We think of orbits as being nice and elliptical and closed, whether it be a planet around a star or a star around a galaxy. However, things are not so neat in reality and especially within galaxies.* This has been further demonstrated in our own Milky Way with the discovery of stars streaming away from the Galactic Center in the Radial Velocity Experiment (RAVE).
RAVE is a survey to learn as much about the stars in our general neighborhood as we can from velocities, distances, and contents of stars. The current work uses data from over 200,000 stars within 6500 light years of our solar system. By 2012, they hope to have cataloged 1 million stars!
Stellar orbits are not at all circular, but often have interesting rosette shapes in the plane of the galaxy and can cross the plane to be above and below it. The whole aggregate of stars moving in this way gives the galaxy its overall thickness and shape, which then in turn affects the orbits of the stars in it.
The stars in question mostly lie in the fourth Galactic Quadrant. (No, not those Quadrants. These Quadrants.) In studying the components of the stars' velocities, the astronomers found the stars moving a bit away from the center of the Galaxy. In fact, the closer they were to the Galactic Center, the faster they were moving away from it.
This kind of motion is created by some sort of asymmetry in the Galaxy. It is by no means a perfect disk of stars. Spiral arms and a central bar, stellar traffic jams though they are, can affect the motions of stars throughout the Galaxy and guide them into their non-circular orbits. The stellar disk itself is warped a bit, making the picture more complicated. If the halo of dark matter in which the Milky Way sits is not perfectly spherical, that too could affect stellar orbits.
It is not yet sure which cause is dominant, or if they are all conspiring together to move the stars as they do. As the RAVE survey and other large projects gather data on millions of stars, we are sure to get a clearer picture of the very Galaxy in which we live.
Image: Illustration of the Milky Way with the Galactic Quadrants labelled and a yellow arrow to indicate the approximate location and radial component of velocity of the RAVE stars. Credit: Original image by NASA, improved on WikiMedia Commons, and further mucked about with by the author.
This research will be published in the Monthly Notices of the Royal Astronomical Society, and a preprint is available on arxiv.org.
*And I have a whole textbook to prove it. Raise your hand if you have a copy of Binney & Tremaine!