Emission nebula and illuminating star
Animation of a flight into an emission nebula, showing the massive hot young star whose radiation causes the nebula to glow. Emission nebulae are starbirth regions, where huge clouds of gas contract through gravity until regions become hot and dense enough for stellar nuclear reactions to begin. This process can produce massive stars that burn far more brightly than the Sun, emitting a huge amount of ultraviolet radiation. This radiation excites atoms in the remaining gas in the nebula, causing it to emit light as it returns to its ground state. These starbirth regions typically appear red/pink, as the ultraviolet light ionises hydrogen atoms, and their resulting relaxation produces light with a wavelength of 656.3 nanometres, in the red part of the electromagnetic spectrum.