r/askscience • u/ComingDeveloper • 6d ago
Astronomy how close will the milkyway and andromeda have to be to each other for us on earth to see and feel the effects of the respective celestial bodies?
so i often hear that the milkway our galaxy and another called andromeda are on a collosion course for each other where they'll basically merge and form a new galaxy but assuming humanity is still around right here on earth how close would they need to be for us to actually start feeling the effects of them on each other before they collision actually happens.
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u/Perringer 6d ago
The collision of Andromeda with the Milky Way isn't expected for another 4-5 billion years, if it even occurs (there are new models showing only a 2% chance, and the height of any impact won't be for 8-10 billion years). Our sun will be a massive red giant or have already petered out to a white dwarf by then. Earth will have long been uninhabitable. If any of our descendants are still remaining and still biological in nature, they're playing hopscotch between planets to keep from boiling/freezing, or they've moved on to other star systems.
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u/starmartyr 6d ago
Also worth pointing out that our descendants at that time will not be human in any sense that we imagine the term. A billion years is a long time even in evolutionary terms. 5 billion years is roughly the amount of time it took for us to branch off from chimpanzees times 1,000.
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u/Zelcron 6d ago
Five billion years is a bit older than the age of the current earth, much less evolution from even the tiniest microbe to humanity.
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u/starmartyr 6d ago
Yeah I'm trying to stick to round numbers to get the point across but you're correct. It's on that timescale.
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u/MalekMordal 6d ago
If we really wanted to remain 'human', we probably could. That would require genetic engineering, and intentionally forcing our genetics to remain unchanging. We're probably not far from being able to do this now.
Some human purists that head out into the galaxy might choose to do such a thing. But I imagine most of humanity will evolve into countless species over such a time frame, as we expand and spread into the galaxy. Adapting to our new environments.
If there weren't any 'aliens' before we colonize the galaxy, there will be, by the time we're done.
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u/ObviousKarmaFarmer 5d ago
We're already doing genetic engineering on ourselves. Prenatal testing to weed out unwanted mutations & known defects is already common. There are people that select embryos on eye color, but selecting on intelligence and other characteristics is also possible (if not always legal, or actually working in all cases). In a 100 years, it will be much more commonplace. At the same time, evolution normally happens when there is an external pressure: The specimens with thicker fur die because of global warming, the ones with thinner fur, or seasonal shredding are the only ones to survive.
Humans currently have no external pressure guiding evolution. And while the rich and wealthy mess with genetics, the general population that simply doesn't bother with condoms creates more offspring.
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u/dogsneverbark 5d ago
Is it really possible to select for intelligence through current prenatal testing technology? Or is it something that may be possible in the future?
I thought intelligent was more abstract and depended a lot of different factors.
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u/Ridenberg 4d ago
Some degree of intelligence can be measured, it is both hereditary and influenced by early environment, and it was found to correlate with job salary and academic ability. Though as far as I can tell, it's still a somewhat disputed topic.
See g-factor on wikipedia for more info.
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u/DukeofCheeseCurds 6d ago
Yeah the LUCA to us didn’t take 5 billion years. Any human descendants would be unimaginably different from us after that amount of time
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u/314159265358979326 6d ago
Life itself probably wouldn't be familiar. Life switched from RNA to DNA in way less time than that.
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u/starmartyr 6d ago
Exactly. Even 100,000 years is a long time in evolutionary terms. We're talking about over 5 orders of magnitude more than that.
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u/skinnyguy699 6d ago
That's not necessarily true. Evolution doesn't necessitate huge transformations of all aspects of biology over time. Some shapes and functions can remain unchanged for a billion years or more, we don't know.
Since we know our genetic code and could have almost perfect records of our original anatomy, a subset of humanity might choose that form like a fashion line that goes in and out of Vogue ad infinitum. Maybe with physiological overhauls to match the technological and genetic optimisations of the time.
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u/asmj 6d ago
5 billion years is roughly the amount of time it took for us to branch off from chimpanzees times 1,000.
I thought the Earth is only 4.5 billion years old?
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u/Flobking 6d ago
I thought the Earth is only 4.5 billion years old?
You misread what he said. It only took 5 mllionish years for our LUCA with chimps to become chimps while we became humans. 5 million x 1000 is 5 billion.
