# space - that final frontier
**URL:** https://boards.straightdope.com/t/space-that-final-frontier/600407
**Category:** Factual Questions
**Created:** [October 22, 2011, 12:11am UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407 "2011-10-22T00:11:52Z")
**Posts on this page:** 19
**Page:** 2
### Author:  [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)
#### Post date: [October 22, 2011, 6:47pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/21 "2011-10-22T18:47:30Z")
To go back to question 1 in the OP
What we view as a expansion velocity isn’t a real speed. Its a is caused by the expansion of space between the galaxies.
Thus the red shift is caused by the light-wave actually getting longer as that space grows increasing the wavelength.
Locally, galaxies and other bound objects are basically at rest.
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [October 22, 2011, 6:53pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/22 "2011-10-22T18:53:21Z")
> [@Grey](#):
>
> V of Sun/V of Space Sun resides in = 3.97324387 × 10-22
Since you’re looking at collisions, you need to look at the relative area per star, not relative volume. So Area[sub]star[/sub]/Area[sub]starSystem[/sub] = (3.97324387 × 10-22)^(2/3) = 5.4E-15.
Andromeda has about 10^12 stars, and the Milky Way about 3_10^11. Since Andromeda is bigger, we’ll assume all of the Milky Way passes through Andromeda. For each star in the Milky Way, it has a 5.4E-15 chance of colliding with one of Andromeda’s stars, based on the assumptions above. Since there are 3_10^11 stars, the chance of a collision is about 0.16 percent.
There are some big error bars in that calculation, but under a 1 percent chance seems likely.
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### Author:  [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)
#### Post date: [October 22, 2011, 7:36pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/23 "2011-10-22T19:36:02Z")
> [@Der\_Trihs](#):
>
> No. If during an explosion two chunks of shrapnel happen to collide with each other that doesn’t negate the fact that the explosion is in fact expanding.
And for a better analogy, don’t think of two chunks of shrapnel from the same metal shell casing. Instead think that some of the shrapnel chunks are metal, some wood, some plastic, etc. – variations in the mass & density of the chunks. So while they are all expanding outward from the explosion, some are moving faster or slower than other chunks, so they can run into each other.
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### Author:  [@robert\_columbia](https://boards.straightdope.com/u/robert_columbia)
#### Post date: [October 22, 2011, 10:50pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/24 "2011-10-22T22:50:05Z")
> [@automagic](#):
>
> The vastness of space is beyond human comprehension. The stars are so far apart that the possibility of one star colliding with another is so small you can call it zero.
>
> The Milky Way is 100,000 light years across and Andromeda is not much bigger. Assuming each galaxy has 100 billion stars with the average size of the star being the size of our sun(1,000,000 miles across) and the average distance between the stars is 4 light years (distance to Alpha Centurai) then the ratio of the average distance between the stars and the average size of a star becomes about 23,000,000.
>
> Imagine you are walking blindfolded in an extremely large field full of people but the average distance between the people is 4,300 miles (23,000,000 feet) than what are the chances that any one is going to collide with anyone?
But in our galaxy’s case, it won’t be like one person in that extremely large field of people. It will be BILLIONS of blindfolded people wandering into that field.
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### Author:  [@Candyman74](https://boards.straightdope.com/u/Candyman74)
#### Post date: [October 22, 2011, 11:27pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/25 "2011-10-22T23:27:54Z")
> [@robert\_columbia](#):
>
> But in our galaxy’s case, it won’t be like one person in that extremely large field of people. It will be BILLIONS of blindfolded people wandering into that field.
Yes; but there is still 4,300 miles between each of them. The large number of people in the field isn’t in question; it’s just that it’s a friggin’ large field!
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [October 23, 2011, 1:11am UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/26 "2011-10-23T01:11:33Z")
> [@robert\_columbia](#):
>
> But in our galaxy’s case, it won’t be like one person in that extremely large field of people. It will be BILLIONS of blindfolded people wandering into that field.
A bunch of people wandering around really isn’t that good of an analogy. Better would be billions of people at the South end of a field walking North, while Billions of people at the North end walk South. Then see my post above. The chance for any _one_ person running into someone coming the other way is around 5E-15, so even multiplied by billions, it’s still a small chance.
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### Author:  [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)
#### Post date: [October 23, 2011, 3:00am UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/27 "2011-10-23T03:00:02Z")
correct me if I’m wrong, but if two very big objects in space get close together, don’t they start orbiting each other, rather than bump into each other.
