Early-type stars are hot, luminous stars of spectral type O, B, A, and F0 to F5. They were originally thought to be at an earlier stage in their evolution than late-type stars, such as those that are cooler than the Sun. However, it is now known that their high temperature is an outcome of their high mass. Early-type stars are not considered good candidates in the search for inhabited extrasolar worlds, on account of their relatively short lifetimes on the main sequence (see stars, lifetimes). More generally, the term early is used to described any star that is at the hotter end of its spectral class: for example, a G2 star is said to be earlier than a G5 star.