Our Galaxy is Being Invaded

If you shake off the bright lights of the city and head out into the country, you’ll see the bright band of the Milky Way stretching across the sky. The Milky Way is the galaxy we live in, our home metropolis populated by more than 100 billion stars, and it’s part of the Local Group: an imaginatively-named, gravitationally-bound group of galaxies, hanging out close to each other in space. We’ve known about most of our galactic neighbours for decades, like the Large and Small Magellanic Clouds and the Andromeda galaxy - but in 1994, astronomers realised that there was another one lurking right next door.

Image Credit: R. Ibata, R. Wyse & R. Sword

They were studying the stars at the centre of the Milky Way, where there’s a large bulge of high concentrations of stars, when they realised that some of these stars weren’t moving as expected. This odd group of stars were all moving together at the same speed, and it was soon realised that they didn’t belong to the Milky Way at all - they’re part of a dwarf galaxy chilling out on the other side of our galaxy’s central bulge. Dubbed the Sagittarius Dwarf Spheroidal Galaxy, it’s just 50,000 light years from the centre of the Milky Way, which isn’t very far when you consider that our galaxy is 100,000 light years in diameter.

But here’s the interesting part: our neighbour doesn’t just keep to itself. It’s a satellite galaxy of the Milky Way, orbiting us every billion years, and it can get pretty friendly, actually plunging through the plane of the Milky Way as it orbits. It’s orbited us at least 10 times in the past, and is going to pass through us again in the next 100 million years. In the first image above, you can see the blue spiral of the Milky Way with the orbit of the dwarf galaxy traced out in red.

Eventually, the tidal forces from the much bigger Milky Way will probably tear the Sagittarius Dwarf Spheroidal Galaxy apart, and we’ll absorb all of its stars into our own galaxy in an act of immensely cool galactic cannibalism.