New study casts doubt on the likelihood of Milky Way collision with Andromeda
71 comments
·June 3, 2025didgetmaster
When simulating an actual merger with the two galaxies; any estimates on the number of stars that might collide?
400+ billion stars per galaxy might make each one seem 'dense' but the distance between stars is enormous.
I have heard that it might be possible for one galaxy to pass through another without any stars colliding with each other. I don't have any idea if that is actually true.
teamonkey
As others have mentioned, the space between stars is vast, but also it's actually very hard for stars to directly collide.
The reason for this is because the dynamical systems of gravity, conservation of momentum etc. tend to pull approaching stars into orbits around each other or make them slingshot away after a close approach, rather than collide directly. Even though intuitively you feel gravitational attraction would make it so that they pull together, the mechanics tend to prevent that from happening. It's the same reason that, unintuitively, it takes a lot of energy to bring a rocket from a stable orbit down to Earth.
That's not to say that direct collisions won't happen, the circumstances will surely be there for them to happen, with all those millions of stars, just less likely than you'd think.
When two stars collide it's usually because two stars are in close orbit and something causes an orbital decay, such as one leaching matter from another, or another star passing close enough to disrupt it. This last point is probably more of a catastrophic risk here; even more so the possibility of a passing star slingshotting planets away into open space.
Source: this was part of my undergrad thesis.
m4rtink
Still close enough passes could disrupt orbits of all the stuff orbiting the stars far out in the Oort Cloud (basically leftovers from star formation) and result in comet bombardment of the inner system/free mas delivery for mega projects.
nandomrumber
Sort of random question: is there some estimate of the kinetic energy in the rotating mass of an entire galaxy.
Rooster61
IIRC, this concept is where the idea of dark matter came from. Given the mass of a galaxy, they have more angular kinetic energy (or rather they spin faster) than they should given detectable mass alone. Gotta be something making galaxies spin faster than they should, and that something is what was labeled dark matter.
Dark matter is just a placeholder until we find whatever that "something" is, or a better model of why this is happening arises. All of that is predicated on the estimate of kinetic energy you inquired about
ahazred8ta
Well, the mass of the Milky Way is very roughly 2 × 10^42 kg, and most of the matter is orbiting at 200,000 meters per second. My napkin says ~ 4e52 joules.
exe34
I think direct collision depends just on the geometric cross-section, no? Gravity can't repulse, so if you're on a direct collision path, nothing will move you off it. The whole slingshot thing is that you get close, you change path, but just because you pass close doesn't mean gravity will make you collide. I might be missing something...
teamonkey
If you have two stars moving directly and perfectly towards each other, and there are no other influences, then yes, they will collide.
In reality there will be always some sideways motion for each star relative to each other, especially in this kind of situation with a lot going on. The gravity of the bodies pull on each other as they approach, accelerating them towards each other, but their momentum is strong, and so gravity pulls them into an elliptical orbit (or hyperbola) and not into each other. There only needs to be a tiny relative sideways motion for this to happen.
In other words, gravity will not cause two stars passing very close to each other to collide, as one might expect. The stars really would to be aimed perfectly at each other to collide, which would be extremely unlikely with both stars feeling the faint pull of many distant neighbours.
I think I was being overly cautious when I suggested a direct collision was likely to happen. It's more accurate to say that an accidental confluence of the many complex gravitational forces and sheer number of stars could maybe allow collisions to happen somewhere in that system.
