The luminosity class of NGC 646 is III.[1] It has surface brightness equal to 14.69 mag/am2.[5] NGC 646 is alow surface brightness galaxy (LSB).[5] LSB galaxies are diffuse (D) galaxies with a surface brightness less than one magnitude lower than that of the ambient night sky.
The Hubble distance of PGC 6014, the galaxy to the east of NGC 646, is 106.4 ± 7.5 Mpc (~347 million ly).[6] A distance of approximately 45 million light years therefore separates these two galaxies which appear to be neighbors in the image. Their gravitational interaction, if there is any interaction, should therefore be of short duration.[3]
