The prodigious radiation from hot (25kK), luminous (log 5) stars in galaxies allows the investigation of massive (25) stellar evolution within different environments since they can be directly observed at large distances, and so represent `standard candles' and tracers of young stellar populations. Luminous stars also crucially affect the energy and momentum balance of the interstellar medium because of their powerful stellar winds. They are considered to play a key role in the evolution of starbursts, with an understanding of the properties and evolution of massive stars critical for starburst initial mass functions etc. From this, understanding the physics of massive stars, their atmospheres, radiation, and evolution is important for many aspects of astrophysics.
Included amongst post-main sequence hot, luminous stars are Luminous Blue Variables (LBVs), Of stars, hybrid Of/WN stars and Wolf-Rayet (W-R) stars. In particular, W-R stars, showing a strong, broad emission line spectrum, represent the chemically evolved descendants of O stars in which the products of nuclear burning are revealed at their surface. Distinct sequences of W-R stars exist, showing either core CNO cycle products (WN sequence) or the subsequent triple- products (WC and WO sequences).