A bright emission nebula surrounding the multiple star Theta1 Orionis θ1 Ori in the sword of Orion. The luminous nebula is just part of a complex region of interstellar matter at a distance of 1,300 light years occupying much of the constellation of Orion. The Orion cloud is the largest such dark cloud known in the Galaxy. Millimetre-wave observations of the emission from the molecules CO carbon monoxide, HCHO formaldehyde, and many others, reveal the presence behind the visible part of a large molecular cloud, known as the Orion Molecular Cloud OMC-1. This is an important region of star formation, and the group of four young hot stars that make up θ1 Orionis, also known as the Trapezium, are believed to be less than 100,000 years old. The Becklin-Neugebauer object and the Kleinmann-Low Nebula, both detected through their infrared emission, are sites of current star formation. The Trapezium stars are creating an expanding spherical cavity near the edge of the dark cloud. Their ultraviolet radiation is ionizing the gas and blowing away the dust. Relatively recently, in astronomical terms, the bubble broke through on our side of the dark cloud, revealing the stars and ionized hydrogen within. The sharper edges of the nebula are produced by remnants of dust. M43 NGC 1982 is a northern section of the nebula separated from the larger part M42; NGC 1976 by a lane of dust. In photographs, the dominant colour of the luminosity is red from the hydrogen alpha light. Observed visually, the nebula appears greenish because of the eye’s low sensitivity to red light. The green emission is due to oxygen. The nebula occupies an area of sky about one degree across and is faintly visible to the naked eye. It has the highest surface brightness of all nebulae.