During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue supergiant and then a red supergiant. Particularly massive stars (exceeding 40 solar masses, like Alnilam, the central blue supergiant of Orion's Belt) do not become red supergiants due to high mass loss. These may instead evolve to a Wolf–Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss, or from stripping of the outer layers.