A star is a luminous and gaseous body that makes energy by nuclear fusion reactions, which occur in the core. All stars must exceed .05 solar masses in order to have the correct amount of central pressure and temperatures within the core. Only a few stars are known to have a mass greater than 50 times of our sun. Many things depend on the stars size, from its life time to luminosity. A star's characteristics can only be known as soon as its distance is known.
Greater distances require different methods.
A neutron star is a star that has had a gravitational collapse to which most of its material is compressed into neutrons. These stars are thought to form when the mass of the stellar core after a supernova, exceeds 1.44 solar masses, the core is not stable. As of now, the core isn't as stable as a white dwarf's. This allows the star to withstand any further contraction. These stars only have diameters of 20-30 km but have been found to be less than ten miles in diameter.