A silhouette of an amateur astronomer standing in awe of the gigantic Orion Nebula (some 1500 light years away) on a mountaintop. The Orion Nebula (M42; NGC 1976) is a huge glowing cloud of dust and gas, most of which is hydrogen, about 15 light years across and 1600 light years away. It is just visible to the naked eye as a luminous pink patch in the center of Orion's sword. The gas is illuminated by ultraviolet radiation produced by a cluster of extremely hot, young stars in the centre of the nebula (not visible). The radiation ionizes hydrogen atoms, causing them to emit light of a distinctive wavelength (color). The Orion Nebula contains many sources of infrared radiation, which are thought to be sites of stellar formation. Composite image.