Spectral classification of stars

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 What is a spectra?
 What is an absorption spectrum ?
 Can we see what stars are made up of ?
 Can we measure how hot they are ?
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 For the other two questions the answer is :
 Yes we can !!! Using spectral classification of
the stars.
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 Surface temperature of the star is associated
with specific spectral classification.
 The spectral classification includes 7 main
types:
O, B, A, F, G, K, M.
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 This is called “Morgan-Keenan spectral
classification”
 The classes, listed from hottest to coldest
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
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 Notice that hottest stars are blue, while
coldest stars are red.
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 Naos (in the constellation Puppis)
 These have prominent ionized and
neutral helium lines and only weak
hydrogen lines.
 Class O stars emit most of their
radiation in ultra-violet.
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 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.
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 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.
 As with all class A stars, they are white. Many white dwarfs
are also A.They have strong hydrogen lines and also ionized
metals.
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 Class F stars are still quite powerful..
 Fomalhaut in Pisces Australis.
 Their spectra is characterized by the weaker hydrogen lines
and ionized metals, their color is white with a slight tinge of
yellow.
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 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 and have
neutral helium lines but along with the ionized metals, they
have neutral metals.
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 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.
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 Class M is the most common class by the number of stars.
 All red dwarfs, such Proxima Centauri, the closest star to
our Solar System
 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.
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 M stars may be dwarf stars or supergiant stars, and A stars can
be white dwarfs or white giants as well.
 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.
 A number of other spectral types have been taken into use for
rare types of stars: these areW, L,T, S, and C (that includes R
and N).
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Hertzsprung-Russell diagram
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Spectral classification of stars

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Spectral classification of stars