Supergiant stars are massive (> 10 - 20 solar masses), extremely luminous (perhaps as bright as a million Suns and found at the top of the colour magnitude diagram. Since there can be both red and blue (and even yellow) supergiants, they occupy a roughly horizontal region with 'surface' temperatures from 3,500 K to 20,000 K. These are the stars that will go on to become supernovae leaving behind exotic remnants such as black holes and neutron stars.

Figure 1: Artist's impression of a blue supergiant.
Credit: NASA Goddard Space Flight Center/S. Wiessinger