.org/wiki/Tadpole_Galaxy)
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