A Q-star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. Such a star can be smaller than the progenitor star's Schwarzschild radius and have a gravitational pull so strong that some light, but not all photons, can escape.[citation needed] The Q stands for a conserved particle number. A Q-star may be mistaken for a stellar black hole.[1]