H-alpha Imaging of the Trifid Nebula

This comparison between an unfiltered CCD image and one taken through a narrow-band hydrogen-alpha filter shows how the appearance of the Trifid nebula (M20) is dominated by hydrogen-alpha emission from its three major gas clouds. The graphs illustrate the spectral range of each image, modified by the quantum efficiency of the CCD as a function of wavelength. The red tint of the H-alpha image is artificial, but it is similar to the true color.

The central star HD 164492 is a multiple system, with its two brightest components resolved in the H-alpha image. The one at upper left has been classified O7 III((f)) and is the main source of energy that causes the nebula to glow.

The H-alpha filter has a 3 nm passband, so it partially includes two emission lines from ionized nitrogen in addition to the hydrogen line. An exposure through another narrow-band filter at a nearby wavelength to correct for the dusty nebular continuum was foiled by clouds.

Playtime: In a 3D plot with elevation proportional to H-alpha brightness, the Trifid becomes a magical landscape with three bright hydrogen islands separated by dark rivers of interstellar dust and cool gas. The central star becomes a castle with the nearby bright ionization rim as its fortified wall.