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).