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Does everything get smashed to bits when two galaxies collide?
Category: Space
Published: June 25, 2024
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
No. When two galaxies collide, the stars, planets, dwarf planets, and moons from one galaxy do not crash into any stars, planets, dwarf planets, or moons from the other galaxy. No astronomical bodies get smashed to bits during a galaxy collision. When astronomers say that two galaxies are colliding, they mean that the two galaxies are moving toward each other and are progressively occupying the same region of space. Throughout the entire collision, zero astronomical bodies from the one galaxy run into astronomical bodies from the other galaxy. You need to realize that there are unimaginably huge amounts of empty space in a galaxy—where the word "empty" in this context means devoid of stars, planets, dwarf planets, and moons. (Outer space is never absolutely empty because it contains light, neutrinos, vacuum fluctuations, and so forth.) There are such vast stretches of empty space between one astronomical body and the next within a galaxy that it is extremely unlikely that astronomical bodies from the one galaxy will crash into any astronomical bodies from the other galaxy during a collision of galaxies. Here I am taking "astronomical body" to mean a star, planet, dwarf planet, or moon. I'm not including asteroids, meteoroids, or comets as astronomical bodies in this article because they are almost always far too small to smash an astronomical body to pieces. The huge amounts of empty space between neighboring astronomical bodies makes it extremely unlikely that they will ever collide.
Let's make a rough estimate to demonstrate this. Let's assume that our galaxy is representative of the average galaxy, that every star has a solar system similar to ours, and that the stars in a galaxy are spread out more or less uniformly. Our solar system contains 1 star, 8 planets, 5 dwarf planets, and 293 moons, giving a total of 307 astronomical bodies. Let's round that up to 400 for the sake of the argument. There are 100 billion to 400 billion stars in our galaxy. However, the exact number is hard to know. For the sake of the argument, let's say that that there are 400 billion stars in our galaxy. According to these assumptions and estimates, there are thus 160 trillion stars, planets, dwarf planets, and moons in our galaxy.
A conservative estimate for the volume of our galaxy is about 17 trillion cubic light-years. Divide this by our number of astronomical bodies in the galaxy and we find that, on average, each astronomical body has about 0.106 cubic light-years of empty space all to itself. This is the volume of a cube with an edge length of 0.474 light-years. It may not sound like much, but you need to keep in mind that a light-year is a huge distance. Let's convert this volume to units where the volume of the sun is one unit. After doing some math, we find that, on average, each astronomical body in our galaxy has a region of space all to itself, devoid of other stars, planets, dwarf planets, and moons, that has a volume that is equal to the volume of about 6.4×1019 suns. In order for one astronomical body from one galaxy to crash into an astronomical body from the other galaxy, it has to find this other body within this huge region of empty space that the one body has all to itself. To be clear, this region is equivalent in size to the space taken up by 64,000,000,000,000,000,000 suns. To crash into each other, the two objects have to find each other within this huge volume of empty space and then must have their paths cross at just the right moment. The chances of this happening are effectively zero.
To give you a better idea of this, imagine that we uniformly shrink down the universe so that the sun becomes the volume of a tennis ball. On this scale, the amount of empty space surrounding each astronomical body that it has to itself would be, on average, equal to the volume of the air covering the entire earth's surface from the ground level or sea level up to 19 m high. It would be as if only two tennis balls existed on the whole earth. We then randomly place each ball somewhere on earth's surface and then have someone throw each ball. Would these two balls ever collide? Obviously not. (Even if you and a friend were only a few meters apart and you both threw a tennis ball at the same time with your eyes closed, the chances of the two balls colliding would be extremely small. Don't believe me? Try if for yourself!)
Let's assume that the two colliding galaxies are near the end of the collision process so that they almost completely occupy the same space. This means that each astronomical body from the one galaxy is sharing its local region of empty space with one astronomical body from the other galaxy, on average. Detailed mathematical analysis tells us that for two locations uniformly randomly chosen within a cube of edge length 1, the average distance between them is the Robbins constant, which is about 0.6617. If we apply this to our situation, where we assume uniform random placement, this means that at any given moment, on average, the distance between an astronomical body from the one galaxy and the nearest astronomical body from the other galaxy will be about 0.3 light years. This is equivalent to the distance spanned by two million suns lined up in a single-file row with surfaces touching. In other words, at any given moment, each star, planet, dwarf planet, and moon from the one galaxy is two million sun-diameters away from the closest star, planet, dwarf planet, or moon from the other galaxy, which is nowhere near close enough to collide.
Although astronomical bodies from one galaxy do not collide with any astronomical bodies from the other galaxy during a galaxy collision, the two galaxies still affect each other. The gravitational field of an astronomical body extends far beyond its surface. The total gravitational field inside a galaxy, which arises from all of its astronomical bodies and other matter, extends throughout the entire galaxy. Therefore, as two galaxies collide, their gravitational fields will whip around each other's astronomical objects, thereby deforming the shapes of the galaxies and changing the relative locations of the stars and other bodies within them. A galaxy collision is violent in the sense of making major changes to the arrangements of their stars and other bodies, and not in the sense of large bodies of matter smashing into each other. Note that unless the velocity vectors of two nearby astronomical objects are almost exactly antiparallel, the gravitational attraction between the two bodies will just whip each other around and will not make them collide.
Because such huge distances are involved within and between galaxies that are colliding, a galaxy collision happens extremely slowly by human standards. Typically, a single galaxy collision requires millions of years to hundreds of millions of years to progress from beginning to end. This is so slow that, to humans, two galaxies that are in the process of colliding look like they are frozen in place.