Another definition of a relativistic star is one with the equation of state of a special relativistic gas. This can happen when the core of a massive main-sequence star becomes hot enough to generate electron - positron pairs. Stability analysis shows that such a star is only marginally bound, and is unstable to either collapse or explode. This instability is believed to limit the mass of main-sequence stars to a couple of hundred solar masses or so. Stars of this size and larger are able to directly collapse into a black hole of either intermediate or supermassive size.