Astronomers designate a star's brightness in visible light by its apparent magnitude. The faintest star you can see with the naked eye is about magnitude 6. Most stars brighter than this are numbered from 5 to 0, with each difference equivalent to a jump in brightness of 2.512 times.
Apart from the Sun, four stars are even brighter than magnitude 0, and have negative magnitudes: Alpha Centaun (-0.01), Arctums (-0.04), Canopus (-0.7), and Sinus (-1.5).
Apparent magnitude does not tell us how luminous a star truly is, because the stars lie at various distances from Earth. A star's true luminosity is defined in terms of absolute magnitude, which is how bright it would appear at a standard distance of 32.6 light-years. The Sun's nearness yields an apparent magnitude ot —26.7, but its absolute magnitude is a humble 4.8.