The Orion nebula, also known as M42, is a galactic nebula consisting of gas and dust, which because of its brightness and
angular size has been much studied by astronomers. The nebula is located at a distance of about 1,500 light-years. It has a strong emission-line spectrum. The main sources of its radiative energyÑwhich is absorbed and reradiated by gas in the nebulaÑare the two brightest stars in an area known as the Trapezium, a small group of six stars. Photographs taken with large telescopes show that the gas has a patchy, irregular structure.The Orion nebula also has a weak continuous spectrum, with absorption lines. It is formed by light from the stars in the Trapezium, scattered by the
interstellar dust in the nebula. Strong infrared
radiation is emitted by this dust, which is heated by the shorter-wavelength radiation it absorbs. Many interstellar molecules are present in the dense condensations of interstellar matter in the nebula.