Andromeda/Milky Way collision

nails1

Hi guys, when the Milky Way and Andromeda eventually come into contact, will there be any collisions between the planets and stars from each galaxy or would the huge distance between these celestial objects mean that no such incident could even potentially occur?

shedweller

I saw something on this before and it was said that there would be almost nothing happening that would change your day to day living. More lightshows at night, slightly increased radiation and thats about all afaik.
Which is just downright weird considering the size of both objects. I suppose it has to be remembered that the photos we see of them are long exposures and in reality they are much dimmer. Galaxies are mostly empty space.

Mr Benevolent

Very little would happen. Galaxies are 99.9999999999999999999% empty space.

who_ru

nails1 wrote: »

Hi guys, when the Milky Way and Andromeda eventually come into contact, will there be any collisions between the planets and stars from each galaxy or would the huge distance between these celestial objects mean that no such incident could even potentially occur?

doubt anyone will be around to see it.

2 stroke

If the universe is expanding outward from the big bang, haw can something as big as a galaxy be heading our way?

tricky D

The sum of the Universe is expanding, yes, but that doesn't mean that all its components are uniformly receding from each other. Discrete components are attracted due to gravity all the time, which is why we have galaxies, stars, solar systems, planets, etc... These components would not have happened under your assumption.

In terms of physics laws: the entropy of the Universe as a whole increases, but not necessarily in a uniform way. Localised entropy can decrease.

Captain Chaos

The two supermassive blackholes at the centre of each galaxy will collide and merge to from an even larger blackhole. That should cause some chaos. But as said above, by the time that happens the sun will have entered it's final stages of life in its red giant phase and destroyed all life on Earth in the process by stripping away the atmosphere and boiling off the oceans.

ps200306

nails1 wrote: »

Hi guys, when the Milky Way and Andromeda eventually come into contact, will there be any collisions between the planets and stars from each galaxy or would the huge distance between these celestial objects mean that no such incident could even potentially occur?

We could do a quick back of an envelope calculation. The stellar disk of the Milky Way is often cited as having a radius of 50,000 light years. Actually, it doesn't have an edge as such, it just gets sparser and sparser. But suppose we just averaged the stars out across such a disk.

The sun's nearest neighbours are about four light years away. Suppose we surround each star with a bubble of two light years radius and spread them, one thick, across the disk of the Milky Way. That way, we would get roughly two thirds of a billion stars.

Well, that's about 300 times too few, according to some estimates of stars in the galaxy. We have two problems. First the stars aren't evenly distributed across the galactic disk -- they are denser toward the centre. Second, the disk isn't infinitely thin, nor is the thickness uniform. In fact, it isn't even the same thickness for all types of stars -- the disk also fades out vertically with a different "scale height" for different types of stars. (Bigger ones die before they can travel too far from where they were born).

So instead of all that complication, let's just paste the stars uniformly across an infinitely thin sheet. Let's assume that Andromeda is going to pass face-on through that sheet, in which case a projection of all the stars into a single plane is a reasonable assumption. (Of course, Andromeda could pass edge on through a narrow stripe of our galaxy, vastly increasing the chances of collision. In practice, a galaxy isn't a rigid object so neither extreme is likely. And Andromeda has two or three times as many stars as the Milky Way, but we'll ignore that).

So - we have 200 billion stars distributed across roughly 8 billion square light years, for a cross sectional area of 0.04 square light years per star on average. Converting to square miles, that's about 1.4 trillion trillion square miles (24 zeros).

Our sun is something less than a million miles across. Most stars are smaller. On the other hand, what are we calling a "collision" -- you wouldn't need direct contact to have massive tidal disruption. Lets just stick a finger in the air and take the square of a million miles as the collisional cross section for our average star. That a nice round trillion square miles.

Divide that into our galactic disk's area per star and we get 1.4 trillion. That's the odds against a star colliding with another star randomly tossed into it's patch of the galactic disk. Now divide by the number of stars. We end up with odds of only 7 to 1 against.

However, we already figured that the stellar density in our region of the galaxy is less than the galactic average by 300 divided by the number of stars "thickness" in our region of the disk. So it's likely that the odds are greatly lower in our vicinity, and even more so for the far outer reaches of the disk, and greatly higher for the denser, inner regions.

So while I've heard it said that collisions are very remote possibilities, I don't think the figures bear it out. Collisions near the galactic centre are very likely indeed. But it'll be only a handful out of many hundred of billions of stars. The odds for any given star are orders of magnitude worse than winning the lottery. And there'll be a lot more new stars than banjaxed crashed ones. Depending on how much gas is left in our galaxy for forming new stars at that time, the disruption caused by the galaxy collision could trigger lots of gas clouds to collapse, causing a "star burst".

