Download Picture: Alnitak is the left-hand star in Orion`s Belt. Image: NASA

G star
A yellowish star of spectral type G, with a surface
temperature in the range 5,000 to 5,800°C and mass
of 0.8 to 1.1 solar masses. G stars have spectra
containing many absorption lines of neutral and
ionized metals, together with some molecular
absorption bands. The H and K lines of ionized
calcium are particularly strong. Main sequence G
stars, of which the Sun and Alpha Centauri A are
examples, are the prime targets of searches for
extrasolar planets and targeted SETI programs. Gtype giant stars, such as Capella, are slightly cooler
but more luminous than their main sequence
counterparts, while G-type supergiants have a mass
of 10 to 12 solar masses and a luminosity of 10,000
to 300,000 times that of the Sun.
Picture: Our Sun
Types of Stars
Stars can be classified by their surface temperatures as determined
from Wien's Displacement Law1, but this poses practical difficulties
for distant stars. Spectral characteristics offer a way to classify stars
which gives information about temperature in a different way particular absorption lines can be observed only for a certain range
of temperatures because only in that range are the involved atomic
energy levels populated. The standard classes are:
Spectral TypeTemperature of surfaceType of Star
O
30000 - 60000 K
Blue stars
B
10000 - 30000 K
Blue - white stars
A
7500 - 10000 K White stars
F
6000 - 7500 K Yellow - white stars
G
5000 - 6000 K Yellow stars
K
3500 - 5000 K Yellow - orange stars
M
less than 3500 K
Red stars
The commonly used mnemonic for the sequence of these
classifications is "Oh Be A Fine Girl, Kiss Me".
Apart from spectral type the scientists specify so-called
luminosity class.
Luminos
Ia
Very luminous supergiants
Ib
Less luminous supergiants
II
Luminous giants
III
Giants
IV
Subgiants
V
Main sequence stars (dwarf stars)
VI
Subdwarf
VII
White Dwarfs
O stars
A massive blue luminous star, of spectral type O, with a surface temperature of
about 30,000 to more than 50,000°C and a mass of 20 to about 100 solar
masses. Its spectrum is dominated by emission lines of singly ionized helium
(these lines forming the so-called Pickering series). Most other lines are from at
least doubly ionized elements, though H and He I lines are also present. O stars
are powerful ionizers of the surrounding interstellar medium, giving rise to large,
bright emission nebulae and virtually defining the spiral arms of the Milky Way.
Most O stars are very fast rotators. They have lifetimes of only 3 to 6 million
years. Well-known examples include Alnitak (Zeta Orionis) and Naos (Zeta
Puppis).
Oe stars have prominent hydrogen emission lines. Oef stars are early-type O
stars that show double emission lines in He II at 4686 Å. Of stars are peculiar O
stars in which emission features at 4634 to 4641 Å from N III and 4686 Å from
He II are present. They have a well-developed absorption spectrum, which
implies that the excitation mechanism of the emission lines is selective, unlike
that of Wolf-Rayet stars. The spectra of Of stars are usually variable, and the
intensities of their emission lines vary in an irregular way. Of stars belong to
extreme Population I. All O stars earlier than O5 are Of.
Picture: Alnitak is the left-hand star in Orion's Belt. Image: NASA
B star
A large, luminous, blue-white star of spectral type B
with a surface temperature of 10,200 to 30,000°C.
The spectrum is characterized by absorption lines of
neutral or singly ionized helium, with lines of atomic
hydrogen evident, especially at the cooler end of the
range.
Main sequence B stars, examples of which include
Spica and Regulus, have a mass in the range 3 to
20 solar masses and a luminosity of 100 to 50,000
times that of the Sun. Often they are found together
with O stars in OB associations since, being
massive, they are short-lived and therefore do not
survive long enough to move far from the place
where they were formed. Their brief main sequence
careers, measured in tens of millions of years,
probably allows too little time for even the most
primitive forms of life to develop on any worlds that
circle around them (assuming that life could exist at
all in such an environment).
B-type supergiants, of which Rigel is a familiar
example, may be up to 25 times as massive and
250,000 times as luminous as the Sun.
Picture: Rigel (right) and the Witch Head Nebula.
A star
A star of spectral type A, white in color, with a
spectrum dominated by the Balmer series of
hydrogen. Lines of heavy elements, such as
iron, are noticeable at the cooler end of the
range. Main sequence A stars have surface
temperatures of 7,500 to 9,900 K, luminosities
of 7 to 80 Lsun, and masses of 1.5 to 3 Msun;
familiar examples include Sirius, Vega, and
Altair. A-type supergiants, such as Deneb,
may be as hot as 11,000 K and have masses
up to 16 Msun and luminosities of up to
35,000 Lsun. Among A-type peculiar stars are
Ae stars, Am stars, and Ap stars. Also, two of
the main kinds of pulsating variables, RR
Lyrae stars and Delta Scuti stars, have
surface temperatures in the A-star range.
Picture: Altair - The brightest star in the
constellation Aquila, the twelfth brightest star
in the sky.
