<P> If the mass of the stellar remnant is high enough, the neutron degeneracy pressure will be insufficient to prevent collapse below the Schwarzschild radius . The stellar remnant thus becomes a black hole . The mass at which this occurs is not known with certainty, but is currently estimated at between 2 and 3 M . </P> <P> Black holes are predicted by the theory of general relativity . According to classical general relativity, no matter or information can flow from the interior of a black hole to an outside observer, although quantum effects may allow deviations from this strict rule . The existence of black holes in the universe is well supported, both theoretically and by astronomical observation . </P> <P> Because the core - collapse supernova mechanism itself is imperfectly understood, it is still not known whether it is possible for a star to collapse directly to a black hole without producing a visible supernova, or whether some supernovae initially form unstable neutron stars which then collapse into black holes; the exact relation between the initial mass of the star and the final remnant is also not completely certain . Resolution of these uncertainties requires the analysis of more supernovae and supernova remnants . </P> <P> A stellar evolutionary model is a mathematical model that can be used to compute the evolutionary phases of a star from its formation until it becomes a remnant . The mass and chemical composition of the star are used as the inputs, and the luminosity and surface temperature are the only constraints . The model formulae are based upon the physical understanding of the star, usually under the assumption of hydrostatic equilibrium . Extensive computer calculations are then run to determine the changing state of the star over time, yielding a table of data that can be used to determine the evolutionary track of the star across the Hertzsprung--Russell diagram, along with other evolving properties . Accurate models can be used to estimate the current age of a star by comparing its physical properties with those of stars along a matching evolutionary track . </P>

What are the different stages of a star