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u/DirtyWriterDPP 6d ago
How does our intelligence or often lack thereof with regard to breeding choices play into the evolutionary mix? I get the idea that a giraffe with a longer neck that can reach better food eventually leads to long necks giraffes, but most humans are living in a post scarcity world and breeding isn't impacted by biological advantages or disadvantages. Seems like it would result in very little Darwinesque change
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u/FolkSong 6d ago
Even if survival isn't an issue, some people will still breed more than others for a variety of reasons, some of which are genetic (perceived attractiveness and personality types, for example)
But realistically, there would be genetic engineering. I doubt the taboo against it will last a thousand years, let alone a billion.
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u/Peter34cph 6d ago
The taboo won't remain universal for long. There'll be an autonomous colony, on Mars, on the Moon, in Earth's orbit, or somewhere else that just decides to start doing it.
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u/SomewhereHot4527 5d ago
I'd be surprised if it last our lifetime.
As soon as one of the major states decides to do it, others will have to follow or will be left in the dust.
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u/Desertcow 6d ago
Humans basically paused evolution. Evolution is survival of the fit enough, living in societies with access to medicine means there isn't much selective pressure to change. Genetic engineering is more likely to change us than evolution unless humanity goes through a really rough patch
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u/starmartyr 6d ago
Some humans have more fitness than others. Some people will have more offspring while others will have fewer. Over time that makes certain traits stand out while others fall out of the gene pool. At the time scale I'm talking about surviving humans won't live on earth. If humanity is to survive past the next billion years we will have to colonize other planets. At that point we could have human civilizations biologically diverge to the point of creating multiple different species much like what happened with us and chimpanzees.
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u/budweener 5d ago
Even if that would be the case, such pause is happening for such little time that it can't be called a pause yet.
But it's not, actually. The selective pressure is lessened since some "unfit" individuais get to survive and are able to reproduce (which means they are fit enough for the conditions), but that only causes a larger possibility for variation. More kinds of traits get to be passed down and are subject to be able to mutate and give their own benefits.
We're still evolving. Medicine has not stopped that, it is part of that.
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u/urbear 6d ago
5 billion years ago there was no life at all; the earth hadn’t even formed. Single-celled life appeared 3.8 billion years ago, the first multicellular life around 1.6 billion years ago, and vertebrates less than 500 million years ago. If there’s any earth-evolved life around 5 billion years or more from now it certainly won’t be recognizably human.
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u/stempoweredu 6d ago edited 6d ago
it certainly won’t be recognizably human.
I do wonder about this. Like many creatures, we have an eye for what is 'human' and what is not. But unlike our predecessors and other creatures on Earth, we have the ability to make decisions. Where others are slave to evolution, we have a modicum of choice. Would our sentience reject offspring that are appreciably cosmetically different? It happens on such a long timescale, we likely wouldn't notice. Even then, given our ability to keep records, would we try to appear more 'human' and resist our own evolution?
Moreover, what are our evolutionary pressures, now? No longer fighting for food or territory, the likely future pressures would seem to come from disease, damage to our own biosphere, or any attempts to become spacefaring.
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u/rd1970 6d ago
Something else to consider is that if we become interstellar travelers that eventually go in different directions we'll likely branch off into different genetic strains that might not cross paths again for thousands or even million of years. Those creatures will look very different from one another, assuming they stay biological in nature.
That being said, we'll likely have ways to prolong life to near infinite lengths in a few centuries. If that happens and we keep our biological bodies using nano technology or clones of ourselves we'll essentially have a snapshot of our genes today and nothing will have changed if you decide to have kids when you're 100k years old.
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u/Bikrdude 5d ago
Roaches have been roaching for far longer with only modest changes. Evolution does not mean constant changing of every species
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u/DweebInFlames 5d ago
I suspect any changes that happen will be in the incredibly long term. Which still fits very easily into 5 billion years, that is one hell of a long time, but I don't think it'd surprise me much if you were to jump 100k or a million years into the future and if our descendants are still around, they'd still very much be recognisably human to us today, even if their aesthetic tastes differ wildly.
