The Orion Nebula, which appears to surround the central star in Orion's Sword , is a vast cloud of interstellar gas and dust some 20 light years across that is lit by a quartet of hot stars (the Trapezium) at its apparent center. Some 1500 light years away, the Trapezium and Nebula lie in front of a cold, dusty "giant molecular cloud" that hosts regions of intense star formation. The stars of the Trapezium electrify a blister on the front edge of the molecular cloud, which makes it glow.

More technically, energetic ultraviolet starlight (most of which comes from Theta-1 Orionis C) ionizes a portion of the molecular cloud, that is, it strips electrons from the nebula's atoms, which are mostly hydrogen. When the charged ions recapture the free electrons, the energy is given back up as (in part) optical light in the form of emission lines.

University of Illinois Prairie Observatory.

By Jim Kaler. Return to STARS.