There are no strict upper limits for giant stars, but early O types become increasingly difficult to classify separately from main sequence and supergiant stars, have almost identical sizes and temperatures to the main-sequence stars from which they develop, and very short lifetimes. A good example is Plaskett's star, a close binary consisting of two O type giants both over , temperatures over 30,000 K, and more than 100,000 times the luminosity of the Sun (). Astronomers still differ over whether to classify at least one of the stars as a supergiant, based on subtle differences in the spectral lines.