Why is the Andromeda galaxy so special?

Andromeda has a bright disk that scientists believe spans as much as 260,000 light years almost twice the size of the bright disk seen in photographs. The outer disk emits nearly 10 percent of the galaxy's total light and may be made up of stars stripped from smaller galaxies that strayed too close. In 2007, a team of astronomers announced the discovery of low-metallic, red giant stars up to some 500,000 light years from Andromeda's core. This discovery suggests that the galaxy is much larger than scientists originally thought, and that Andromeda's glowing halo may actually overlap with that of the Milky Way.