# Why won't the Milky Way colliding with Andromeda affect the Solar System?

According to Wikipedia

Such an event would have no adverse effect on the system and the chances of any sort of disturbance to the Sun or planets themselves may be remote.

I'm imagining that this is because the distance between stars is so big, it'd be highly unlikely for any star to actually collide.

But the Oort Cloud is 2 light years in diameter. Given that the average distance between stars in the Milky Way is about 5 light years, that's almost half way. If Alpha Centauri had an Oort Cloud of the same size, we'd already almost be overlapping each other.

Assuming that we're not the only star with an Oort Cloud, isn't there a high probability that the Sun will actually be inside the Oort Cloud of a passing star?

I would imagine this would because the Sun and our planet would be bombarded with lots of comets which have been thrown off their orbits by either the gravity of the Sun or the passing star.

• It's a wikipedia article. What do you expect? The cited references do not support the claim made in the article. – David Hammen Jun 14 '15 at 12:31

Outside my field, but collecting information from the Wiki article, I can make an attempt nevertheless:

You're right that we will probably pass through Oort-like clouds of other stars. But the largest of the planetesimals of the Oort Cloud is measured in kilometres, whereas the distance between these objects is measured in AU, so the total cross section of object compared to that of the cloud is still miniscule. Smaller objects are larger in number, but still it's a very low probability.

Gravitationally, the clouds also won't affect us. The total mass of the Oort Clouds is probably of the order of 10 Earth masses, i.e. much smaller than the Sun.

The outer parts of the Oort Cloud is only loosely bound to the Sun. I suppose the best evidence that we won't have any major collisions is that then we would already have similar objects bombarding us from our neighboring stars' clouds.

• To get an idea of how thin the Oort cloud is, keep in mind that we can easily see right through it; in fact, it's never even been seen. – Keith Thompson Jun 17 '15 at 2:27

I've always found that somewhat strange when they say no stars will hit out of billions, but I also trust the scientists that they know their stuff.

Lets look closer at your numbers. If the Oort Cloud has a diameter of 2 light years (I think it's a bit less than that, but numbers vary), that's a radius of 1 light year, and the Solar System to the Oort Cloud has a volume of $\frac{4}{3} \pi R^3$, or $\frac{4}{3} \pi$ cubic light-years.

With 5 light-years average distance from stars, that's an area 125 times larger. So our Sun fills up (to its Oort Cloud) 1/125th of its region of space, and 5 light-years might be low too. The average closest star might be a bit more than that.

But not all stars are as big as our Sun; many are smaller and if two Oort Clouds pass through each other, not much happens. A star has to pass within our Oort Cloud for the gravitational changes to be interesting, and that doesn't happen very often.

Another thing to consider is that the Earth is in a spiral arm of our galaxy, and both the Milky Way and Andromeda are flat disk-shaped galaxies. Are we crashing head on or at an angle where our spiral arms won't be directly hit?

I don't think we'll care much 4 billion years from now. I like to think we'll have traveled beyond the Solar System by then, but the danger to the Sun from Andromeda probably isn't huge. That said, I'd guess that some stars will pass near other stars, even if the experts say direct hits are unlikely (from the article)

If the Sun were a ping-pong ball, Proxima Centauri would be a pea about 1,100 km (680 mi) away, and the Milky Way would be about 30 million km (19 million mi) wide. Although stars are more common near the centres of each galaxy, the average distance between stars is still 160 billion (1.6×1011) km (100 billion mi). That is analogous to one ping-pong ball every 3.2 km (2.0 mi). Thus, it is extremely unlikely that any two stars from the merging galaxies would collide

If stars are ping pong balls every two miles and we use our Solar System as an example (it's really not, because the ping-pong-ball-every-two-miles example is in the center of the Solar System), but if the Sun is a ping pong ball, the Earth orbits at a radius of 4.3 meters and Neptune at a bit over 120 meters and the outer edge of the Kuiper Belt about 200 meters. That's still small compared to the average distance between stars, even in the galactic center, but not so small that interaction would never happen.

