Stellar spectra


On the classification of stars

The spectra of stars appear to be a continuous band of colours with a series of dark absorption lines. But how are these features explained?

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The solar spectrum: solar continuum with absorption lines. (Dados spectrograph; 20.09.2015; 25 um slit; 11 images; ISO 800; t: 1/20s)

When you have a closer look at the stars, they do not all appear white. Some stars, as for example Betelgeuse in Orion, appear to be rather orange than white. These differences in colours indicate differences in surface temperatures of the corresponding star. The energy emitted as radiation of different frequencies (by a black body at thermal equilibrium) depends on the temperature of the star and is described by Planck's law. The surface temperature of the star can be calculated by knowing the frequency of the maximum of the energy distribution in the spectrum (Wien's law). Our Sun, with a surface temperature of 5770 K has a peak in the spectrum at 503 nm.

And now to the second part; where do the absorption lines come from? Remember Kirchhoff's laws (two of them) [1]:
· A hot, dense gas or solid object produces a continuous spectrum with no spectral lines - this is the part we've just seen.
· A cool, diffuse gas in front of a source of a continuous spectrum produces dark spectral lines, the absorption lines in the continuous spectrum.
The light produced and emitted by the sun has to pass through the outer, transparent layers. The chemical elements in these layers can absorb light of specific wavelengths and each element creates its own set of spectral (absorption) lines. The absorbed energy is used to excite the electron from the ground state to an excited state or to ionize the atom.

Our sun is made up mainly of two elements: hydrogen (H) and helium (He). 71% of the total mass of our sun are composed of hydrogen; 27.1% of helium. Elements, heavier than helium contribute to only 1.9% to the total mass of our sun [2]. These are mainly oxygen O, carbon C, neon Ne, nitrogen N, iron Fe and magnesium Mg.

The current spectral classification scheme, developed at Harvard Observatory in the early 20th century, is based on lines which are mainly sensitive to stellar surface temperatures. The main stellar classes are O, B, A, F, G, K, and M and correspond to a decreasing temperature sequence. O-class stars correspond to stars with hot surface temperature, whereas M-class stars correspond to stars with low surface temperatures. The following table gives a summary of the main characteristics of the different classes [1].

classcolorsurface temperaturecharacteristics
Oblue > 25 000 K singly ionized He II lines; He I and faint H lines; strong blue and ultraviolet continuum
Bblue 11 000 K - 25 000 K H I becoming stronger, He I lines strongest at B2; absence of He II lines
Ablue 7 500 K - 11 000 K H (strongest for A0 type) and Ca II lines; He I and He II lines absent
Fblue to white 6 000 K - 7 500 K H lines; metallic lines (Fe, Ca II, ...) become noticeable
Gwhite to yellow 5 000 K - 6 000 K H lines; lines of neutral metallic atoms and ions grow in strength
Korange to red 3 500 K - 5 000 K metallic lines dominate; weak blue continuum
Mred < 3 500 K molecular bands of titanium oxide TiO


Class O

O6.5 I - HD210839

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HD210839 Top: For instrumental response corrected and uncorrected spectrum.
Bottom: Spectrum with indication of hydrogen absorption lines.

O9.7 I - Cyg X1

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Class B

B0 I - Alnilam

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B8 V - Albireo B

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Class A

A0 V - Vega

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A0 V - Alioth

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A1 V - Sirius

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A2 I - Deneb

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Class F

F5 IV-V - Procyon

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Class G

G5 - Sun

G7 IIIb - Algieba B

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Class K

K0 III / G1 III - Capella

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K0 III - Algieba A

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K1.5 III - Arcturus

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K3 II - Albireo A

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K5 III - Aldebaran

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Class M

M0 I - AZ Cas

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M2 I - Betelgeuse

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M2 I - VV Cep

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M7 III - CH Cyg

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Extended spectral types


Carbon stars

C 6.5 - T Lyr

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Wolf-Rayet stars

WN5 - V1676 Cyg

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WC8 - V1042 Cyg

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Some references