Captain Chaos

Captain Chaos wrote: »

The two supermassive blackholes at the centre of each galaxy will collide and merge to from an even larger blackhole. That should cause some chaos. But as said above, by the time that happens the sun will have entered it's final stages of life in its red giant phase and destroyed all life on Earth in the process by stripping away the atmosphere and boiling off the oceans.

Don't know about that. Time frames being what they are it is plausible that either SMBH may no longer exist. . . . . Lots of stuff on the birth of BH's. . . Not heard much postulated about their demise. . . . .

Still searching for gravitational waves. Primordial BH's and much more. My guess is that as instrumentation improves its quite likely that in 100 years much of what must be taken at face value in physics today will be seen as misinterpretation. Understandable given the limitations of our science at this time. Although technology seems to advance rapidly it can never advance fast enough to keep up with human inqusitivness.:)

ps200306

Deleted User wrote: »

Don't know about that. Time frames being what they are it is plausible that either SMBH may no longer exist. . . . . Lots of stuff on the birth of BH's. . . Not heard much postulated about their demise. . . . .

Still searching for gravitational waves. Primordial BH's and much more. My guess is that as instrumentation improves its quite likely that in 100 years much of what must be taken at face value in physics today will be seen as misinterpretation. Understandable given the limitations of our science at this time. Although technology seems to advance rapidly it can never advance fast enough to keep up with human inqusitivness.:)

Good points, but some of the evidence for BHs is very compelling. In our own galaxy the orbits of central stars indicate 4 million solar masses confined within a volume that would be difficult to explain without a BH.

Distant quasars cannot be observed photometrically in the same way, but reverberation mapping (i.e. looking at how long it takes different parts of the ionisation stratified accretion disk's spectrum to light up when the quasar brightens) indicates 10 to 1000 million solar masses in a region no more than a few light days wide.

And our present understanding of the physics is that black holes can only disappear by Hawking radiation evaporation ... something that will take trillions, rather than billions of years. (Actually, trillions is a massive understatement -- for the supermassive quasar black holes it would be more than ten to the power of ninety years).

Rubecula

Steven Hawking does an interesting series of shows on the Discovery channels explaining this happening.

Apparently the reasons that galaxies will come together is not gravitational They are all moving about and collisions occur between members of local groups. Bigger galaxies are usually lenticular types which it is believed is what will happen to the Milky Way/ Andromeda joint galaxy.

(Galaxies should by all reasoning fly apart anyway, but it is believed Dark Matter binds them. No Super Massive Black Hole known has enough gravity to hold it's galaxy together alone. .... apparently anyway)

ps200306

Rubecula wrote: »

Apparently the reasons that galaxies will come together is not gravitational They are all moving about and collisions occur between members of local groups.

I wonder is meant by that. The universe is expanding and the expansion carries the galaxies away from each other. On smaller scales, gravity binds the galaxies together and makes them move together. There isn't any other force in operation. I suppose what might be meant is that the clusters of galaxies are "virialised" -- moving under gravity they have attained an equilibrium in which there is an overall balance of kinetic and gravitational potential energy. It's still all down to gravity though.

ps200306

ps200306 wrote: »

I wonder is meant by that. The universe is expanding and the expansion carries the galaxies away from each other. On smaller scales, gravity binds the galaxies together and makes them move together. There isn't any other force in operation. I suppose what might be meant is that the clusters of galaxies are "virialised" -- moving under gravity they have attained an equilibrium in which there is an overall balance of kinetic and gravitational potential energy. It's still all down to gravity though.

I don't doubt the existence of BH's for a minute. They are there all right. Hawking radiation is S.H.s postulation about how the B.H. must go through entrophy and meet a condition of the Laws of Thermodynamics I presume. As an idea it is new enough (as are all human ideas) and only an idea/theory/supposition. 99.9% of an atom is nothing. 99.9% of each of us is empty space.
I presume about 99.9% of space is also nothing.

Figuring out just exactly what nothing is, that is the big conundrum for physics. When (and if) we are able to pin down exactly what nothing is, then we might have a better understanding of the universe/multiverse whatevereverse.

The observable universe is approx 13.5 to 15 billion years old. So we are continually told, without a proper emphasis being placed on the word "Observable".
To use this as a milestone for the size or age of the universe is a crazy supposition.
Limitations of astronomical/observational instrumentation mean effectively that there are no known limits to the size of the universe so in my book that means that there is no known age only an age older than a given specific, i.e. as far as we can judge light has travelled from. Perhaps one day when photons/ quanta are better understood then too, we will have a better idea.