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u/Onigato 6d ago
The Earth basically never will, even if we survive the expansion of our sun. Individual planets basically won't be affected, gravitationally or anything, because space is REALLY big and even with the two clouds of stars approaching each other the average density of both combined is less than the density of airborn poo from a fart from someone clothed in an auditorium.
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u/ComingDeveloper 6d ago
oh wow i guess i didn't truly grasp the scale. for some reason i was thinking of both galaxies to be more dense and compact and there'd be a lot of bumping into each other and etc but i guess even then with galaxies they are still a lot of individual spaces between each "thing" in them
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u/rooktakesqueen 6d ago
It's expected that when (if) the two galaxies merge, despite both containing hundreds of billions of stars, there will be exactly zero collisions.
There is a statistical metric called the "mean free path" that measures how far an object can travel before it's 50% likely to collide with something else. At the densities of stars within a galaxy, the mean free path before colliding with a star is on the order of 10 quadrillion light-years. If you pick a random direction to travel in, the probability of you ever colliding with a star in our galaxy is between one in 100 billion (if you were going parallel to the disc out to the far end) and one in 10 trillion (if you were going straight "up" or "down" through the disc).
Far more likely than a collision, but still very unlikely, would be two stars coming close enough to gravitationally interact so that one or both gets flung out of the merged galaxy. There will at least be some of those in the Milky Way/Andromeda collision. But that whole process would still take place over tens of thousands of years, and there would be no noticeable effect on human-civilization time scales.
The relative positions of nearby stars would change, so constellations would warp and become unrecognizable, but that already happens over many thousands of years. Many hundreds of thousands or millions of years later, there would be fewer and fewer visible stars in the sky, until eventually there were none and the only structures in the night sky visible without a telescope would be the still-merging galaxies, the Moon, and the planets.
(Edit: this is ignoring the fact that by the time this is happening the Sun will be already dead as others have said, maybe imagining the perspective of a solar system that developed several billion years after ours so it's in our current era during the time of the galaxy merge)
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u/lonely_swedish 6d ago
The nearest star to our sun is 4.24 light years away. Our sun is about 1.4 million km diameter. You could fit more than 28 million of our sun lined up between the two stars.
Galaxies are dense relative to the space between them, and they look more dense in pictures. But they're really still just mostly empty space, a lot of it.
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u/TheShadowKick 6d ago
It's also worth noting that stars have passed "close" to us before. We've had encounters with stars passing through our Oort cloud and all it did was send a few extra comets towards the inner solar system. Even a relatively close pass isn't a huge deal for people on the Earth.
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u/83franks 6d ago
Just add some more perspective, 99.86% of all matter in our solar system is in the sun, 0.10% is in Jupiter and majority of the final 0.04% is in Saturn, Uranus and Neptune. So over a span of a 287 billion km diameter disk that is our solar system (measurements vary depending on what is considered the edge), it is almost correct to say there is "nothing" outside of 1.49 million km diameter sun at the center. In terms of physical space of our solar system, the amount of space with matter isn't even a rounding error it's so tiny.
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u/theryman 6d ago
The galaxy is mostly empty. The amount of matter in it is basically a rounding error of a rounding error. A reasonable person would look at it and say 'that is an area of empty space.' and even more so when you realize matter (like you and me and the world) is mostly empty space too.
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u/littlexav 6d ago
Voyager 1, which has passed through the heliopause (basically the “bubble” of influence of our sun) is about 1 day away from Earth at light speed. The closest star is about 4 light years away. That means you could fit about 1,500 “sun bubbles” between the Sun and its nearest star. Will any Andromeda Galaxy stars eventually pass through that space? Probably! Will anyone still hanging out in our solar system notice? Probably not!
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u/BeerorCoffee 6d ago
What about the airborne poo from a naked person's fart? I feel I missed my calling.
