Definitions from the Collins Dictionary of Astronomy (1994)
Early-type Stars
Hot stars of *spectral type O, B, and A. They were originally thought, wrongly,
to be at an earlier stage of evolution that *late-type stars.
O
hottest blue stars
ionized helium (HeII) lines dominant
strong UV continuum
B
hot blue stars
neutral helium (HeI) lines dominant
no HeII
A
blue, blue-white stars
hydrogen lines dominant
ionized metal lines present
Of stars
Young massive O stars that show selectively enhanced emission lines of
ionized helium (HeII) and nitrogen (NIII) in addition to a well-developed
absorption spectum. The emission lines arise in an unstable atmosphere
that is being lost from the star. Of stars are the hottest, most luminous,
and probably the most massive stars in the Galaxy; the current record holder
is HD 93129A, near *Eta Carinae, with a temperature of 50000 K, a luminosity
of 3 million suns, and a mass of probably 120 solar masses ( see also
supermassive stars). Of stars are thought to be the evolutionary precursors
of *Wolf-Rayet stars.
Wolf-Rayet Stars
A small group of very luminous very hot stars, with temperatures possibly as
high as 90000 K, that have anomalously strong and broad emission lines of
ionized helium, carbon, oxygen, and nitrogen but few absorption lines. Since
their discovery (by C.J.E. Wolf and G. Rayet, 1867), over 300 have been
found in our Galaxy and its neighbours. The majority either have lines of
He, C, and O -- WC stars -- or He and N -- WN stars ;
both types have anomalously low abundances of hydrogen. A further rare group,
called WO stars, have recently been identified with very strong
O emission lines. The emission lines are thought to arise in an expanding
stellar atmosphere moving at very high speeds of up to 3000 km/s so that the
star is continuously and rapidly losing mass. The average mass for Wolf-Rayets
is 10 solar masses. About half are known to be binary stars, usually with O
or B stars as companions, an example being Gamma Velorum (WC8 + O7).
These unusual properties provide clues that Wolf-Rayets are the centres of
very massive *Of stars stripped of their outer envelopes with the products
of interior nuclear burning being revealed. Since the Wolf-Rayets are the
more evolved members of each binary, they must originally have been the more
massive partner, a star of at least 20 solar masses. Half that mass has thus
been lost in their stellar winds, whose high outflow rate would strip this
mass off the star in only 100000 years. This gas is in fact often visible as
a ring nebula surrounding the Wolf-Rayet star. In a close binary the
companion's gravity may assist the stripping, but for single stars the cause
of the high mass loss is still uncertain, although *radiation pressure
probably plays a major role.
Luminous blue variables (LBVs)
A class of very massive luminous blue stars known for sporadic mass ejections
(eruptions); subclasses include *P Cygni stars and *Hubble-Sandage variables.
They are generally found near the upper luminosity limit in the observed
*Hertzsprung-Russell diagram, and most are thought to have evolved from stars
with initial masses exceeding 40 solar masses. The mass ejections are most
likely due to instabilities in their stellar envelopes caused by *radiation
pressure. LBVs show different types of variations occuring on a wide range
of timescales. The largest variations are associated with sudden brightenings
by more than 3 magnitudes lasting for several hundred to several thousand
years. The smallest variations (< 0.5 mag) last from several months to
several years. Examples of LBVs are *Eta Carinae, AG Carinae, *P Cygni, and
*S Doradus.