Our laws are based on our observations and empiricism, however is is quite plausable that as the body of knowledge increases these laws and theories will become obsolete (e.g. Newtonian gravity to Einsteinian gravity to the conflicts between classical physics and the quantum world).

As far as I know there is no such thing as matter, matter is merely our perception of the various effects of known and unknown forces and field theories.

I have even heard various contradictions as to the true accuracy/validity and interpretation of the various red shifts in relation to the expansion of the universe.

So I guess if I could say anything it would be that insofar as Physics goes, nothing is a done deal.

It is a scientific given that xx + rr + aa = y.

Decades later we take y, extrapolate it into a new equation and hey presto we have a new insight : that y + our new equation = u.

We took the old givens... no one bothered with better science and tech to go back rechecking whether xx + rr + aa actually did = y.

So therefor u is actually completly untrue.

Laziness/costings equals false positives................. l+c = fp :):)

Rubecula

Rubecula wrote: »

Steven Hawking does an interesting series of shows on the Discovery channels explaining this happening.

Apparently the reasons that galaxies will come together is not gravitational They are all moving about and collisions occur between members of local groups. Bigger galaxies are usually lenticular types which it is believed is what will happen to the Milky Way/ Andromeda joint galaxy.

(Galaxies should by all reasoning fly apart anyway, but it is believed Dark Matter binds them. No Super Massive Black Hole known has enough gravity to hold it's galaxy together alone. .... apparently anyway)

Spot on the button............

ps200306

I don't think there was ever any suggestion that a supermassive black hole held its host galaxy together by gravity. Even the very most massive are only a fraction of a percent of the mass of the host galaxy. It is the mutual gravity of all the galaxy's contents that holds it together. Nowadays that is believed to also include a dark matter halo ... but the dark matter's contribution is still by gravity.

ps200306

ps200306 wrote: »

I don't think there was ever any suggestion that a supermassive black hole held its host galaxy together by gravity. Even the very most massive are only a fraction of a percent of the mass of the host galaxy. It is the mutual gravity of all the galaxy's contents that holds it together. Nowadays that is believed to also include a dark matter halo ... but the dark matter's contribution is still by gravity.

Perhaps, perhaps not.

There is a suggestion that the total gravitational effect of all mass within any given galaxy is noticibly greater that that which would/should account for the galaxy holding its shape for any great length of time (although what "any great length of time" actually is, is anyone's guess) when in actual fact it should (the galaxy) be compressing/shrinking in size much more appreciably and that some of the "nothing" as I like to call it is actually anti-gravity which has yet to be detected and which too is playing its part in the cosmic game.
i.e. Stablising the galaxy and stopping it from folding in on itself.

Just playing Devils Advocate.:)

Sorry ps....

I was replying to the u/m insofar as I do not doubt the existence of B.H.'s for a minute.

"Good points, but some of the evidence for BHs is very compelling. In our own galaxy the orbits of central stars indicate 4 million solar masses confined within a volume that would be difficult to explain without a BH".

ps200306

Deleted User wrote: »

Perhaps, perhaps not.

There is a suggestion that the total gravitational effect of all mass within any given galaxy is noticibly greater that that which would/should account for the galaxy holding its shape for any great length of time (although what "any great length of time" actually is, is anyone's guess) when in actual fact it should (the galaxy) be compressing/shrinking in size much more appreciably and that some of the "nothing" as I like to call it is actually anti-gravity which has yet to be detected and which too is playing its part in the cosmic game.
i.e. Stablising the galaxy and stopping it from folding in on itself.

Just playing Devils Advocate.:)

Any references for that anti-gravity suggestion? The reason I ask is that we haven't even detected the dark matter that is supposed to hold the galaxy together and explain its rotation rate. We've just inferred how much is there from how much is needed. So I don't see how we could simultaneously think there is too much mass, when we infer the dark matter from the fact that there is too little.

ps200306

ps200306 wrote: »

Any references for that anti-gravity suggestion? The reason I ask is that we haven't even detected the dark matter that is supposed to hold the galaxy together and explain its rotation rate. We've just inferred how much is there from how much is needed. So I don't see how we could simultaneously think there is too much mass, when we infer the dark matter from the fact that there is too little.

As a genreal rule of thumb and as can be seen with the naked eye most of the sky contains nothing. What there is, is mostly "clumped" which we can see with telescopic aid.

I personaly cannot confirm or deny galatic ratio's viz a viz the amount of "matter", the density of galaxies within the universe, the size and gravitational effect of a SMBH, the amount of "Dark Matter" but more importantly the type and/or effect produced by various units of "Dark Matter" and lastly the importance and existence or non existence of "Dark Energy" and its effects, heretofore not mentioned.

