The Orion Nebula
by Rob Gendler
Bright nebulae like the Orion nebula are illuminated by stars that recently condensed from their gas and dust. The illumination takes two forms: Parts of the cloud simply reflect starlight (and so are called "reflection" nebulae), while in other areas gas in the cloud is also "excited" by the light of nearby stars—absorbing and re-emitting energy just as happens with the gas in a neon sign (these parts are called "emission" nebulae). Advanced-color astrophotos like this one typically are made with a black-and-white CCD digital camera, chilled to reduce the effects of heat on the sensitive electronics during long exposures. Separate pictures are taken through red, green, and blue filters, and then combined to create a color composite. The red light in the Orion nebula comes from excited hydrogen, the blues from the reflected light of its most massive and hottest stars.