NGC 604

photo: A Hubble Space Telescope (HST) image of NGC 604.
Credit: HST/NASA/ESA.

NGC 604 is an H II region inside the Triangulum Galaxy. It was discovered by William Herschel on September 11, 1784. It is one of the largest H II regions in the Local Group of galaxies; at the galaxy's estimated distance of 2.7 million light-years its longest diameter is roughly 1500 light-years (460 parsecs), over 40 times the size of the visible portion of the Orion Nebula. It is over 6300 times more luminous than the Orion Nebula, and if it were at the same distance it would outshine Venus. Like all emission nebulae, its gas is ionized by a cluster of massive stars at its center.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.