English: Emission nebula NGC 604 lies in a spiral arm of galaxy M33, 2.7 million light-years away in the constellation Triangulum. This is a site where stars are being born. Though such nebulae are common in galaxies, this one is particularly large, nearly 1,500 light-years across. At the heart of NGC 604 are over 200 hot stars, much more massive than our Sun (15 to 60 solar masses). They heat the gaseous walls of the nebula making the gas fluoresce. Their light also highlights the nebula's three-dimensional shape, like a lantern in a cavern. The image was taken on January 17, 1995 with Hubble's Wide Field and Planetary Camera 2.