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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 d
by teamonkey 1y ago
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.
- exe34 1y agoI 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 1y agoIf 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.
- simondotau 1y agoRelated 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?
- teamonkey 1y agoIf 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.
- nandomrumber 1y agoOne 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.
- ahazred8ta 1y agoThe approach speed is 1 ly per thousand years, so in the neighborhood of 50 - 100 million years.
- nandomrumber 1y agoSort of random question: is there some estimate of the kinetic energy in the rotating mass of an entire galaxy.
- Rooster61 1y agoIIRC, 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
- DiggyJohnson 1y agoMy 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?
- andrewflnr 1y agoThat was one of the early ideas, yeah.
- mystified5016 1y agoProbably 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
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- m4rtink 1y agoStill 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.
- xhkkffbf 1y agoWhat? I was led to believe there would be collisions. I feel ripped off.