Abstract :
Astronomers who study the Milky Way don\´t have it easy. Bound to the solar system, they\´re on the inside looking out, all the while whipping around the galactic center at roughly 900 000 kilometers per hour. That\´s made it more than a little tricky to pin down fundamental details. It\´s still unclear, for example, how massive the Milky Way is and whether it\´s on a collision course with the nearby Andromeda galaxy. And there\´s still a lot of uncertainty about its basic structure. "There\´s been a pretty active debate in the last couple of years whether there are two or four spiral arms," says Mark Reid, a radio astronomer at the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Mass. "That\´s pretty basic".
