To our feeble eyes, Andromeda appears as a dim smudge of light in the night sky. With decent binoculars, its elliptical form sharpens into view. The light we see when peering up at Andromeda comes from the hundreds of billions of stars that compose the galaxy and its spiral arms. If each of those stars were to shine brighter, the image above is what it would like to us on Earth—a beautiful galactic whorl for all to see.
The image above was created by Tom Buckley-Houston, who superimposed the Andromeda galaxy on a picture of the night sky with a moon for comparison.
“The GALEX image of Andromeda is in the ultraviolet, and the extent of this image is close to the size of the optical image of Andromeda that everyone is familiar with,” says astronomer Alan McConnachie of the National Research Council Herzberg in Victoria, Canada. “The moon is about 0.5 degrees in diameter; the Andromeda galaxy, measured on its longest axis, is approximately 2-3 degrees long.”
Thus, if possible for Andromeda to be more luminous, it would appear roughly six times larger than our moon. But at 2.5 million light-years from Earth, the galaxy is not as easily seen as the crescent in our sky—which is only 238,900 miles (384,400 kilometers) from Earth.
“As a side note, its worth noting that the "size" of the Andromeda galaxy only really corresponds to that part of Andromeda that everyone is familiar with,” says McConnachie. “That part of Andromeda is most of the disk, which is where the overwhelming majority of the stars are located. However, if you include the much fainter outer parts of the galaxy, then the actual spatial extent of Andromeda is much, much, much larger.”
The full extent of Andromeda, according to McConnachie, is at least 20 degrees across, or the equivalent of more than 40 full moons!