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At a distance of 2.9 million light-years, M31, also known as the Andromeda Galaxy, is the nearest large galaxy to our own Milky Way. Its diameter of 200,000 light-years is about double that of our own galaxy. For decades, M31 was thought to be the most massive galaxy in the Local Group, but recent data suggest that its mass is about 300 billion to 400 billion solar masses, somewhat  less massive than the 600 billion solar masses of the Milky Way which is thought to have a larger halo of dark matter. At magnitude 3.4, M31 is visible with the naked eye even through moderate light pollution and is the farthest object distinctly visible with the naked eye after M33, which requires a very dark sky to be seen. From a dark site M31 is an impressive naked-eye object, and it is spectacular in binoculars or small telescopes. With a large telescope, some of the galaxy's star clusters can be seen. M32, located left of the nucleus of M31, and M110, located to the lower right, are satellite galaxies of M31. M32, although smaller than M110, has a higher surface brightness and is easily visible in small telescopes. M32 is about 8,000 light-years in diameter and contains about 3 million solar masses. M110, a bit more difficult to observe than M32, is about 17,000 light-years in diameter and has eight known globular clusters associated with it.

Equipment: ST-10XME/Nikkor 300mm f/4 lens/Losmandy G-11/Gemini/Astrodon RGB filters
F-ratio: f/5.6
Exposure: LRGB: L 18 x 5 minutes, binned 1x1: R 16 x 5 minutes: G 16 x 5 minutes: B 20 x 5 minutes: RGB binned 2x2
Date: October 8, 2004 and October 29, 2005
Location: Landers, California, USA
Technical Notes: Individual exposures were sigma combined. The RGB images were combined in Registar 1.0. Unsharp Mask was selectively applied to the brightest portions of the luminance image at 100% with a radius of 1 pixel and a threshold of 0 and then repeated. A Gaussian Blur of 1 pixel was applied to the dim portions of the luminance image and to the RGB image.