Rigel is a blue supergiant star, while the others are either red giants (Aldebaran, Betelgeuse, Arcturus) or a red dwarf (Proxima Centauri). Rigel is exceptionally hot and luminous, with a blue-white color, making it spectrally different from the cooler red-colored stars in this list. Its classification as a blue supergiant makes it the clear outlier.