This is a Hubble Space Telescope image of a vast nebula called NGC 604, which lies in the neighboring spiral galaxy M33, located 2.7 million light-years away in the constellation Triangulum. This is a site where new stars are being born in a spiral arm of the galaxy. Though such nebulae are common in galaxies, this one is particularly large, nearly 1,500 light-years across. The nebula is so vast it is easily seen in ground-based telescopic images. At the heart of NGC 604 are over 200 hot stars, much more massive than our Sun, which heat the gaseous walls of the nebula making the gas fluoresce.