Q671450

NGC 7331, also known as Caldwell 30, is an unbarred spiral galaxy about 13.427 megaparsecs (43.79 million light-years) away in the constellation Pegasus. It was discovered by William Herschel on 6 September 1784.

The galaxy appears similar almost in size and structure to the Milky Way, and is sometimes referred to as "the Milky Way's twin". However, discoveries in the 2000s regarding the structure of the Milky Way may call this similarity into doubt, particularly because the latter is now believed to be a barred spiral, compared to the unbarred status of NGC 7331. In spiral galaxies the central bulge typically co-rotates with the disk but the bulge in the galaxy NGC 7331 is rotating in the opposite direction to the rest of the disk. In both visible light and infrared photos of the NGC 7331, the core of the galaxy appears to be slightly off-center, with one side of the disk appearing to extend further away from the core than the opposite side.