Neutrinos, the He-4 abundance, and stellar evolution

Constraints on the number of neutrino flavors N(nu) are discussed. Cosmological limits on N(nu) are enumerated, and it is pointed out that they imply that N(nu) is less than 6600. Astrophysical constraints based on stellar evolution are discussed, including the photoneutrino process, the bremsstrahlung process, and the properties of red giants. A safe limit for N(nu) of 760 is obtained considering only the latter factor, while a much lower but more uncertain limit of 23 is obtained by considering the two processes. The major source of uncertainty is the lack of observational data regarding carbon burning stars. A third source for a limit on N(nu) comes from considering the cooling rates of neutron stars. Neutron emission processes from these stars lead to limits which, while not as strong as the ones derived from carbon burning stars, again indicate that N(nu) is less than 6600.