If the Milky Way were to collide with another galaxy, how would it affect day-to-day life on Earth?
Herman D鈥橦ondt
Sydney, Australia
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The short answer to the question is 鈥減robably not at all鈥�. The long answer is more complicated.
The average distance between stars in our galaxy is about 5 light years (ly). This is roughly the same as the distance of 4.25 ly to our nearest stellar neighbour, Proxima Centauri. Stars don鈥檛 significantly affect each other gravitationally unless they are less than 2 ly apart. That means that, when another galaxy collides with ours, the stars will typically be too far apart to affect each other鈥檚 planets.
If a star did get close enough to us, it could change the orbit of Earth and push us either closer to the sun or further from it. Any shift like that would make global warming seem trivial. If this did happen, we would see it coming for many thousands of years. The Andromeda galaxy, the closest spiral galaxy to ours, is moving towards us at about 110 kilometres per second, and the collision is about 4 billion years away, so we have plenty of time to take action.
Another possible problem concerns the gas clouds in both galaxies. When these collide, the resulting pressure waves trigger furious star creation. Some of these new stars will be massive and will quickly burn out and go supernova. If such an explosion were to happen within 50 ly of Earth, the radiation released would profoundly affect life. However, even the most massive stars take a few million years to go 鈥渟uper鈥�. So, again, there is time to prepare.
Finally, there are the supermassive black holes at the centre of each galaxy. Even if we don鈥檛 collide with the black hole at the centre of the incoming galaxy (which would be uniformly bad for life as we know it), the two galactic cores would eventually merge. That may cause the resulting black hole to become active and spew out radiation that could affect life. The collision would also affect the orbits of stars around the galaxy.
However, by the time any of this happens, the sun will have started to run out of fuel and will have swollen so much that Earth will be burned to a crisp. In other words, we will have more pressing things to worry about.
Guy Cox
Sydney, Australia
Galaxies are mostly empty space, so the chances of anything hitting us are slim. Earth would continue to rotate around the sun, and our seasons would be unchanged. What would be different would be the night sky. The constellations would be totally changed. If you believe in your birth zodiac sign you would be in trouble, since it would no longer exist.
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