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STAR TYPES
Star Classification
Stars are classified by their spectra (the
elements that they absorb) and their temperature. There are seven
main types of stars. In order of decreasing temperature, O, B, A,
F, G, K, and M.
O and B stars are uncommon but very bright; M stars are
common but dim..
An easy mnemonic for remembering these is: "Oh be a fine girl,
kiss me."
The Sun is a
as a G2V type
star, a yellow
dwarf and a
main
sequence star.
Hertzsprung - Russell Diagram
The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color
(spectral type or surface temperature) vs. its luminosity (intrinsic brightness or
absolute magnitude). On it, astronomers plot stars' color, temperature,
luminosity, spectral type, and evolutionary stage. This diagram shows that there
are 3 very different types of stars:



Most stars, including the sun, are "main sequence stars," fueled by
nuclear fusion converting hydrogen into helium. For these stars, the
hotter they are, the brighter. These stars are in the most stable part of
their existence; this stage generally lasts for about 5 billion years.
As stars begin to die, they become giants and supergiants (above the
main sequence). These stars have depleted their hydrogen supply and are
very old. The core contracts as the outer layers expand. These stars will
eventually explode (becoming a planetary nebula or supernova,
depending on their mass) and then become white dwarfs, neutron stars,
or black holes (again depending on their mass).
Smaller stars (like our Sun) eventually become faint white dwarfs (hot,
white, dim stars) that are below the main sequence. These hot, shrinking
stars have depleted their nuclear fuels and will eventually become cold,
dark, black dwarfs.
Spectral Classes
Star Color Approximat Averag Averag
Average
Main
Examples
Typ
e
O
B
A
e Surface e Mass
e
Luminosit Characteristic
Temperatur (The Radius y (The Sun
s
e
Sun =
(The
= 1)
1)
Sun =
1)
Blue
over 25,000
K
Blue
11,000 25,000 K
Blue
7,500 11,000 K
60
18
3.2
15
Singly ionized
helium lines (H
I) either in
emission or
1,400,000
absorption.
Strong UV
continuum.
7
20,000
Neutral helium
lines (H II) in
absorption.
Rigel
Spica
80
Hydrogen (H)
lines strongest
for A0 stars,
decreasing for
other A's.
Sirius,
Vega
6
Ca II
absorption.
Metallic lines
become
noticeable.
Canopus,
Procyon
Sun,
Capella
2.5
10
Lacertra
F
Blue
6,000 - 7,500
to
K
White
G
White
to
5,000 - 6,000
Yello
K
w
1.1
1.1
1.2
Absorption
lines of neutral
metallic atoms
and ions (e.g.
once-ionized
calcium).
K
Orang
3,500 - 5,000
e to
K
Red
0.8
0.9
0.4
Metallic lines,
Arcturus,
some blue
Aldebaran
continuum.
M
under 3,500
K
0.4
0.04
(very
faint)
Some
molecular
Betelgeuse
bands of
, Antares
titanium oxide.
Red
1.7
0.3
1.3
Subtypes
Within each stellar type, stars are placed into subclasses (from 0 to 9) based on
its position within the scale.
The Yerkes Luminosity Classes: (by William Wilson Morgan and
Philip Keenan)
TYPE
Star
Ia
Very luminous supergiants
Ib
Less luminous supergiants
II
Luminous giants
III
Giants
Luminosity is the total brightness of a
star (or galaxy). Luminosity is the total
amount of energy that a star radiates
each second (including all wavelengths
of electromagnetic radiation).
In the Yerkes classification scheme,
stars are assigned to groups according to
V
Main sequence stars (dwarf stars) the width of their spectral lines. For a
group of stars with the same
VI
Subdwarf
temperature, the luminosity class
VII
White Dwarf
differentiates between their sizes
(supergiants, giants, main-sequence
stars, and subdwarfs).
IV
Subgiants
Main Sequence Stars - Young Stars
Main sequence stars are the central band of stars on the Hertzsprung-Russell
Diagram. These stars' energy comes from nuclear fusion, as they convert
Hydrogen to Helium. Most stars (about 90%) are Main Sequence Stars. For
these stars, the hotter they are, the brighter they are. The sun is a typical Main
Sequence star.