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simondotau
Related to the OP’s hypothetical, if we did cross paths with Andromeda, for how long would that intersection last? Or to put it another way, how long could Earth be within both galaxies, assuming a hypothetical perfect aim?
ahazred8ta
The approach speed is 1 ly per thousand years, so in the neighborhood of 50 - 100 million years.
nandomrumber
One potential outcome would be that the galaxies merge, but by the time the collision is well underway our sun will have aged to a point where Earth will already be quite crispy.
teamonkey
If we as a species survive to witness the event, I'm sure astronomers will still be arguing about where the boundary of a galaxy lies.
xhkkffbf
What? I was led to believe there would be collisions. I feel ripped off.
banana_giraffe
https://web.archive.org/web/20140701085917/http://www.nasa.g... :
> Although the galaxies will plow into each other, stars inside each galaxy are so far apart that they will not collide with other stars during the encounter. However, the stars will be thrown into different orbits around the new galactic center. Simulations show that our solar system will probably be tossed much farther from the galactic core than it is today.
raverbashing
Very low probability times massive number of stars: yes I think there will be a non-zero number of actual collisions
bruce511
You are confusing one big number with another big number, and treating them as the same.
Yes there are a lot of stars. It's a big number. But there's a lot of space. That's a number that's in a whole different league.
For example, the whole solar system is about 2 light-day diameter. But it's 4 light years from the nearest star in any direction. Empty space is thus many orders of magnitude more than solar systems, never mind suns.
Sure there's a probability of a collision. But even multiplied by the number if stars, it's still really tiny.
penteract
If we assume that the stars of one galaxy (A) are distributed uniformly at random within a circular area in the galactic plane, and the other galaxy (B) is moving perpendicular to the plane of A and passes entirely through the circle containing stars, and assume that stars in galaxy B are point-like, then:
Expected Number of collisions = Number of stars in galaxy B * cross sectional area of star in A / average area of galactic circle per star in A.
https://www.wolframalpha.com/input?i=%28number+of+stars+in+m...
says it comes to 0.5 - 1.0 (the uncertainty comes from number of stars in the milky way)
My assumptions are bad enough that it could be off by a factor of 100 one way or the other (there should be a few factors of 4/pi, it looks like Andromeda is about twice the size of the Milky Way, the average star is smaller than the sun, no stars are point like, gravity probably does something, stars are much more densely packed towards the galactic center, I'm calculating the result one galaxy passing through another, not a merger in which they might partially intersect more than once).
rwmj
But they don't need the stars to physically collide for it to be a problem. A star coming anywhere near the orbit of Jupiter would pull planets away from the Sun, which would make dramatic changes to the Solar System.
jerf
The question is about stellar collisions.
If you reframe the question as "will the collision of the galaxies cause problems for some hypothetical civilizations who may be living there" the probability of that is simple, it's 1. The good news for such civilizations is that they'll have literally hundreds of thousands of years of warning to deal with them. Planets may have problems but if a civilization is based on space stations and other off-planet structures they'll hardly notice the problems since they'll be so slow to occur.
simondotau
Correct me if I’m wrong, but isn’t the scale of our solar system minuscule compared to the distance between stars? Presumably if another star gets close enough to affect Jupiter, it’s going to affect everything to varying degrees.
superfish
Wikipedia has a nice size analogy: https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_co...
> […] if the Sun were a ping-pong ball, […] the average distance between stars […] is analogous to one ping-pong ball every 3.2 km (2 mi).
Intuitively this visualization actually makes it seem like stars are pretty close? Usually with galactic dimensions it’s hard for our mere monkey minds to grasp the scales but this is actually pretty easy to imagine.
petee
We really can't grasp just how large the sun is either; the earth would be a grain of sand in that scale. To me that still feels pretty vast
snickerbockers
That's because you're significantly larger than the pingpong ball.
nandomrumber
Sol is about 1.39 million kilometres in diameter.
A table tennis ball is 40mm in diameter.
That makes the sun about 34,750,000,000 times bigger than a pingpong ball.
kristianc
Infinitesimally small. Like throwing a grain of sand from either end of a football pitch and expecting them to hit each other small.
lIl-IIIl
The article:
>Such a collision would be devastating for both galaxies which would be destroyed, leaving behind a spheroidal pile of stars known as an elliptical galaxy.
https://en.m.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_...:
>The stars involved are sufficiently spaced that it is improbable that any of them will individually collide,[6] though some stars will be ejected.
