Just yesterday Fraser wrote about the Milky Way’s demotion from a 4-arm spiral galaxy to a 2-arm. This isn’t the only change we’ll have to accept about our home galaxy: a Milky Way mapping project has discovered stars in the galaxy moving slower and in more elliptical orbits than predicted. This means we might have to redraw the map we have of our own neighborhood yet again.

Astronomers using the Very Long Baseline Array (VLBA) – a collaboration of ten radio telescopes across the United States – tracked the positions of masers in a dozen star-forming regions in the Milky Way. They used parallax to determine the distance to the masers, then combined this information with how the masers shifted in the plane of the sky, giving a 3-dimensional model of their movement.

Drawing a map of the Milky Way is a challenging task, as we only have an edge-on view of the galaxy in which we reside. To top it off, it’s full of dust and gas that muck up the view in the visible light spectrum. Using the VLBA’s radio antennae, though, has made it possible to track radio-emitting bodies as they move across the sky because radio waves travel more easily through matter than does light. Since the VLBA functions as one huge telescope, it can track the position of stars with great accuracy.

“Right now, our map of the Milky Way still has large areas marked ‘Here there be dragons.’ Ten years from now, those areas will be filled in,” said Mark Reid, of the Harvard-Smithsonian Center for Astrophysics. Reid presented these findings at a press conference at the 212th American Astronomical Society meeting.

Instead of neatly circling the galactic center, the stars mapped by Reid and his colleagues are tracing an elliptical orbit. Previous maps of the Milky Way have assumed that the material in our galaxy orbits the center in a circular fashion, so stars that don’t follow this path come as somewhat of a surprise.

The stars are moving slower likely because of the loss of angular momentum when they interact gravitationally with other matter in the galaxy, traveling through what is called a ‘density wave’. The best description of a density wave I’ve run across has to be Phil Plait’s over at Bad Astronomy:

If you were in a helicopter over a traffic jam on the freeway, it would look like the jam is a permanent fixture of the traffic. But in reality, cars leave the jam at the same rate as cars entering it. So while the jam itself stays put, the cars making it up always change. So it is with spiral arms: they are places where the matter in the galaxy is compressed, but stars enter the jam and stars leave. The arm looks permanent, but over time its resident stars, gas, and dust change

This probably won’t be the last time the map of the Milky Way gets edited. The European Space Agency’s Gaia satellite is set to launch in 2011, and will provide a 3-dimensional map of 1 billion stars located as far as 30,000 light-years away from Earth.

Source: CfA Press Release