We’ve known for decades that our closest large galactic neighbour, the Andromeda Galaxy (M31), is heading toward our galaxy, the Milky Way, at around 120km per second (approx 75 miles per second).
But until recently, astronomers were unsure whether it would score a direct hit, a near miss or just deliver a glancing blow.
The problem is that it’s quite easy to measure the relative approach speed of M31, but very difficult to measure its exact direction. Our best estimate says that M31 will collide almost head-on with the Milky Way in about 4 billion years.
However, recent research has suggested that the collision may already be underway. This is because both galaxies are surrounded by enormous halos of gas that stretch far beyond the visible stars, possibly accounting for up to 70 per cent of their mass.
In this case, the halos of M31 and the Milky Way are already interacting.
This article is an answer to the question (asked by Lewis Irwin, Shrewsbury) 'When will the Milky Way and Andromeda galaxies collide?'
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