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Table: Luminosity Classes
: The main bands on the
Hertzsprung-Russell (HR) diagram
of various kinds of
stars
are the
luminosity classes
. Note that a
luminosity class
is
NOT
a band of the same
luminosity
:
luminosities
can vary quite widely within a
luminosity class
. Note also, there are actually
NO
gaps between the bands, except that
white dwarfs
form a rather isolated band:
0 :=
hypergiants
: See
Wikipedia: List of most massive stars
, where the current record most massive star is ∼ 300
M_☉
.
Ia := more luminous
supergiants
Ib := less luminous
supergiants
II :=
bright giants
III :=
giants
IV :=
subgiants
V :=
main-sequence stars
: These are also called
dwarf stars
, but that name now seems a disfavored term,
except that
main-sequence
K stars
and
M stars
are called, respectively,
K dwarfs
and
M dwarfs
, and collectively
red dwarfs
. The
main sequence
extends down to
brown dwarfs
(divided into
L dwarfs
,
T dwarfs
, and
Y dwarfs
) which are
NOT
considered
stars
at all in that they never have significant
nuclear burning
.
VI :=
subdwarf stars
: They are
main-sequence stars
that lie a bit below
the ordinary
main-sequence stars
because of low
metallicity
. Their
chemical abundance ratio [Fe/H]
<∼ 0.3. Note
chemical abundance ratio [Fe/H]
is a
proxy
for
metallicity Z
. They are probably all
Population II stars
with ages >∼ 12 Gyr and Z <∼ 0.004 (see
Table: Stellar Population Metallicity for the Milky Way
). Recall the
age of the observable universe = 13.797(23) Gyr (Planck 2018)
, and so
subdwarf stars
formed relatively near the time of the
Big Bang
.
VII :=
white dwarfs
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.
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.