Mu Cephei, also known as Herschel's Garnet Star due to its red, garnet-like color, is a red hypergiant star in the Cepheus constellation. With a radius of 1,420 solar radii, or a diameter of 1,977,000,000 kilometers, it is one of the largest stars known. Mu Cephei is also the "prototype" of the semiregular variable star type.