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