Nebula NGC 604 is an H II region inside the Triangulum Galaxy. It was discovered by William Herschel in 1784. The glow of the nebula is because it is an emission nebula, so unlike a reflection nebulae, where light from neighboring stars reflect off it, or dark nebulae, which are completely unilluminated, NGC 604 is lit by stars being formed inside it. The stars, that form from the compaction and heating of dust and gas due to gravity, ionize the gas of the nebula, causing it to emit light.