The Andromeda galaxy (also known as M31) is the nearest large neighbour to the Milky Way, composed of an almost unimaginable 1 trillion stars. To count them all at a rate of one a second would take almost 32,000 years. It's the most distant object we can easily see with the naked eye. At 2.5 million light years away the light we see now set out when our ancestors looked something like this.
It's said that the Andromeda galaxy is as big as 6 full Moons in the sky but is this the whole story?
To the naked eye it never appears this large. Its central core is much brighter than the spiral arms due to the higher concentration of stars. It's only this region that's visible. The darker your skies the easier it is to pick out from the background, and the larger it appears, but it's no bigger than the Moon.
Binoculars or a telescope gather more light and produce a brighter image, allowing more of the galaxy to be seen. With a medium-sized telescope (perhaps a reflector with a 6" mirror) in good conditions it becomes possible to make out the spiral structure.
Long-exposure photographs reveal more of the galaxy's faint outer regions, as the camera can be used to collect light over minutes or hours.
In a long-exposure image the Andromeda galaxy does indeed have the same apparent size as 6 full Moons. The image below was taken using a small telescope and cooled camera. The 2 hour 30 minute exposure time reveals the faint glow from the sparse population of stars in its outer reaches.
A bridge of stars connects M31 with its satellite galaxy, M110. The immense gravitational pull is distorting its smaller companion.
An even longer exposure, such as this exceptionally deep image by Olly Penrice, shows the outer regions more clearly. However, galaxies don't have sharply defined limits. The 300 ton Keck telescopes in Hawaii have found a sparse population of stars, extending the diameter of the disc from 120,000 light years to 220,000 light years. From our perspective, the full extent of the disc is 5 degrees, 10 times the apparent width of the full Moon.
Beyond the disc is an even fainter halo of stars, and beyond that a huge envelope of tenuous gas spanning two million light years. The gas slowly accumulates onto the disc, contributing to the formation of future generations of stars.
Whatever Andromeda's actual size, it's getting bigger. It's heading this way and will collide and merge with our own galaxy in a few billion years.
But several nebulae in our own galaxy have an even larger apparent size than the Andromeda galaxy, such as the Angelfish nebula on the shoulders of Orion.