The Milky Way Just Got Larger

An annotated map showing the Milky Way's structure, based on measurements to distant stars and other objects. (Credit: NASA) Despite residing in it, it’s hard for us to know exactly how big the Milky Way is. But new research has found that our galaxy is bigger than previously thought. Using a large survey of stars instead of just models (as previous researchers did), astronomers have now determined the disk of our galaxy to be 200,000 light-years across — twice as large as was believed a decade ago. Astronomers know the Milky Way to be a spiral galaxy with a flat central disk composed of spiraling arms and a sparse outer cloud of stars called the halo. Most of the stars are found in the disk, which rapidly thins out at the edges. The size of the galaxy is measured from the point at which the number of stars in the disk significantly drops, so finding disk stars is key to determining the Milky Way’s size. Fortuitously, disk stars tend to be much younger than the halo stars. Since stars gradually build up elements over generations, the disk stars’ composition can be differentiated from the halo stars, allowing astronomers to figure out which stars belong to which population.