Above, a wide-angle view of M31 (the Andromeda Galaxy), the nearest large galaxy to ours, a little over 2 million light years away. M31 is very similar to our own galaxy, but a little larger and more massive. Because of its large size and relatively close distance, the galaxy is easily visible as a faint fuzz-ball without optical aid in a dark sky; but in city lights, is only a faint smudge even in a large telescope. The actual size of the galaxy spans several degrees, but is only visible with long exposures in dark skies. The fuzzy elliptical balls near M31 (M32, above, and NGC205, below) are dwarf elliptical galaxies, which are companions or satellites of M31. The image below is a closer view, emphasizing the various colors of regions with different kinds of stars, by using different choices for the colors assigned to black-and-white images taken at various wavelengths (click on the image for a still closer view).

     In the outer portions of galaxies, stars are further apart than in the inner regions, making the surface brightness too low for easy detection. Using longer exposures, as in the above image of M31, shows that the disc extends considerably further than might usually be thought. Very long exposures can even reveal the halo, which typically extends two to three times the distance occupied by the disc. Conversely, short exposures can reveal details in the core of the galaxy, which is grossly overexposed in most images. It is in this way that we are able to estimate the distribution of stars, which are typically light weeks apart near the center of the nucleus, light months apart in the outer parts of the nucleus, light years apart in the disc, and light decades apart in the halo.

Andromeda Gökadası (ayrıca Messier 31, M31, ve NGC 224 olarak da bilinir) Andromeda Takımyıldızında bulunan bir sarmal gökadadır. Spitzer Uzay Teleskopundan ele edilen verilere göre 1 trilyon yıldıza ev sahipliği yapmaktadır. 2006 ölçümlerine göre Samanyolu, Andromeda'nın kütlesinin ancak ~80%'ine sahiptir. Andromeda'nın bir diğer özelliği ise, çıplak göz ile Yer'den görülebilen en uzak gök cismi olmasıdır. Ayrıca Samanyolu'na en yakın büyük gökadadır.