For a long time the most popular suspect in the ageing process was mergers: two gas-rich spiral galaxies collide and form a new one whose stars are scrambled out of their elegant structure. Astronomers call the young spiral galaxies “blue”, because young stars tend to be blue in colour, and the old blobby galaxies “red and dead”, because old stars appear red.

That leaves a transition zone between blue and red galaxies called the “green valley”, in part because green light falls between blue and red in terms of energy, although the stars don’t actually look green.

“The green valley is a mess,” says Daniel McIntosh of the University of Missouri-Kansas City. “It’s like a smorgasbord of all kinds of processes. We have a general picture that galaxies are becoming more red and dead, but how?” Read more.