On January 5 this year, NASA released an image of a portion of Andromeda (aka M31), the nearest galaxy to our Milky Way. It’s the largest picture ever taken, a 1.5 billion pixel image (69,536 x 22,230) requiring 4.3 GB disk space. The full image is made up of 411 images captured by the NASA/ESA Hubble Space Telescope. 

The largest NASA Hubble Space Telescope image ever assembled, this sweeping bird’s-eye view of a portion of the Andromeda galaxy (M31) is the sharpest large composite image ever taken of our galactic next-door neighbor. Though the galaxy is over 2 million light-years away, The Hubble Space Telescope is powerful enough to resolve individual stars in a 61,000-light-year-long stretch of the galaxy’s pancake-shaped disk. It’s like photographing a beach and resolving individual grains of sand. And there are lots of stars in this sweeping view–over 100 million, with some of them in thousands of star clusters seen embedded in the disk. (via Hubble’s High-Definition Panoramic View of the Andromeda Galaxy | NASA)

A video http://youtu.be/udAL48P5NJU takes you through the photo (thanks to Chuck Cole for spotting this). Andromeda probably has a trillion stars, ten thousand times what is shown in the photograph.

Andromeda and the Milky Way will collide in about 4 billion years. Although more than a trillions stars are involved, the distances between them are so great that it is unlikely that any of them will individually collide.