THE ORION NEBULA
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.