November: The Great Galaxies

Abstract

The Great Andromeda Galaxy, M31, is the farthest object that most of us will ever see with the naked eye. At a distance of over 2 million light-years, this galaxy is so huge that it occupies an area in the sky several times larger than the full moon! Although similar to our own Milky Way Galaxy, M31 may be twice as large, containing 300 billion suns. Billions of stars are packed tightly together at the galaxy’s core, creating the bright central glow, concealing a central massive black hole. Two satellite galaxies of M31 are in the same view. M32, a dwarf elliptical galaxy containing a mere 3 billion solar masses, is tucked in tightly on the lower left of M31. M110 is slightly larger, seen in the upper right of this image. Imaging. The Andromeda Galaxy should be one of the first targets attempted by a novice imager. If your CCD chip is small, acquire your image through a camera lens to capture a wide field of about 2°. Routine RGB imaging or single-shot color imaging at maximum resolution is easy with this bright large object. Consider LRGB only to reveal the dimmer portions of the outer arms with luminance exposures. Advanced imagers may try mosaics through longer focal lengths to boost the resolution. Processing. The interesting dark lanes of the Andromeda Galaxy are largely in the dimmer areas. Stretch your histogram with either digital development or levels and curves. Sharpen around the dark lanes to emphasize their contrast. If you have saturated the core, avoid sharpening in the center of the core, or you may end up with abrupt transitions in brightness where you would prefer a smoother appearance. Enrich your color gently to distinguish the yellow core from the brown dark lanes and the blue outer arms. Subtract light-pollution gradients if needed (Fig. 11.1).