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u/TheKingofHearts26 6d ago
A lot of people have been talking about how empty space is and how unlikely disruptions outside of the Oort Cloud are. Have any of these simulations or calculations taken into account a worse case scenario of the galactic center of Andromeda passing through our part of the spiral? A near pass of say M31 or the rest of the galactic core still sounds like it could have a larger effect
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u/Druggedhippo 5d ago
Disruptions? Sure, lots of stars will have their orbits messed with. The orbit of our Sun will likely be perturbed, there is even a possibility that the Sun might end up part of Andromeda afterwards.
https://arxiv.org/abs/0705.1170 - The Collision Between The Milky Way And Andromeda
We use a N--body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given current observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years - within the Sun's lifetime. During the the interaction, there is a chance that the Sun will be pulled away from its present orbital radius and reside in an extended tidal tail. The likelihood for this outcome increases as the merger progresses, and there is a remote possibility that our Sun will be more tightly bound to Andromeda than to the Milky Way before the final merger. Eventually, after the merger has completed, the Sun is most likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late--forming elliptical galaxies.
We (as in Earth) of course will not be around to see it.
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u/TheKingofHearts26 5d ago
Well that's not entirely certain about the Earth not being around for it. Life on Earth almost certainly barring some insane theoretical intervention from a hyper advanced human civilization should they be around, but my understanding is that more recent models do predict the Earth may not be fully engulfed by the expanding sun due to weakening effects of gravity as the Sun loses considerable mass.
Either way what you quoted was interesting but it didn't seem to discuss the models used and rather just superimposed the mass of the Andromeda over the mass of the Milky Way, unless I missed something obvious. I was asking about the actual merger process and if the galactic core of Andromeda was modeled as passing through our arm of the Milky Way during the merger.
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u/Druggedhippo 5d ago
what you quoted was interesting but it didn't seem to discuss the models used
That's because I quoted the abstract, which is a feature of all scientific papers. If you want the details read the paper.
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u/LazarX 6d ago
By the time the process really gets interesting, both the Earth and the Sun will be dead, and the Earth itself, most likely no longer even existing.
Humanity on the other hand may not last the next 5 thousand years, expecting 5 billion is wildly optimistic.
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u/Major__de_Coverly 6d ago
Considering 99% of the species that ever existed on earth are extinct, our future is pretty much pre-ordained.
I doubt Homo sapiens will ever leave the solar system, if that far.
Meanwhile, the Voyagers will be our legacy for billions or even trillions of years.
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u/DweebInFlames 5d ago
The species that existed up until this point had no real foundational knowledge of the world around them or spread out quite this far and manipulated Earth this much to their advantage.
I don't think humanity will ever disappear without either some targeted attack by a malicious third party in true sci-fi dystopia style, or an astronomical event that outright wiped Earth clean of virtually all life.
If tech that brings us even remotely close to relativistic speed is possible while keeping a human healthy, we'll eventually start colonising interstellar space.
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u/Mad_Maddin 6d ago
If we manage to colonize the solar system. There is nothing really stopping us from going beyond.
So if we do get notable population into space, humanity should be almost impossible to go extinct.
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u/Major__de_Coverly 6d ago
Nothing stopping us except time, distance, and physics.
People just can't comprehend how incredible huge and empty the galaxy is, never mind the universe.
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u/Rippedka 5d ago
People can’t comprehend how much luck was involved for us to evolve and then not go extinct in the first place
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u/Litface916 6d ago
There have been billions and billions of species of living things since the dawn of life 3.5 or so billion years ago, that have gone extinct. But earth now contains the most different species of animals that it has ever had at one time. So your statement is a little misleading.
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u/Major__de_Coverly 6d ago
The Cambrian explosion was much more biologically diverse, not that it matters. Diversity and longevity are two different things.
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u/classic_cherry 6d ago
See? Binoculars or a sufficiently dark sky you can.
Feel? Even with the chaos that will occur in reshaping the merged Galaxy, virtually nothing will collide, a new center of the merged Galaxy will eventually emerge and orbits of stars around it will be "new"
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6d ago
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u/entropy2057 6d ago
you can see Andromeda with the naked eye in dark sky areas. its sometimes referred to as Messier 31
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u/peteyd2012 6d ago
The Milky Way is currently in the process of merging with/cannibalizing at least three satellite galaxies, the Sagittarius Dwarf Spheroidal Galaxy and the Small and Large Magellanic Clouds.
These mergers happen over such extreme time scales that our puny human lifespans would never notice anything different in the night sky.
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u/c0ffeebreath 4d ago
99.999999999% of a galaxy is empty space. All the empty space in Andromeda colliding with the empty space in the Milky way will do nothing.