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### Author:  [@Bearflag70](https://boards.straightdope.com/u/Bearflag70)
#### Post date: [October 23, 2011, 3:16am UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/28 "2011-10-23T03:16:33Z")
Am I the only one who reads the thread title while picturing Joe Piscopo impersonating Frank Sinatra?
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [October 23, 2011, 3:22am UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/29 "2011-10-23T03:22:02Z")
> [@Peter\_Morris](#):
>
> correct me if I’m wrong, but if two very big objects in space get close together, don’t they start orbiting each other, rather than bump into each other.
If they don’t collide, they’ll keep going past each other with altered direction. They have too much energy to be bound, and won’t go into orbit unless there’s a third body involved to take away some of that energy.
> [@Bearflag70](#):
>
> Am I the only one who reads the thread title while picturing Joe Piscopo impersonating Frank Sinatra?
I’m going to go with “yes”.
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### Author:  [@njtt](https://boards.straightdope.com/u/njtt)
#### Post date: [October 23, 2011, 4:17am UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/30 "2011-10-23T04:17:10Z")
> [@Candyman74](#):
>
> I don’t have a cite handy, but I’m 99.999999% sure that the general view is that the galaxies in our local cluster will all eventually combine due to gravitational attraction.
Well, I think I’d rather see that cite than rely on your 99.999999% certainty, but even if true it does not really invalidate what I said. When you are talking about the universe, “eventually” is a very long time indeed, and until we get much closer to that time, the momentum of galaxies - which is huge, because they are incredibly massive and move very fast - is likely to have more effect on their motion than local gravitational interactions, which are relatively weak because, despite their huge mass, galaxies (even the ‘local’ ones like Andromeda) are also incredibly far away from one another. The strength of gravity falls off with the square of distance, remember.
There are some gravitational effects of course, and they will get stronger the closer that Andromeda gets to us, but while it is still very far away, their strength is small. Maybe it is true (I don’t know) that the local group is gravitationally bound together, rather than being just a random collocation. (It is my understanding that the nearby dwarf galaxies, such as the Magellanic clouds, _are_ gravitationally bound to, i.e., orbiting around, the Milky Way, but Andromeda, though in the ‘local group’ is a lot further away than they are, and has a lot more momentum.). However, even if that is so, I still maintain that the principal reason that Andromeda is coming towards us is not the relatively tiny mutual gravitational attraction that exists between it and the Milky way, but the fact that, in its random motion (and ours), it just happens to be coming this way.
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### Author:  [@Candyman74](https://boards.straightdope.com/u/Candyman74)
#### Post date: [October 23, 2011, 5:05pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/31 "2011-10-23T17:05:54Z")
> [@njtt](#):
>
> Well, I think I’d rather see that cite than rely on your 99.999999% certainty, but even if true it does not really invalidate what I said. When you are talking about the universe, “eventually” is a very long time indeed, and until we get much closer to that time, the momentum of galaxies - which is huge, because they are incredibly massive and move very fast - is likely to have more effect on their motion than local gravitational interactions, which are relatively weak because, despite their huge mass, galaxies (even the ‘local’ ones like Andromeda) are also incredibly far away from one another. The strength of gravity falls off with the square of distance, remember.
>
> There are some gravitational effects of course, and they will get stronger the closer that Andromeda gets to us, but while it is still very far away, their strength is small. Maybe it is true (I don’t know) that the local group is gravitationally bound together, rather than being just a random collocation. (It is my understanding that the nearby dwarf galaxies, such as the Magellanic clouds, _are_ gravitationally bound to, i.e., orbiting around, the Milky Way, but Andromeda, though in the ‘local group’ is a lot further away than they are, and has a lot more momentum.). However, even if that is so, I still maintain that the principal reason that Andromeda is coming towards us is not the relatively tiny mutual gravitational attraction that exists between it and the Milky way, but the fact that, in its random motion (and ours), it just happens to be coming this way.
I found this:
> **[Guide to the Universe: Stars and Galaxies](https://books.google.co.uk/books?id=BuyIHbwsm0sC&pg=PA165&lpg=PA165&dq=andromeda%27s%2Bmotion&source=bl&ots=jQKBubZY_r&sig=uqI-OqkzY3zRtJtqPnV3OAppLsE&hl=en#v=onepage&q=andromeda's%20motion&f=false)**
>
> This up-to-date volume offers student researchers an unexcelled primer on current scientific knowledge about stars.This volume in the Greenwood Guides to the Universe series provides the most up-to-date understanding available of the current...
“Our own Milky Way is a member of an interacting pair of galaxies.”
“Because of its proximity to the Milky Way, its motion towards us is significant.”
I’m inclined to see that as supportive of a gravitational effect which is greater than you posit.
