What happens when galaxies collide?

Astronomers already knew that our galaxy, in which our Sun is one of a hundred billion or more stars, is on a collision course with its slightly bigger neighbour, Andromeda. Now, thanks to detailed measurements of stellar motions by the Gaia space telescope, we’ve got a clearer idea of when the crash is due to occur: in around 4.5 billion years. That’s roughly the age of the Earth, so there’s no immediate cause for alarm.

Even when the collision happens, it will be a sedate affair hardly noticeable to any creatures still around in that far distant future – apart from giving them some spectacular night-sky views. In fact ‘collision’ is a misnomer. A galaxy is mostly empty space, so it’s closer to the truth to say they ‘pass through’ each other. There’s little chance of an individual star actually crashing into another star.

We know what happens during a galactic collision as telescopes can see them in action elsewhere in the universe. The most striking effect is on the shape of a galaxy. What started out as a neat disc or elegant spiral can be contorted beyond recognition by tidal forces. If the collision speed is slow, the two galaxies may not have enough energy to separate again, resulting in an eventual merger into a single giant galaxy. Astronomers believe that will happen with the Milky Way and Andromeda – and they’ve coined the name ‘Milkomeda’ for the future combined galaxy.

Galactic tides 

As distance between galaxies gets smaller, they begin to distort each other via tidal stresses. These arise because the pull of gravity on one side of the galaxy is stronger than on the other, analogous to the way the Moon pulls more strongly on one side of the Earth. Just as the Moon raises tides by distorting the shape of the ocean, so one galaxy can alter the shape of another – on a vaster scale. Stars inside the galaxy can be flung onto completely different orbits, sometimes forming long ‘tidal tails’. According to one estimate, the Sun has a small chance of ending up in such a tail, but if so, it’s likely to take its retinue of planets with it, so the Solar System would survive unscathed.

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