Our Milky Way Galaxy Grows By Eating Other Galaxies

We know this for many reasons. For one, we can see other galaxies colliding and merging all over the sky. For another, we can track stars on our galaxy that were once part of another galaxy, but were absorbed into our own. These are usually relatively recent events involving a much smaller galaxy, the SYFY reported.

But a new result changes that. By mapping huge numbers of stars in the galaxy, astronomers have found compelling evidence that the Milky Way ate a galaxy that was, at the time, about a quarter its size. And that time was a staggering ten billion years ago.

The stars in question were mapped by Gaia, a European Space Agency satellite. It is in the process of mapping the positions, motions, colors, and most importantly the distances of well over a billion stars. Yes, a billion.

Gaia has provided nothing short of a revolution in astronomy. How can mapping stars be so important? In some cases it can solve long-standing puzzles that have irritated astronomers for decades. In others it can reveal hidden denizens of the Milky Way. It can resolve uncertainties in distances to critical stars called Cepheids — like Polaris, the North Star — that are the bottom rung of the distance ladder, where each rung is a single step, but by the time you get to the top you're measuring distances on a cosmic scale.

And in this new case, it reveals the remnants of a long-dead galaxy, one used as sustenance for our own.

What Gaia found was a vast stream of stars, 30,000 strong, that travel along similar orbits around the center of the Galaxy: highly elongated, tipped to the plane of the galaxy's disk, and weirdly in a direction backward relative to other stars. This structure is so big that we're actually inside it — literally, it surrounds the Sun in all directions — and can be seen stretching nearly across the entire sky. This stream is the remnant of the now-eaten galaxy.

The astronomers even gave that galaxy a name: Gaia-Enceladus, after the observatory that found it, of course, and the name of one of the Giants, which in Greek mythology were created when the Titan Cronus castrated his father Uranus and the blood was "received" (um, yeah) by the Earth-goddess Gaia. Yeah, I know, but Greek mythology is as gruesome as it is just plain weird. Anyway, the myth is a rough fit to what happened to the galaxy, so why not. My only complaint is that we already have a moon of Saturn named Enceladus, but given the relative scales they're not likely to be confused with each other. - Istanbul