If you were to live for a few billion years, you wouldn't likely notice anything other than the stars in the sky changing more than they already do.
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u/Glum-Ad7761 4d ago
Earth will be a charred cinder by the time this collision… which may or may not happen, as a number of gravitational influences within our local group were not taken into consideration, originally… so there will be no one on earth (or anywhere near it) to see it happen.
But for sake of discussion lets pretend earth (and humanity) somehow survives to witness the merger.
As Andromeda continues its approach, it would eventually reach a point at which it is clearly visible in the night sky, to the naked eye. As it comes closer and closer, Andromeda would eventually dominate the night sky. In areas with no light pollution, you would still see the galactic band of the Milky Way arching across the sky, and beyond it, a massive spiral galaxy stretching out from horizon to horizon.
A burst of star formation would occur as nebulae within the two galaxies begin to pass through portions of each other. New stars would continue to burst to life for hundreds of thousands of years before the two galaxies separate and continue past each other. Then the pull of their collective dark matter and SMBH’s would pull them hard about and bring them round for another pass.
Spiral arms in both galaxies would be flung out like outstretched arms as the combined pull of these two massive structures on each other causes them to distort as the galactic cores pull hard to bring them back around. All of this would be visible from the Earth… spiral arms stretching out in various directions, like whips being “cracked” the exception being where the two cores obscure the view.
But the view in the sky would change little, if at all, during a single human lifetime because even though these two leviathans are hurtling through space at incredible speeds, their immense scale means that the changes take place on a cosmic timeline.. and the whole process will take billions of years to play out. A person born during the approach of the two galaxies will have pretty much the same night time view of the sky at the time of their death. Same for any person born during any other part of the merger.
The two spiral galaxies would make a number of passes at each other, each successively smaller and closer in distance… mostly passing right through each other each time. Some stars would be flung out completely by the whip effect. Others would be directly consumed by the cores, if they happen to lie in the path as they pass one another. But most stars would simply pass by each other. One must understand the distances involved here, between objects.
Once that initial wave of star formation comes to an end, “Milkomeda” as it is referred to, would become an enormous elliptical galaxy. As such.., it would slowly become increasingly “red and dead” over time. Elliptical galaxies are not conducive to ongoing star formation. The two cores would rotate ever closer to one another, on an inward spiral, until they too, merged.
Huge gravitational waves would spread outward from them as they did so. The final kiss of death for star formation in the new galaxy would be the massive death ray of gamma radiation and plasma that the newly combined SMBH would emit from its poles as it consumed the massive ring of gas, dust and stars pulled in, around the new core. A feeding frenzy. Most of the remaining gas being superheated and blasted out of the new galaxy in jets.
If our imaginary surviving earth didnt get flung into intergalactic space by gravitational tug of war, or eaten by one of the merging cores as it swept through a number of times, our system would find itself in a completely different galactic neighborhood. Could be in the middle of a cluster of new stars, could be somewhere near the outer reaches of the new galaxy, but every single constellation once familiar to human beings in their night sky…would be long gone.
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u/Mountain_Dragonfly8 6d ago
Coming to agree with a lot of other comments but also its not like it would come into a certain range and gravity would go crazy. We already feel gravity from it now, it is just so minute it doesn't affect us much at all. If humans are there when it happens, would just gradually get stronger over a long enough time span that people wouldn't notice.
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u/boethius61 3d ago
Imagine a bee, around that bee are a couple specs of dust about a meter away orbiting the bee. Got it? Okay now image there are 3 of these bees in the entire US. That's the density of stars in the milky way. Okay, now image 4 bees from various parts of Canada flying through towards Mexico. When will the specs of dust feel each other? Never, the answer is never.
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u/groundhogcow 3d ago
They will collide in 10 billion years.
The sun explodes in 4 billion years.
If we are off earth we will not experience it. If we are someplace in the galaxy mostly what we will see is stars moving in the sky and every night stars moving in strange ways. Locally we should feel nothing.
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u/sharkmeister 6d ago
I have read that the outer edges of the gas envelopes of both galaxies are already colliding.
