If you’re lucky enough to live under a sky without light pollution, you can see the Milky Way as a faint band of light (like a pathway of spilled milk, hence the name) streaking across the sky through some well-known constellations like Scorpius and Cassiopeia.

IF YOU'RE lucky enough to live under a sky without light pollution, you can see the Milky Way as a faint band of light (like a pathway of spilled milk, hence the name) streaking across the sky through some well-known constellations like Scorpius and Cassiopeia. We actually live inside that streak of light; it’s a thin disc, and the streak is what we see looking lengthwise through the disc.

Ours is but one of two trillion galaxies visible to us. We now know that galaxies are the building blocks of the universe, formed just a billion years or so after the Big Bang. But after they were formed they started running into each other, and the smaller ones accreted on to the larger ones. Is it possible to actually map out where and when these mergers occurred in our own galaxy?

Thanks to the Gaia satellite which has measured the precise location and distances of the stars in our galaxy, and some clever mathematical modelling, Khyati Malhan and his colleagues have attempted to do just that. Based on the measured motions of 170 globular clusters, 41 different streams of stars and 46 smaller galaxies orbiting our own galaxy, they have been able to identify six different times a smaller galaxy was merged into ours. Given the huge time span they’re attempting to cover, it’s more than history: it’s archaeology.

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