Milky Way shakes: The cosmic collisions that made our galaxy

This composite image of X-rays from Chandra (colored light blue) and optical data from the Canada-France-Hawaii Telescope (yellow, red, white, and blue) shows a beautiful new look at the compact group of galaxies known as Stephan???s Quintet. One galaxy is thought to be passing through the others at almost two million miles per hour. This generates a shock wave that heats the gas and creates the ridge of X-ray emission detected by Chandra.

A galaxy passes through other galaxies in a group known as Stephan’s quintet

X-ray (NASA/CXC/CfA/E.O’Sullivan); Optical (Canada-France-Hawaii-Te​lescope/Coelum)

HUMANS have always been captivated by the Milky Way. To the ancient Greeks, it was a squirt of breast milk, while the Mayan farmers of Mesoamerica saw it as a growing maize stalk. Today, astronomers understand that what we see when we gaze up at the hazy streak of light stretching across the night sky is actually just one part of our host galaxy – one of four spiral arms emanating from an enormous disc of dust and gas that together contain at least 100 billion stars.

But even as we have traced our cosmic origins on the grandest scales, from the big bang to the growth of trillions of other galaxies entangled in vast webs, the history of our galactic backyard has remained opaque. It isn’t for want of trying. The problem is that when you are inside something, it can be hard to see what is really going on.

Now, a clearer picture of how our galaxy, the Milky Way, came to look the way it does is beginning to emerge thanks to data gathered by the European Space Agency’s Gaia satellite. Hidden patterns in the movements of stars have taken astronomers by surprise, unveiling in rich detail a turbulent history of cataclysmic collisions and aftershocks.

Some say that soon we will even be able to pinpoint the specific events that led to the formation of our sun – the reason we are here to ponder such things. “It’s our heritage, a way of knowing …