Hide and seek, galaxy style.

The Milky Way Galaxy, home to our Sun and a few hundred billion other stars, is definitely a place to be proud of. As galaxies go, it's one of the larger ones, and, although opinions are sure to vary, it is also one of the loveliest.

Galaxies come in many different shapes: some are spherical, some are sort of blobby and disorganized, and some are disk-shaped, with elegant spiral arms of young stars radiating out from their centers. We belong to this last class, the so-called "grand design" spiral galaxies, which seem almost too beautiful to believe. But the Milky Way, of course, is only one of countless billions of other galaxies that we know about, and astronomers recently got a pretty big surprise when they tried to figure out how the Milky Way fits into the larger scheme of things.

Take a fairly basic question: what is the closest galaxy to our own? A lot of people might say the Andromeda Galaxy, which is, in fact, the closest large spiral to us, about two million light years away. Andromeda is so close, in fact, that it is gravitationally bound to the Milky Way, and is one of the only galaxies in the sky that is moving toward us.

Other, somewhat more astronomically savvy people might name the Magellanic Clouds as the closest galaxies. The Large and Small Magellanic Clouds are two small, blobby galaxies that actually orbit the Milky Way, and are about 180,000 and 210,000 light years away, respectively. And up until the last few years, everyone seemed pretty pleased with this answer; after all, these were the closest galaxies we could see in the sky. But we were wrong to trust our eyes.

As it turns out, there are at least two other galaxies that are much, much closer. Part of the reason we missed them, in fact, is that they are so close; they are actually colliding with the disk of the Milky Way. In a very real way, they snuck up from behind us, hiding behind the stars, dust, and gas that fill the volume of the Milky Way's disk.

So how could we have missed two entire galaxies that are colliding with us? The answer has a lot to do with how we know we live in a spiral galaxy in the first place. I mean, you can't look up into the sky and see the spiral structure of our galaxy, so how do we know it's there? Most people are familiar with the dim, blurry band of light that crosses the sky, which ancient people from several cultures likened to a path of milk. This "Milky Way" is actually the combined light of billions of stars in the plane of our galaxy, and people from at least the eighteenth century have realized that our galaxy has a flattened shape.

But what about the spiral arms? That was a bit tricky. The plane of our galaxy is thick with stars, gas, and dark clouds of dust, and our Solar System is embedded inside all that material. There's so much stuff in the Milky Way's disk that we can't actually see very far into our own galaxy. Even the core of the Milky Way, a region where millions of stars are crammed together in tight orbits around a massive central black hole, is invisible to us, blocked by a thick dust cloud that sits inconveniently between us and the center.

So, at first, the most we were able to do was map the positions of the nearest and brightest stars to the Sun. When we did, we found that many stars, most notably the brightest, blue-hot, young ones, were distributed in layers of arcs that traced out the boundaries of the spiral arms. We still can't see the entirety of the arms, and we're not even sure how many spiral arms the Milky Way has. But even by mapping the small number of stars we're able to see, we can still deduce our place in the galaxy: we live about 30,000 light years from the center of the Milky Way, about 20 light years above the center of the plane of the disk, along a spur of stars called the "Local" or "Orion" arm, which is actually a minor arm that connects two larger spiral arms, the Perseus and the Sagittarius arms.

Simply observing the numbers, distribution, and motion of the stars has gotten us a long way, and that's how the first of the new galaxies was found back in 1994. Astronomers were studying the distances and velocities of stars near the central region of our galaxy, when they noticed something strange.

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