Radius and absolute magnitude of a Cepheid variable

Cepheid variables are used with confidence for the determination of the distances of all stellar systems as two independent methods agree on their absolute magnitudes. Unfortunately, they are too faint to be seen beyond the local group of galaxies and are of no use for the distances of galaxies that are more remote. On the other hand, Supernovae are up to 1000,000 times brighter then Cepheids and can, consequently, be seen and recognized in stellar systems that are far beyond the local group. They would be ideal distance indicators if the authors uses a luminosities were more accurately known