See the color spectrum.

North is up and a bit to the left. The Orion Nebula (Messier 42), 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. The small nebula that seems attached at the upper left edge of the main nebula is Messier 43, which is separated from M 42 by a dust lane. The stars and nebula are no more than two or so million years old. The glow at the bottom is coming from the massive multiple star Iota Orionis. (The weak squarish feature at lower right and the streak at upper left are caused by internal reflections in the optics.)

See full resolution.


Above is the visual spectrum of the Orion Nebula from a wavelength just to the long side of the Hydrogen-Alpha line at 6563 Angstroms to just shortward of H-Delta at 4101 Angstroms. (The wavelength scale is in nanometers; multiply by 10 to get Angstroms.) Energetic ultraviolet starlight (most of which comes from the hot class O6 star 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 recombine with the free electrons, the energy is given back up as light in the form of emission lines. The Hydrogen Alpha line helps give much of the nebula its reddish color. From the strong neutral helium line at 5876 Angstroms we can get the abundacnce of helium relative to that of hydrogen.

Additional "forbidden" emission lines (forbidden under simplified theory, but strong in nebular spectra) are produced by collisions between electrons and ions, which elevate the ions' electrons to excited states from which they descend, again releasing energy. The strong forbidden lines (indicated by square brackets) of doubly ionized oxygen ([O III}) just left of give a greenish glow to the nebula's central portion, while those of ionized nitrogn that bracket H-Alpha add to the general red color of the outer portions of the nebula. Not shown, to the right of the H-Alpha and [N II] lines are a pair of strong forbidden lines of ionized sulfer ([S II]), while to the left of H-Delta is a pair of strong forbidden llines of ionized oxygen ([O II]), as seen in the spectra of the planetary nebulae BV-1, the Ring Nebula, NGC 7009, and IC 418. Weak forbidden lines of neutral nitrogen show up near dead center.

Photo: University of Illinois Prairie Observatory; spectrum: Okayama Astrophysical Observatory, NOAJ.

By Jim Kaler. Return to STARS.