DWARF STARS
Dwarf stars are relatively small stars, up to 20 times larger than our sun and up
to 20,000 times brighter. Our sun is a dwarf star.
YELLOW DWARF
Yellow dwarfs are small, main sequence stars. The Sun is a yellow dwarf.
RED DWARF
A red dwarf is a small, cool, very faint, main sequence star whose surface
temperature is under about 4,000 K. Red dwarfs are the most common type of
star. Proxima Centauri is a red dwarf.
Giant and Supergiant Stars - Old, Large Stars
RED GIANT
A red giant is a relatively old star whose diameter is about 100 times bigger
than it was originally, and had become cooler (the surface temperature is under
6,500 K). They are frequently orange in color. Betelgeuse is a red giant. It is
about 20 times as massive as the Sun about 14,000 times brighter than the Sun,
and about 600 light-years from Earth.
BLUE GIANT
A blue giant is a huge, very hot, blue star. It is a post-main sequence star that
burns helium.
SUPERGIANT
A supergiant is the largest known type of star; some are almost as large as our
entire solar system. Betelgeuse and Rigel are supergiants. These stars are rare.
When supergiants die they supernova and become black holes.
Faint, Virtually Dead Stars:
WHITE DWARF
A white dwarf is a small, very dense, hot star that is made mostly of carbon.
These faint stars are what remains after a red giant star loses its outer layers.
Their nuclear cores are depleted. They are about the size of the Earth (but
tremendously heavier)! They will eventually lose their heat and become a cold,
dark black dwarf. Our sun will someday turn into a white dwarf and then a
black dwarf. The companion of Sirius is a white dwarf.
BROWN DWARF
A brown dwarf is a "star" whose mass is too small to have nuclear fusion occur
at its core (the temperature and pressure at its core are insufficient for fusion).
A brown dwarf is not very luminous. It is usually regarded as having a mass
between 1028 kg and 84 x 1028.
NEUTRON STAR
A neutron star is a very small, super-dense star which is composed mostly of
tightly-packed neutrons. It has a thin atmosphere of hydrogen. It has a diameter
of about 5-10 miles (5-16 km) and a density of roughly 10 15 gm/cm3.
PULSAR
A pulsar is a rapidly spinning neutron star that emits energy in pulses.
Binary Stars:
DOUBLE STAR
A double star is two stars that appear close to one another in the sky. Some are
true binaries (two stars that revolve around one another); others just appear
together from the Earth because they are both in the same line-of-sight.
BINARY STAR
A binary star is a system of two stars
that rotate around a common center of mass (the barycenter).
About half of all stars are in a group of at least two stars.
Polaris (the pole star of the Northern Hemisphere of Earth) is part of a binary
star system.
ECLIPSING BINARY
An eclipsing binary is two close stars that appear to be a single star varying in
brightness. The variation in brightness is due to the stars periodically obscuring
or enhancing one another. This binary star system is tilted (with respect ot us)
so that its orbital plane is viewed from its edge.
X-RAY BINARY STAR
X-ray binary stars are a special type of binary star in which one of the stars is a
collapsed object such as a white dwarf, neutron star, or black hole. As matter is
stripped from the normal star, it falls into the collapsed star, producing X-rays.
Variable Stars - Stars that Vary in Luminosity:
CEPHEID VARIABLE STARS
Cepheid variables are stars that regularly pulsate in size and change in
brightness. As the star increases in size, its brightness decreases; then, the
reverse occurs. Cepheid Variables may not be permanently variable; the
fluctuations may just be an unstable phase the star is going through. Polaris and
Delta Cephei are examples of Cepheids.
MIRA VARIABLE STAR
Some Mira Variable Stars
R Carinae
R Centauri
Mira
(Omicron Ceti)
Magnitude Range
3.9-10.5
5.3-11.8
Period (days)
308.7
546.2
3.4-9.3
332.0
A Mira variable star is a variable star whose brightness and size cycle over a
very long time period, in the order of many months. Miras are pulsating red
giants that vary in magnitude as much as a factor of many hundred (by 6 or 8
magnitudes). Mira variables were named after the star Mira, whose variations
were discovered in 1596.