F star
A white or yellowish-white star of spectral type F
whose spectrum shows strong absorption lines of
ionized calcium which are more prominent than
the hydrogen lines. Moderately strong lines due
to iron and other heavier elements are also in
evidence. Main sequence F stars, of which
Procyon is an example, have a surface
temperature of 5,800 to 6,900°C, a mass of 1.2
to 1.6 solar masses, and a luminosity of 2 to 6
times that of the Sun. Relatively nearby, late-type
members of this category are generally included
in the list of target stars for extrasolar planet
searches and targeted SETI programs. F-type
supergiants, of which Canopus and Polaris (the
Pole Star) are examples, have a mass up to 12
solar masses and a luminosity up to 32,000 times
that of the Sun.
Picture: Canopus - The brightest star in the
constellation Carina, the second brightest star in
the sky, and one of the principal stars used in
spacecraft navigation. It was known in antiquity
as the
G star
A yellowish star of spectral type G, with a surface
temperature in the range 5,000 to 5,800°C and mass
of 0.8 to 1.1 solar masses. G stars have spectra
containing many absorption lines of neutral and
ionized metals, together with some molecular
absorption bands. The H and K lines of ionized
calcium are particularly strong. Main sequence G
stars, of which the Sun and Alpha Centauri A are
examples, are the prime targets of searches for
extrasolar planets and targeted SETI programs. Gtype giant stars, such as Capella, are slightly cooler
but more luminous than their main sequence
counterparts, while G-type supergiants have a mass
of 10 to 12 solar masses and a luminosity of 10,000
to 300,000 times that of the Sun.
Picture: Our Sun
K star
An orange-red star, of spectral type K. The spectra
of K stars are dominated by the H and K lines of
calcium and lines of neutral iron and titanium, with
molecular bands due to cyanogen (CN) and
titanium dioxide (TiO) becoming increasingly
prominent at the cooler end of the range. K-type
main sequence stars (that is, K-tye dwarfs) are
intermediate in size and temperature between Mtype red dwarfs and Sun-like stars (type G), with a
mass of 0.5 to 0.8 Msun, a temperature of 3900 to
5200 K (3600 to 4900°C), and a luminosity 0.1 to
0.4 Lsun.
Nearby examples include Epsilon Indi, Epsilon
Eridani, and Tau Ceti, the latter two having been the
target stars of Project Ozma. Early type main
sequence K stars within a few tens of light-years of
the Sun are generally included in the list of target
stars (see FGK stars) of searches for extrasolar
planets and targeted SETI programs, since if they
have planets orbiting within their habitable zones
there is the possibility that these worlds support life
of some kind. Giant K types are typically 100 to
400 K cooler, and have luminosities of 60 to 300
Lsun and masses of 1.1 to 1.2 Msun. Familiar
examples of K-type giant stars include Arcturus
(K1), Aldebaran (K5), and Pollux.
Picture: Pollux - The brightest star in the
constellation Gemini and the 17th brightest in the
sky.
M star
A cool, red star, of spectral type M, with a surface
temperature of less than 3,600°C. Molecular
absorption bands are prominent in the spectrum,
with bands of titanium oxide becoming dominant
at the lower end of the temperature range. Main
sequence M stars, known as red dwarfs, have a
mass of less than 0.5 Msun and a luminosity of
less than 0.08 Lsun; examples include the nearby
Proxima Centauri and Barnard's Star. M-type
giant stars, known as red giants, occur in the
mass range 1.2 to 1.3 Msun and may have
luminosities exceeding 300 Lsun. The largest
stars of all are M-type supergiants, such as
Betelgeuse and Antares, of mass of 13 to 25
Msun and luminosity of 40,000 to 500,000 Lsun.
Picture: Antares (upper left), Rho Ophiuchi (blue),
and Sigma Scorpii ( red).
R star - an obsolete name for what is now classed as a
hotter carbon star and N star - an obsolete name for
what is now classified as a cool carbon star (classes C6
to C9).
A red giant whose spectrum is dominated by strong
absorption bands of carbon-containing molecules; the
Swan bands of C2 are especially prominent, with
absorption by CN, CH, C3, SiC2, and CaII present to
varying degrees, with often a strong sodium D line.
Carbon stars, also known as C stars, have
carbon/oxygen ratios that are typically four to five times
higher than those of normal red giants and show little
trace of the light metal oxide bands that are the usual
red giant hallmark. They resemble S stars in their
relative proportion of heavy and light metals, but
contain far more carbon in their upper layers. The
carbon is likely the dredged-up ashes of nuclear helium
burning in the stellar interior. Carbon stars lose a
significant fraction of their total mass in the form of a
stellar wind which ultimately enriches the interstellar
medium – the source of material for future generations
of stars. Carbon stars were previously classified as
stars of spectral type R (hotter, with surface
temperatures of 4,000 to 5,000 K) and N (up to 10
times more luminous but cooler, with a temperature of
about 3,000 K). They are typically associated with some
circumstellar material in the form of sooty shells, disks,
or clouds.
S star
A red giant of spectral type S, similar
to an M star except that the dominant
oxides in its spectrum are those of the
metals of the fifth period of the
periodic table (zirconium, yttrium, etc.)
instead of the third (titanium,
scandium, vanadium). S stars also
have strong cyanogen (CN) molecular
bands and contain spectral lines of
lithium and technetium. Pure S stars,
also called zirconium stars, are those
in which zirconium oxide bands are
very strong and titanium oxide bands
are either absent or only barely
detectable. Almost all S stars are longperiod variables. So-called SC stars
appear to be intermediate in type
between S stars and carbon stars (C
stars) and have a carbon/oxygen
abundance ratio near unity.