Spectroscopy

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Spectroscopy the study of the colors of light
(the spectrum) given off by luminous objects.
Stars have absorption lines at different
wavelengths where the energy is precisely correct
to excite the electrons to a new level.
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Different elements show different absorption
lines, so the composition can be determined by
the spectrum of the light produced.
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However, differences in the absorption spectra of
different stars is not due to differences in
composition, but due to differences in
temperature.
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Stars over 25,000K show intense lines of
singly-ionized helium and multiply-ionized
heavier elements (O, N, Si). There are no
Hydrogen lines because the hydrogen is mostly
ionized, so no lines due to excited electrons.
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Only blue stars are hot enough to completely
ionize hydrogen.
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Stars about 10,000K show mostly H-lines. These
lines are produced by electrons moving between
the 2nd and 3rd orbitals eliminating those
wavelengths.
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Stars about 10,000K show no He, O, or N lines
because these electrons are too tightly held in
their orbitals. Calcium and Titanium lines are
common because they easily lose their electrons.
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Stars about 6000K, like our Sun, have few strong
lines of ionized elements the elements are too
cool to ionize. They have few H-lines.
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Stars about 3000K, red stars, have weak
H-lines, show weak lines for neutral heavy
atoms. No lines are seen from ionized elements.
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Spectral Classifications Early researchers
designed a scheme of classification based on the
spectra of stars.
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At this time atomic theory was lacking so all the
lines were not understood. (Most importantly, the
absence of H-lines was not understood.) It was
believed that the abundance of hydrogen varied
from lots to none.
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Stars were classified by Hydrogen-line intensity.
They used a system of letters from A through P,
thinking A had more hydrogen than P due to the
strength of the H-line.
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The abundance of Hydrogen is actually similar for
all stars, the different intensities of H-lines
from one star to another is due to differences in
temperature causing different levels of
ionization.
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Stars are more meaningfully classified by surface
temperature. So, the A to P classes were
realigned by temperature. The result is the
spectral classes OBAFGKM (the other letters
have been dropped from usage)
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In this system, O is the hottest type of star, M
is the coolest. A mnemonic device for remembering
these classes is Oh, Be A Fine Guy, Kiss Me!
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Each letter is subdivided into subclassifications,
0 through 9. The lower the number, the hotter
the star.
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Using this system our Sun is a G2 (cooler than
a G1, hotter than a G3). The star Vega is a A0.
Barnards Star is a M5. Betelgeuse is a M2.
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Prominent Spectral
Surface Absorption Class Temp Lines
Example O 30000K
Strong Ionized He
Multiply-ionized Heavies Faint
Hydrogen lines B 20000K
Neutral He Moderate Singly-ionized
Heavies Rigel (B8) Hydrogen lines
moderate A 10000K
Faint Neutral He
Singly-ionized Heavies Vega (A0)
Hydrogen lines strong Sirius (A1) F
8000K Singly-ionized
Heavies Neutral metals Canopus
(F0) Hydrogen lines moderate
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Prominent Spectral
Surface Absorption Class Temp Lines
Example G 6000K
Singly-ionized
Heavies Neutral metals
Sun (G2) Hydrogen lines moderate
Alpha
Centauri (G2) K 4000K
Singly-ionized
Heavies Neutral Metals Strong
Arcturus (K2) Hydrogen Faint
Aldeberan (K5) M 3000K
Neutral Atoms
strong Molecules moderate
Betelgeuse Hydrogen-very faint
(M2) Barnards Star (M5)
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To construct a Hertzsprung-Russell diagram
astronomers must first find the stars surface
temperature. This can be done using a Plank
curve or the spectrum.
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Second , the luminosity must be found. Finding
the luminosity is either easy, if the distance
to the star and the apparent brightness are known
they can be used to find the luminosity from the
inverse square law or it is impossible, if the
distance isnt known.
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The Main Sequence - The stars are not evenly
distributed on a Hertzsprung-Russell diagram.
Most of the stars range from high temperature and
high luminosity to low temperature and low
luminosity. (In other words, cool stars are faint
and hot stars are bright, duh.)
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Hot stars tend to be larger, cooler stars tend to
be smaller.
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Large, hot, bright stars are found to the upper
left. These are blue giants and blue
supergiants. Small, cool, faint stars are to the
lower right. These are the red dwarfs. Our Sun is
in the middle of the range.
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HR diagrams are biased in favor of blue giants
(so much easier to see) and against red dwarfs
(hard to see). Actually red dwarfs make up
approximately 80 of all the stars in the galaxy.
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Most stars lie on the main sequence, but there
are some notable exceptionsthe red giants and
the white dwarfs.
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While red giants are rare, they are very
visible.The distribution of types of known
stars90 of all stars are on the main sequence,
9 of all stars are white dwarfs, 1 of all stars
are red giants.
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The main sequence can be used to find the
distance to a star using the apparent brightness
and the temperature (color). This use of a
Hertzsprung-Russell diagram to find the distance
of very distant stars is called spectroscopic
parallax.
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Luminosity is comparable to absolute brightness
this is compared to the apparent brightness to
find the distance by the inverse square rule.
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Spectroscopic parallax depends on the Principle
of Mediocrity we assume that distant stars are
similar to nearby stars.
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Spectroscopic parallax is simple to use if the
star is on the main sequence. Fortunately, 90
of all stars lie on the main sequence. But, what
if the star is not on the main sequence?
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The width of the spectral line seen in the
spectra of stars is determined by the density of
the gas producing the light. The densities of
these gases is less for a red giant and more for
a white dwarf.
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This lets astronomers using spectroscopic
parallax to distinguish between red giants and
red dwarfs and between white dwarfs and white
giants. Therefore the distance to the other 10
of stars not on the main sequence can be found.
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Another way to distinguish types of stars is by
luminosity classIa - bright supergiantsIb -
supergiantsII - bright giantsIII - giants IV-
subgiants V- main sequence dwarfsVI - sub dwarfs
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