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?
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) :
· 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 . 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 .
|O||blue||> 25 000 K||singly ionized He II lines; He I and faint H lines; strong blue and ultraviolet continuum|
|B||blue||11 000 K - 25 000 K||H I becoming stronger, He I lines strongest at B2; absence of He II lines|
|A||blue||7 500 K - 11 000 K||H (strongest for A0 type) and Ca II lines; He I and He II lines absent|
|F||blue to white||6 000 K - 7 500 K||H lines; metallic lines (Fe, Ca II, ...) become noticeable|
|G||white to yellow||5 000 K - 6 000 K||H lines; lines of neutral metallic atoms and ions grow in strength|
|K||orange to red||3 500 K - 5 000 K||metallic lines dominate; weak blue continuum|
|M||red||< 3 500 K||molecular bands of titanium oxide TiO|
O6.5 I - HD210839
O9.7 I - Cyg X1
B0 I - Alnilam
B8 V - Albireo B
A0 V - Vega
A0 V - Alioth
A1 V - Sirius
A2 I - Deneb
F5 IV-V - Procyon
G5 - Sun
G7 IIIb - Algieba B
K0 III / G1 III - Capella
K0 III - Algieba A
K1.5 III - Arcturus
K3 II - Albireo A
K5 III - Aldebaran
M0 I - AZ Cas
M2 I - Betelgeuse
M2 I - VV Cep
M7 III - CH Cyg
Extended spectral types
C 6.5 - T Lyr
WN5 - V1676 Cyg
WC8 - V1042 Cyg
- : Modern Astrophysics; B.W. Carroll, D.A. Ostlie; 1996 Addison-Wesley Publ. Company
- : The chemical composition of the sun; arxiv:0909.0948v1; September 2009