The ping pong balls might miss each other but I think it's a safe bet that there will be the occasional star-to-outer planet close encounter. From a local perspective these will be rare, but from an entire galaxy perspective, I'd think there would be quite a few of what could be called, planetary fly-bys/near misses.

• Comment, this is not meant to be a definitive answer, only my thoughts on the subject. – userLTK Jun 14 '15 at 17:41

I've seen some articles, which I've referenced in a recent answer of mine concerning this collision, which make a claim to the tune of "the Earth and Sun will probably not be significantly affected" (other than that the Sun is dead by this point, and Earth possibly consumed by it). This was based on N-body simulations of the merger, themselves based in part on recently collected data on Andromeda's tangential velocity.

In short, this is likely true for the sun because it is true for most of the stars in both galaxies: the distances between stars are so vast that the galaxies are almost entirely empty space (well, okay, there's gas, and there's going to be all sorts of stuff happening with the gas, in the form of a lot of compression leading to new star formation; but stellar density is basically nothing), and as the galaxies merge the stars will mostly just harmlessly pass through the vast open spaces between each other. Indeed, in the neighborhood of our sun stars give an average density of about $4\times 10^{−24} \ g/cm^3.$ The densest parts of our galaxy are around 500 times as dense as this. This gives about 70 solar masses in a cubic parsec. So while the dense regions may have several stars within a cubic light year, that's still a very small fraction of space occupied by stars within that volume. Our sun has a volume of about $1.41\times 10^{18} \ km^3$, whereas a cubic light year is $8.46732407 \times 10^{38} \ km^3$: over 20 orders of magnitude larger! Even the largest stars only have volumes about $5\times 10^{10}$ that of sun, which still makes the cubic light year ten orders of magnitude larger. That's a very tiny fraction of 1% for even the biggest stars, and much, much, much tinier for stars more like our Sun.

The orbital paths of stars themselves may see significant changes, but the disturbing influence generally won't extend to structures on the scale of a solar system. The existing star(s) will still have the overwhelming influence over their own systems, since other stars are simply too far away. Some stars' orbital paths will undergo drastic alterations over the course of the merger (which is expected to take a few billion years), and may be flung into the halo or completely ejected from the merging system. Still, if any of these stars have planetary systems of their own then it is unlikely that those systems will be significantly disrupted. There may be a few exceptions, the most notable being the expected merger of the supermassive blackholes, but in the overwhelming number of cases the structure of a star system won't be altered.

Of course, this is all fairly model-dependent, but the general expectation for any model is that very little will change locally for any given star system (or even star cluster), due to the low density of stars. It's just the global picture that changes a lot.

In a simpler way than the preceding answers, think probability. The probability of a galaxy colliding with another is small, but it affecting a single star greatly is very tiny, especially 3/4 from the center. The most affect we would get is a wonderful show... https://www.youtube.com/watch?v=qnYCpQyRp-4

The stars in our (and all other) galaxies are extremely far apart, plus, our star would probably have exploded most likely by then anyway, so goodbye Earth way before the collision :)

• Actually, the collision with Andromeda (which will happen, so in this case the probability equals 1) is estimated to take place in 4 billion years, whereas the Sun will engulf Earth (it won't explode) in about 7 billion years. – pela Jun 17 '15 at 10:08
• I knew it was 7 something, wasn't quite sure about million, 10 millions etc... – GraniteBear Jun 18 '15 at 16:44
• And yes it will engulf the Earth, but eventually explode, whether or not we're around to see it :) – GraniteBear Jun 18 '15 at 16:46
• Well, I won't really explode, as in a supernova. For an explosion like that to happen, its mass would have to be at least 8 times larger. But it will blow off its outer layers via the pressure created by its central white dwarf's radiation. However, this will be a much more quiescent event than an explosion, happening on timescales of 10,000 years, as opposed to seconds for a supernova explosion. – pela Jun 20 '15 at 11:12