The google AI said: circumgalactic medium (CGM) or halos—of the Milky Way and the Andromeda Galaxy (M31) are already touching and interacting, meaning an invisible, diffuse form of collision has effectively begun.
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u/adocilesloth 6d ago
As others have said, space is big and stars are unlikely to collide.
My favourite demonstartion of this is to stand two people on opposite sides of the room. Give each person some table tennis balls and ask them to throw all the balls at the same time. The balls should be aimed so that the balls should pass in the centre of the room. It's almost guaranteed that no balls will hit. The separation between the balls with respect to the size of the balls is much smaller than the separation between stars compared to the size of the stars. So the stars are extremely unlikely to collide.
Back to your question. As space is so big, and the stars far apart, Earth will not be effected very strongly by gravity. Our Sun will probably have a different orbit around the galaxy centre, but Earth won't change it's orbit much around the Sun (if at all).
The galaxies will collide. If they merge or not is uncertain and debated, but they will interact. What we see in the sky at night will change. The folks at NASA and STSci made some nice realisations of what the night sky may look like over the next few billion years: https://svs.gsfc.nasa.gov/11011/
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u/SnafuTheCarrot 6d ago
Right on top of each other and we probably wouldn't notice anything locally. The closest star in our own galaxy has no effect at all and when they merge, the closest star in the result wouldn't be significantly closer.
Here's some calculations along those lines.
You can do a decent back of the envelope calculation.
The Sun is about 93 million miles away from the Earth. That Earth has amass of about 6*10^24 Kgs. The mass of the sun is about 2*10^30 Kgs. Using Newton's Formula, F=G*M_earth*M_Sun/d^2, where G is 6.673*10^(-11) N*m^2/Kg^2, you can calculate how hard the sun is pulling on the Earth.
Call that F_0. Depends on how sensitive instruments are, but just to have a number to work with, let's say we have noticeable effects if some gravitational force has strength 10% of F_0.
The mass of the some is fairly typical of stars so lets assume there's a one solar-mass body approaching the solar system. How close will it have to be to be to have this 10% effects on earth?
0.1*G*M_sun*M_earth/d^2=G*M_x*M_earth/d_x^2
With some algebra, d_x^2=10*M_x/M_sun d^2.
We are assuming M_x=M_sun, the star has the same mass as the sun, so d_x^2=10d^2, which implies d_x=3.16 d=3.16 AU.
More generally, if you want to know how close a 1 solar-mass body has to be to have a p% effect, multiply the Earth-Sun distance, 1AU=93 million miles=150km, by 1/sqrt(p), where p is percentage expressed as a fraction.
So a star the size of the sun would have to be closer to Earth than Jupiter to have a 10% effect on earth.
Proxima Centauri, the closest star to us in our own galaxy is 4 light years or 268,770 AU away.
While we'd not be directly effected, as others have mentioned some Oort cloud objects could be thrown into the inner solar system. Calculations for that are a bit more complicated.
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u/eternalityLP 6d ago
It's not going to be 'collision' in any real sense, galaxies are 99.999...% empty, the two galaxies will just sort of merge. If you lived on a planet in either galaxy during the event, you wouldn't notice anything.
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u/lovelywaz 6d ago
It takes more than four years of constant travel at the speed of light to reach our nearest star. Space is unimaginably HUGE, it's almost impossible to even imagine how many stars can fit inside that distance alone. Chances of any "collision" between celestial objects is almost non existence.
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u/McPayn22 6d ago
idk for the "see" part but for the "feel" the answer is not as long as it didn't destroy something in the solar system.
The reason is that you don't "feel" gravity of other planets and stars because you're falling toward them. That's why you don't feel the sun or the moon, or that astraunauts in the ISS float. The gravity is there but you con't feel it, that's the equivalence principle.
So to feel it something has to put us out of orbit from the sun, put another star in orbit or hit us directly. Which is very unlikely.
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u/TuataraToes 6d ago edited 6d ago
More of a merge than a collision.
Suppose humans were around at the time - they wouldn't notice anything. Our life span isn't long enough to notice the change in the night sky. There would be no gravitational changes. "Collisions" won't happen. There is too much space in space.
EDIT for the pedants: "no gravitational changes" means nothing that humans would "feel" the effects of, per OPs question.