One of the problems of living inside our galaxy is that it can be devilishly tricky figuring out exactly what the Milky Way actually looks like. We might have discovered a whole new arm of our galaxy...and it's very strange.
It was back in 1852 that College of New Jersey astronomer Stephen Alexander first suggested the galaxy has a spiral shape. Since then, we've identified at least six arms of that spiral, and in the 1990s we found evidence that there is a star-heavy central bar running through the galactic plane. It's not easy figuring all this out because our neighboring stars tend to obscure the ones further away, making it difficult to identify our galaxy's larger structures with precision.
Here's our current understanding of the Milky Way's geography. The central bar has a huge proportion of our galaxy's stars, and jutting off of it are the two main arms of the spiral, which are the Perseus Arm and the Scutum-Centaurus Arm. The other four arms are mostly just gas and relatively unimportant. Our solar system is close to the Perseus Arm, which extends about 300 degrees around galactic center.
But Harvard-Smithsonian Center astrophysicists Thomas Dame and Patrick Thaddeus have discovered something that complicates that picture. They discovered a new arm in a part of the galaxy surprisingly far away from the center. It's an offshoot of the Scutum-Centaurus Arm, both of which are obscured by being on the other side of the galactic center. The discovery of this new arm provides good evidence that the Scutum-Centaurus Arm is just as large and expansive as the Perseus Arm, suggesting we live in an almost perfectly symmetrical galaxy.
Except there is one rather large catch. The reason we've only now identified this arm is because it does not lie on the galactic plane. Instead, it lies at an angle, meaning it's slightly above the rest of the Milky Way, which mostly exists on an effectively two-dimensional cross-section of the universe. The Milky Way then would look rather warped, not unlike a bottle cap right after it's been opened. It's not the most elegant image for our home galaxy, but that's reality for you.
arXiv via Technology Review. Image via.