The Magnificent Messiness of the Milky Way in Perspective
Astronomers have created a new map of the Milky Way's spiral arms using high-mass star-forming regions, bright young stars, and young open clusters as reliable tracers of spiral arms. The team found a total of seven spiral arms, with the Perseus and Norma arms dominating the others. The other five arms don't extend nearly as far as the major two and begin either as forks of the major arms or as loose segments of their own. Spiral arms represent places of greater-than-average star formation, triggered by gravitational interactions with satellite galaxies, that travel through a galaxy at their own speed, like ripples in a pond.