Originally I thought thought those statements were contradictory, but I guess the first statement says the galaxies will be destroyed, and says nothing of the stars.
vikingerik
"Galaxies would be destroyed" is extremely misleading hyperbole. Nothing gets destroyed; at most a relatively tiny proportion of the mass hits the central black holes or gets flung out.
They'll get "destroyed" as much as pouring two drinks into the same cup "destroys" them. Nothing gets destroyed, you just get an intermingled mix.
GoblinSlayer
Muh spiral shapes.
snickerbockers
It's like when your boss calls an emergency meeting to go over the latest company-wide BU reorganization like it's absolute pandemonium but you're just sitting there thinking about how you get paid the same salary to perform the same job with the same colleagues under the direction of the same manager. The total destruction of our entire galaxy is just that but in space.
elihu
I was in a startup that was acquired by a big tech company and it went about like that. They made us use Windows for email and "business stuff". I continued to do all my real work on a low-budget Frankenstein computer under my desk that I think I was supposed to relinquish but I wasn't going to say anything and no one asked.
exe34
Perfect analogy! I've sat through about 5 restructures already and I swear they use musical chairs to decide who's doing what this time around, while having no impact on my life whatsoever.
spenczar5
I could have told you in advance it would have been a Finn! Something in Helsinki has been going on for a while in astronomy. They're so much more willing to challenge the norm, and unusually capable with Bayesian statistics and big computation. It's really remarkable.
gnabgib
Related:
Milky Way may escape fated collision with Andromeda galaxy (9 points, 10 months ago) https://news.ycombinator.com/item?id=41240641
Milky Way and Andromeda galaxies are already merging (2020) (138 points, 2022, 74 comments) https://news.ycombinator.com/item?id=30494523
Our Dazzling Night Sky When the Milky Way Collides with Andromeda in 4B Years (182 points, 2019, 120 comments) https://news.ycombinator.com/item?id=21327269
lisper
Damn, I was really looking forward to seeing Andromeda up close. Oh well.
chopin
The entire luminosity of Andromeda will be dispersed over a much wider angle. It'll be much less spectacular than one might think. It won't be brighter than what we see today.
lisper
Yeah, that was meant to be a joke. Even if we were to collide, it wouldn't happen for a few billion years. Our sun will be a red giant by then. Earth will no longer exist.
braiamp
It only cast doubts in the time frame. Everything in our local region will merge eventually.
jvm___
https://youtu.be/uD4izuDMUQA?si=UMD-FBpBNroELCKE
This video explores what's predicted to happen in the future of the universe. The speed of time passing doubles every 5 seconds. And the video is almost 30 minutes long.
chasil
How long does Triangulum take?
https://science.nasa.gov/asset/hubble/the-fate-of-the-milky-...
ednite
Good point. With my limited understanding of the cosmos, collisions are inevitable, when they happen depends on which rock you're standing on and how long you're willing to wait.
It's comforting to know scientists are out there, keeping an eye and ear on things.
mensetmanusman
Now that you say that, I should cancel my jewelry insurance!
adamgordonbell
Oh no, a plot hole for Alastair Reynolds revelation space series.
svachalek
> Such a collision would be devastating for both galaxies which would be destroyed, leaving behind a spheroidal pile of stars known as an elliptical galaxy.
Where the heck did this sentence come from, especially on a university website?! Is it AI slop or did a human actually write this?
block_dagger
Collide? No; merge.
ChocolateGod
I guess it depends whether you consider the space between stars as part of galaxy, in which case they're both colliding and merging.
aurareturn
They will collide then merge eventually.
tekla
12yos before this article: https://www.youtube.com/watch?v=y_PrZ-J7D3k
>If the Milky Way and Andromeda are to collide and merge, the researchers found that it would most likely happen in 7 to 8 billion years’ time, significantly later than previously predicted.
Aah damn well let me check my calendar for what I'll being doing in 7 billion years instead of 4...