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Star classification
(Stellar classification)
In astrophysics, stars are
classified by their surface temperature, that is associated to specific
spectral patterns. An early schema from the 19th
century ranked stars from A to P, which is the origin of the
currently used spectral classes. After several transformations, today the
spectral classification includes 7 main types: O, B, A, F, G, K, M.
A popular mnemonic for remembering this order is "Oh, Be A Fine Girl,
Kiss Me".
This is called "Morgan-Keenan
spectral classification", even though its form was already by Annie Cannon, also
based on the work of other
astronomers from the Harvard College Observatory. The
classes, listed from hottest to coldest, are:
|
Class
|
Temperature
|
Star Color
|
|
O
|
30,000 - 60,000 °K
|
Blue
|
|
B
|
10,000 - 30,000 °K
|
Blue
|
|
A
|
7,500 - 10,000 °K
|
White
|
|
F
|
6,000 - 7,500 °K
|
White
(yellowish)
|
|
G
|
5,000 - 6,000 °K
|
Yellow (like the Sun)
|
|
K
|
3,500 - 5,000 °K
|
Orange
|
|
M
|
2,000 - 3,500 °K
|
Red
|
Notice that hottest stars are
blue, while coldest stars are red. This seems unusual to most people, who
associate red with hot and blue with cold. This is because we see fire as yellow,
orange or red, but light produced by hotter sources is blue. However, blue
sources are hard to find on Earth because it requires a large amount of energy.
Also notice that this is true for light-emitting objects. However, the color of a common object, like a blue shirt or a piece of red paper, is not related to its temperature. Confusion also arises when one considers how artists or photographers may
refer to the color of light: usually they describe reds as "warm" colors and blues as "cool".
Kelvin Temperature
K means Kelvin degrees, that can
be calculated adding 273 to Celsius degrees. Here are 4 examples of common
temperatures in Fahrenheit, Celsius and Kelvin degrees:
|
Condition
|
oF
|
oC
|
oK
|
|
Water boils
|
212
|
100
|
373
|
|
Room Temperature
|
72
|
23
|
296
|
|
Water Freezes
|
32
|
0
|
273
|
|
Absolute Zero
|
-460
|
-273
|
0
|
However, star temperatures are
much higher, so the following table can be useful:
|
Conditions in different temperatures
|
|
Kelvins
|
Celsius
|
Condition
|
|
1,808 °K
|
1,535 °C
|
Melting point
of iron
|
|
2,013 °K
|
1,740 °C
|
Boiling point
of lead
|
|
3,683 °K
|
3,410 °C
|
Melting point
of tungsten
|
|
3,925 °K
|
3,652 °C
|
Sublimation
point of carbon
|
|
5,780 °K
|
5,500 °C
|
Surface
temperature of the Sun
|
|
5,828 °K
|
5,555 °C
|
Boiling point
of tungsten
|
Spectral types
The seven spectral classes were subdivided into
tenths (for example B0, B1, B2, B3, ..., B9, A0, A1, A2, A3, ... A9, F0, F1,
F2, F3...). The Sun is a G2 star.
- Class O stars are very hot
and luminous, being blue in colour. Naos (in the constellation Puppis) shines with
a power close to a million times solar. These stars have prominent ionized and
neutral helium lines and only weak hydrogen lines.
Class O stars emit most of their radiation in ultra-violet.
- Class B stars are again very luminous, Rigel
(in the great constellation Orion) is a prominent B class blue supergiant.
Their spectra have neutral helium and moderate hydrogen lines. As O and B stars
are so powerful, they live for a very short time. They do not stray far from
the area in which they were formed as they don't have the time. They therefore
appear clustered together in the OB associations, which are associated with
giant molecular clouds. The Orion OB association is an
entire spiral arm of our Galaxy.
- Class A stars are amongst the more common naked eye stars.
Deneb in Cygnus is another very powerful star.
Sirius, that appears the brightest star as seen from Earth, is also an A class star, but not
nearly as powerful. As with all class A stars, they are white. Many
white dwarfs are also A. They have strong hydrogen lines and also ionized metals.
- Class F stars are still
quite powerful and they are average-sized, such as Fomalhaut in Pisces
Australis. Their spectra is characterized by the weaker hydrogen lines and
ionized metals, their colour is white with a slight tinge of yellow.
- Class G stars are probably
the most well known for the reason that our Sun is of this class.
They have even weaker hydrogen lines than F but along with the ionized metals,
they have neutral metals.
- Class K are orange stars
which are slightly cooler than our Sun. Some K stars are giants
and supergiants, such as Arcturus, while others like Alpha
Centauri B are smaller. They have extremely weak hydrogen lines, if they
are present at all, and mostly neutral metals.
- Class M is the most common
class by the number of stars. All red dwarfs, such Proxima Centauri,
the closest star to our Solar Sysem, go in here, and they are plentiful. M is
also host to most giants and some supergiants such as Antares in
Scorpio and Betelgeuse in Orion, as well as Mira variable stars. These red giants are old stars.
The spectrum of an M star shows
lines belonging to molecules and neutral metals but hydrogen is usually absent.
Titanium oxide can be strong in M stars.
M stars may be
dwarf stars or supergiant stars, and A stars can be white dwarfs or white
giants as well. However, not all combinations are possible. For example, F and
G stars must be average-sized stars.
This can be understood through
the Hertzsprung-Russell diagram, that is
very important in astrophysics and relates temperature and spectral
classification of stars with their luminosity and size.
While these descriptions of
stellar colors are traditional in astronomy, they really describe the light as
we see them from Earth, after it has been scattered by the atmosphere. The Sun
is not in fact a yellow star, but has the color
temperature of a body of 5780 K, that is a white with no trace of yellow
which is sometimes used as a definition for standard white.
Spectral type additions
A number of other spectral types have been taken into use for rare types of
stars: these are W, L, T, S, and C (that includes R and N).
Thanks for your interest!
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