New study casts doubt on the likelihood of Milky Way collision with Andromeda | Hacker News Reader

New study casts doubt on the likelihood of Milky Way collision with Andromeda

durham.ac.uk

56 pointslayer870 comments

>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...

I think the significance is the the sun will actually have engulfed the earth by that time

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.

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.

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...

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.

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.

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?

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.

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.

The approach speed is 1 ly per thousand years, so in the neighborhood of 50 - 100 million years.

Sort of random question: is there some estimate of the kinetic energy in the rotating mass of an entire galaxy.

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

My lay question is would the discovery of a fuckton of cold objects like dust or rogue planets or cold sub brown dwarfs clear up the dark matter issue regarding the spin of galaxies?

No. There's too much of it for that.

If this missing mass were in the form of dust the entire night sky (relatively speaking) should be full of dust reflecting light and radiating infrared due to being lit by starlight for billions of years. But we don't.

If the missing mass were in rogue planets, brown dwarfs, or even cold dwarf stars there would need to be so very many of them that we should be detecting them by the millions with our current telescopes as they pass between us and distant stars. But we don't.

The better our telescopes get the more and more certain we are that the missing mass is not normal matter as we know it. We are getting really good at spotting dim objects (or their side-effects) even when hidden by the glare of stars. Normal matter but not radiating much energy just can't hide from us well enough to account for the missing mass.

Probably not. It requires a lot of mass to make galaxies make sense. And we can compute that the extra matter is usually distributed in a halo around the outer edge of the galaxy. The density is such that if it were any kind of normal matter, we should be able to see it in at least one galaxy.

All of our observation tells us that whatever dark matter is, it doesn't interact with the electromagnetic field. That is, it does not interact with light of any frequency, so is completely undetectable to us. As far as we can tell, dark matter appears to not interact with anything other than gravity, which is pretty weird. We think that it doesn't even interact with itself; two dark matter particles should just pass through each other.

We know a lot about what dark matter isn't, but we're still pretty clueless on what it is

That’s one of the first explanations that was considered. It can’t be dust or gas, because those would be backlit by the stars behind them and thus be visible. Larger objects are called MACHO [0] and have been ruled out by observational studies as well. As explained in [1], baryonic dark matter is also contradicted by CMB anisotropy analysis and nucleosynthesis theory.

[0] https://en.wikipedia.org/wiki/Massive_compact_halo_object

[1] https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter

That was one of the early ideas, yeah.

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.

What? I was led to believe there would be collisions. I feel ripped off.

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.

Very low probability times massive number of stars: yes I think there will be a non-zero number of actual collisions

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.

There is no Universal Saftey Insitute where they test crash Galaxis , and we have no idea if the indivual stars in a galactic collision will line up such as there mutual gravitational atractions leads to head on collisions. Both galaxis are rotating and presumably there are sections ,unknown and unknowable, that will interact much like gears meching, where the chances of collision are much higher over time. Reducing the whole thing to a simple probability of something like two fields of dots, pushed strait through each other may not be valid, and I am absolutly cetain that no one has the compute needed to run a simulation on billions of varied gravitational singularities moving and interacting with another set......... which is of course all my lead up to do a pitch for a mega intraferametric space telescope to observe actual coliding galaxies and other phenominon, to be built sooner, rather that later. would also be good for looking at details on exo planets.....give it a realy wide focul length and it could also zoom in on asteroids and comets for research, early warning of impacts and determining potential space mining candidates. current work in space navigation is focused on providing ultra high precision data links between different satelite units in an array that will alow introfermetry to work without a physical conection between the sensors, making for larger, cheaper telescopes, that are orders of magnitude more powerfull than anything built yet

We should all close our eyes and imagine the insanity of two stars directly colliding. My goodness.

I mean spin down collisions are a common enough occurrence and likely happen at higher speeds.

Also watching stars getting disrupted by black holes would be similar.

I would agree, except for some factors:

Gravity tends to bring heavy objects together

Andromeda and the Milky Way are not colliding "like two pancakes on top of each other" but at an angle to each other

Also star surroundings are usually places where you can collide with a lot of stuff (asteroid belts, dust, etc)

We're not shooting a goal on a 4 light-year wide goalpost (btw that's the density on our vicinity, but on other areas the density is higher) but passing multiple stars on an environment that can be perturbed and heavy things attract each other

This entire conversation seems silly without a time frame. Over a long enough time with continually interacting orbits, of course mass will coalesce. I don't think "merge" means "one time interaction that fully disperses the mass", anyway.

If they merge in 7 billion years, you think over the eg next 7 billion there won't be kinetic collisions? I'm skeptical.

If they merge in 7 billion years, I wouldn't count a collision in year 13 billion as a result of colliding galaxies. It would be just a normal behavior of stars colliding in the new galaxy that was formed.

Well, you certainly should—that's how causation works.

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).

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.

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.

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.

Yes you're correct! It's about a factor of 50,000 between the orbit of Jupiter and the distance to the nearest star.

But the orbit of Jupiter is still a lot bigger than the size of the Sun. To give numbers: The radius of the Sun is 700km, the radius of the orbit of Jupiter is 7*10^8 km (approx, it varies a bit), and the distance to the nearest star is 4*10^13 km.

> The radius of the Sun is 700km

700 Mm, or 700000 km.

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.

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

That's because you're significantly larger than the pingpong ball.

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.

I find some of these numbers hard to believe sometimes. 40 mm seems really big for a table tennis ball.

It's 0.00019884 furlongs, if that helps.

Or 0.0008 OSP (Olympic Swimming Pools)

Edit to add: length, not volume

Or 0.000587613 smoots

https://en.wikipedia.org/wiki/Smoot

Infinitesimally small. Like throwing a grain of sand from either end of a football pitch and expecting them to hit each other small.

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.

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

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.

"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.

Once the elliptical is formed star formation stops for the most part so the galaxy slowly 'dies'

Muh spiral shapes.

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.

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.

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.

Damn, I was really looking forward to seeing Andromeda up close. Oh well.

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.

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.

It only cast doubts in the time frame. Everything in our local region will merge eventually.

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.

How long does Triangulum take?

https://science.nasa.gov/asset/hubble/the-fate-of-the-milky-...

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.

Now that you say that, I should cancel my jewelry insurance!

12yos before this article: https://www.youtube.com/watch?v=y_PrZ-J7D3k

Oh no, a plot hole for Alastair Reynolds revelation space series.

Collide? No; merge.

I guess it depends whether you consider the space between stars as part of galaxy, in which case they're both colliding and merging.

They will collide then merge eventually.

> 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?

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