# What will happen to the Earth when Milky Way and Andromeda merge?

Hypothetically, if our solar system survived into the far future and human was still around, is the merge between 2 galaxies were a disaster for lives on Earth?

Were the Sun get thrown away from the galaxy? Or were there a planets-type-heavybombardment? Could human hypothetically survive on Earth or any other planet for that matter? Were the system of planets and star get destroyed?

P.S. Forgive me for inappropriate tags. I couldn't find a better one.

• I read somewhere that the interstellar distances are so huge that the chance of our solar system colliding with another one in such an event is tiny and we probably wouldn't even notice. Not sure if it's true though ... Apr 28, 2015 at 3:28
• You are correct. However, I am actually want to know if gravity effect or something else beside physical collision could fling stars outside the galaxy or rip solar systems apart.
– TBBT
Apr 28, 2015 at 3:37
• The nearby stars would disappear. After a hundred million years, or so, there would be this giant merging galaxy in the night sky and nothing else beyond the planets and moons of the stellar system and small galaxies. After a couple billion years, or so, that galaxy would be the size of the moon. After ten billion years (assuming we are setting around a long lived white or red dwarf star) there would be virtually nothing visible with the naked eye and it would take a good sized telescope to see the few closest galaxies with some resolution. After 100 billion years... nothing to see, at all. Apr 28, 2015 at 4:08
• That is not what I am asking. I just want to know what kind of phenomena, which originated from the merge of 2 galaxies, could effect the lives of human or kill us for that matter? Let just assume that we have already found a good planet to live on.
– TBBT
Apr 28, 2015 at 5:19
• According to the discussion above, if we humans have the technical capacity to survive until the merger ($4 \times 10^9$ yrs), and if we have the ability to colonize space, then the merger will likely not prove disastrous to us, and will afford us the ability to travel to worlds previously bound to Andromeda as well as deep inter-galactic space (for example, by riding on a planet orbiting a star flung out of the galaxy). Anyways, I think the question is veering off from the realm of physics and into the realm of what if's/science-fiction. And @Curious, why do you question the 12%? Apr 28, 2015 at 8:55

It all depends on the closest approach of any stars to the Sun.

When galaxies collide it is not that their stars crash into each other, because their individual cross-sections are extremely small when compared to the space between them. This is dealt with in qualitative terms on the wikipedia page on the likely collision.

The Milky Way disk at the position of the Sun has of order $n=0.1$ stars $pc^{-3}$ and is about $L=200$ pc thick. If we say the cross-section for something catastrophic to happen is a disk of radius 100 au around the Sun (at closer than this separation, another star would start to disrupt the quasi-equilibrium of the solar system planets) - this gives $\sigma \simeq 10^{-6}$ pc$^2$. If we then assume that the Andromeda disk is similar to ours (in terms of stellar density) and they have a disk-to-disk collision, then the "optical depth" for a collison is $n \sigma L \simeq 2\times 10^{-5}$, which is roughly the (small) chance of direct collision.

On the other hand, a significant perturbation to the Oort cloud might occur if a star came within say 0.1 pc, in which case, the optical depth for this is 0.6. So actually, it seems to me that a significant Oort cloud perturbation is rather likely, which could result in a very significant increase in the bombardment of the inner solar system by comets. I don't think the speed of the encounter matters too much: (i) The Oort cloud is supposed to be spherically symmetric; (ii) even for a relative velocity difference of 10 km/s, the encounter would take $<10^{4}$ years, so the target is effectively stationary.

Another potential consequence is a sudden increase in the star formation rate in the solar vicinity. "Starburst galaxies" (e.g. see the Antennae) can result from the merger of gas-rich spiral galaxies. The Milky Way and Andromeda partially fit that description. The caveat being that much of their gas will have been consumed in about 4 billion years. Nevertheless there remains a chance (which I would not know where to begin the calculation), that a burst of nearby star formation could result in the formation of massive stars with their consequent supernovae explosions. This might have a profound effect on any life still existing in the solar system.

• The stars from the Andromeda galaxy would, however, have a larger relative velocity than the stars in the Milky Way, which co-rotate. Wouldn't that make the interaction time between the Oort Cloud objects and the Andromeda stars on average much shorter than the interaction time with Milky Way stars? That would suppress the effective gravitational disturbance considerably, wouldn't it? Moreover, if both galaxies have dense dark matter concentrations around the center plane of their disks, then it will be the disruption of those dark matter clouds that will cause the strongest effects. Apr 28, 2015 at 12:18
• @CuriousOne Spiral galaxies don't have dense dark matter concentrations in their planes. I'm still thinking about the other point. Apr 28, 2015 at 13:27
• You may be right about the dark matter... I remember having read something along those lines but I may have misunderstood. My time scale argument is very hand-waving and obviously depends on some assumptions about the strength of the interaction, so it may very well be pointless... just wanted to throw it in for good measure. Apr 28, 2015 at 13:48

The Wikipedia article has a section on "fate of the solar system" so you should hqve just checked that before asking.

The time line of 4 billion years is not quite the 5.4 billion years before the sun becomes a red giant, but the collision will be an ongoing restructuring and re-formation, and before it's all settled bqck down, the issue will be moot.