Blue giant is not a strictly defined term and it is applied to a wide variety of different types of stars. What they have in common is: a moderate increase in size and luminosity compared to main-sequence stars of the same mass or temperature, and are hot enough to be called blue, meaning spectral class O, B, and sometimes early A. They have temperatures from around 10,000 K upwards, zero age main sequence (ZAMS) masses greater than about twice the Sun (), and absolute magnitudes around 0 or brighter. These stars are only 5–10 times the radius of the Sun (), compared to red giants which are up to .