For this I have to rely on the academics and the scientists.

What we cannot measure we suppose, this gives us supposition and a good deal of Theoretical Physics relys on supposition.


Was Democtitus right, was Arsistarcus right was Copernicus right was Gallileo right ? All were correct up to a point. (excuse my spelling, not sure if correct)

Was Johannes Keppler right ?

Yes until Newton.

Was Newton right ?

Yes until Einstien ?

Was Einstien right ?

Yes until Quantum Mechanics ?????

Is M Theroy right ?

Not sure we will ever know in my lifetime.

There may well be a limitation to even the comprehension of the most talented among us which means in effect the "Theory of Everything" will always be elusive because the power of human comprehension has a limit.

What I would try to say is that Science (i.e. knowledge) is empirically and observationally based. The empiricism is evidence, the observation not. Observation also requires no less an unbiased observer than does empiricism.

Who amongst us is truly unbiased. All I am trying to say is that to question accepted belief and methodology is good.

All that I can do is to relate the suppositions of acedemics. As to reference you might try "The eloquent universe", a book on M theory, I think it was postulated in that, it's been a long time since I read it. Brian Greene, Author.

And I am not suggesting for a minute that M theory has a claim on reality, even if we knew what that was (reality that is, not M theory, however I lay claim to knowing little about either:))

“The fact that we live at the bottom of a deep gravity well, on the surface of a gas covered planet going around a nuclear fireball 90 million miles away and think this to be normal is obviously some indication of how skewed our perspective tends to be.”

Douglas Adams.

My point exactly.............. Supposition.

"dark matter that is supposed to hold the galaxy together and explain its rotation rate."

ps200306

Deleted User wrote: »

All that I can do is to relate the suppositions of acedemics. As to reference you might try "The eloquent universe", a book on M theory, I think it was postulated in that, it's been a long time since I read it. Brian Greene, Author.

It's called "The Elegant Universe" ... I've read it. I'm pretty sure there is no suggestion in there that there is an antigravity force required to explain how galaxies don't collapse. Again, as I say, I can see how we would not easily be able to count up all the matter in the galaxy, although the upcoming Gaia mission will be having a decent stab at it. But I definitely don't see how we can be short of mass and at the same time have too much mass to explain the behaviour of the galaxy. Doesn't it have to be one or the other?

ps200306

ps200306 wrote: »

It's called "The Elegant Universe" ... I've read it. I'm pretty sure there is no suggestion in there that there is an antigravity force required to explain how galaxies don't collapse. Again, as I say, I can see how we would not easily be able to count up all the matter in the galaxy, although the upcoming Gaia mission will be having a decent stab at it. But I definitely don't see how we can be short of mass and at the same time have too much mass to explain the behaviour of the galaxy. Doesn't it have to be one or the other?

OOps your right, Elegant it was, a long time since I read it. Apologies.

Anyway getting back on track... If not in there then I am struggling to recall but if you can remember all the topics and points covered in that book fair play.... however it was a definite supposition by certain academics, who I cannot recall off the top of my head if not that particular book.....

I guess it all depends on who you believe and which science is popular at a given time in history, and sometimes swimming against the current is not so easy.

Me, I just like to read and watch as the science progress's.....

I don't know if it has to be one or the other. Perhaps both are a finely balanced act. Perhaps one or the other has little effect or does not even exist.

As I say, just playing devils advocate and relating what I have read elsewhere.....Sometimes it pays off when we run counterintuitivly.

Anyway keep well and most important of all, keep smiling ......:)

ps200306

A theory of Modified Newtonian Dynamics, combined with new data on a possible previous collision of the Milky Way and Andromeda, rules out dark matter halos on the basis that they would make the galaxies "stick". Milky Way satellite galaxies, rather than being gradually accreted, are reinterpreted as debris objects.

Article here, paper here.

ps200306

ps200306 wrote: »

A theory of Modified Newtonian Dynamics, combined with new data on a possible previous collision of the Milky Way and Andromeda, rules out dark matter halos on the basis that they would make the galaxies "stick". Milky Way satellite galaxies, rather than being gradually accreted, are reinterpreted as debris objects.

Article here, paper here.

On the way to the links..... thanks for that.
I am a complete layman here but I always take each scientific proposition with a pinch of salt.......Richard Feynman would like my attitude but perhaps not my lack of scientific knowledge......

It is not so long ago that any type of life existing elsewhere in the universe was thought to be preposterous outside of sci-fi................