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### Author:  [@cjepson](https://boards.straightdope.com/u/cjepson)
#### Post date: [October 24, 2011, 2:29pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/32 "2011-10-24T14:29:57Z")
> [@Candyman74](#):
>
> So the galaxies move towards each other at 50mpg
My uncle had a [Galaxie](http://en.wikipedia.org/wiki/Ford_Galaxie) and it didn’t get anywhere near 50 mpg.
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### Author:  [@cjepson](https://boards.straightdope.com/u/cjepson)
#### Post date: [October 24, 2011, 2:31pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/33 "2011-10-24T14:31:21Z")
> [@Bearflag70](#):
>
> Am I the only one who reads the thread title while picturing Joe Piscopo impersonating Frank Sinatra?
No, it would have to be “Space – that final frontier, Jack”.
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### Author:  [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)
#### Post date: [October 24, 2011, 4:21pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/34 "2011-10-24T16:21:46Z")
> [@ZenBeam](#):
>
> Since you’re looking at collisions, you need to look at the relative area per star, not relative volume. So Area[sub]star[/sub]/Area[sub]starSystem[/sub] = (3.97324387 × 10-22)^(2/3) = 5.4E-15.
>
> Andromeda has about 10^12 stars, and the Milky Way about 3_10^11. Since Andromeda is bigger, we’ll assume all of the Milky Way passes through Andromeda. For each star in the Milky Way, it has a 5.4E-15 chance of colliding with one of Andromeda’s stars, based on the assumptions above. Since there are 3_10^11 stars, the chance of a collision is about 0.16 percent.
>
> There are some big error bars in that calculation, but under a 1 percent chance seems likely.
Did you calculate p as 5.4e-15 and just multiply it by i=3e11? That’s not the right way to do it. You need to do O=1-(1-p)^i. The former comes out to .162. The latter comes out to .163.
The difference isn’t a big deal because the numbers are huge/infintessimal, but the formula is. I’m noting it in case someone suggests other numbers to plug in.
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### Author:  [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)
#### Post date: [October 24, 2011, 4:34pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/35 "2011-10-24T16:34:00Z")
Also, I don’t know if we can really count the stars as independent events. Imagine if the “I” galaxy collided with the “X” galaxy as below:
```auto
I --> <-- X
I --> <-- X
I --> <-- X
I --> <-- X
```
These two galaxies would have a 100% chance of one star hitting another. However, if they’re like this:
```auto
I I -->
<-- X X
I I -->
<-- X X
```
Then there’s a 0% chance.
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### Author:  [@dracoi](https://boards.straightdope.com/u/dracoi)
#### Post date: [October 24, 2011, 7:23pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/36 "2011-10-24T19:23:15Z")
> [@Chessic\_Sense](#):
>
> Also, I don’t know if we can really count the stars as independent events.
I think stars are close enough to random placement to treat them as independent events. Even if you intentionally lined up stars, you’re still talking about enormous gaps between them and it would take tremendous accuracy to get them exactly right. Using the 1 person per 4300 km analogy, an inaccuracy of just 0.001% would mean you’re trying to hit a 1-meter target with a 43-meter margin of error.
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### Author:  [@Quercus](https://boards.straightdope.com/u/Quercus)
#### Post date: [October 24, 2011, 7:49pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/37 "2011-10-24T19:49:57Z")
> [@Peter\_Morris](#):
>
> correct me if I’m wrong, but if two very big objects in space get close together, don’t they start orbiting each other, rather than bump into each other.
Only if they’re moving slowly. If they’re trucking right along, they might bend slightly toward each other, but keep going mostly in their original directions.
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### Author:  [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)
#### Post date: [October 24, 2011, 8:14pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/38 "2011-10-24T20:14:42Z")
How slowly are we talking about? And what if their original directions are a direct collision course?
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [October 24, 2011, 9:40pm UTC](https://boards.straightdope.com/t/space-that-final-frontier/600407/39 "2011-10-24T21:40:41Z")
> [@Chessic\_Sense](#):
>
> Did you calculate p as 5.4e-15 and just multiply it by i=3e11? That’s not the right way to do it. You need to do O=1-(1-p)^i. The former comes out to .162. The latter comes out to .163.
No, I calculated _p_, multiplied by _i_, then observed that the exact method would be a tiny correction to a number that I only trust to an order of magnitude anyway. (And note that it’s 0.16 _percent_, or 0.0016.)
As mentioned above, the stars being not equally spaced means that we can consider the collisions to be independent events. A correction _could_ be made for the possibility of the two cores lining up. In that case, the higher densities lining up would make the probability of collision larger.
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