Estimating a star's mass without using the gravitational constant (G) is challenging, as most methods rely on it. For main sequence stars, mass can be inferred from spectral class, color, absolute luminosity, and surface temperature. However, any estimation method ultimately connects back to gravity principles, which incorporate G. Observational relationships, like mass-luminosity, are also based on prior measurements that depend on gravitational theories. Clarification of the original question would help in providing a more precise answer.