Spectral Classification
Based on the appearance of the spectra of stars,
a spectral classification
was devised in the late 1800's and the
early part of this century. The initial criteria used to define
the sequence were based primarily
on the strengths (depths) of the hydrogen Balmer lines
but also considered
other features.
Today we realize that other criteria should be used and so the
ordering is rather
obscure. The ordering is O, B, A, F, G, K, M.
Today we know that this is a
temperature sequence starting from the hottest stars at
O and going to the coolest
stars at M.
Examples of stellar spectra follow. Be careful to note the different
lines which are seen
and where the peak of the emission falls. (But be careful when
you look at stars hotter
than A stars.)
Examples of Stellar Spectra
In the various spectra note that different lines are seen and that the
strengths of the various lines are quite different. So how was the
scheme re-ordered? It was re-ordered in terms of temperature. Below is
a rough description of the various spectral classes:
Spectral Classes
- O; 28,000-50,000 K; ionized atoms, especially helium
- B; 10,000-28,000 K; neutral helium, some hydrogren
- A; 7,500-10,000 K; strong hydrogen, some ionized metals
- F; 6,000-7,500 K; hydrogen and ionized metals, such as calcium and iron
- G; 5,000-6,000 K; ionized calcium and both neutral and ionized metals
- K; 3,500-5,000 K; neutral metals
- M; 2,500-3,500 K; strong molecules, e.g., titanium oxide and some neutral
calcium
Given that we know that the chemical compositions of most stars are
roughly the same, why do stars show such grossly different spectra? To
understand this point, we will discuss the hydrogen lines.
Formation of the Hydrogen Lines
The hydrogen lines are weak in cool stars, increase in strength as the temperature
increases, reaches a peak around the A stars, and then weaken at higher
temperatures.
So, why don't all stars show strong hydrogen lines
given that hydrogen is the most abundant element in the Universe?
The reason has to do with the energy level structure of hydrogen and the
temperatures of the stars.
Recall:
- Because electrons are a lazy sort, they like to reside in the lowest
energy level. If they are in a higher state (excited state), they
quickly return to the lowest state (the ground state). The only
way to populate an excited state is for collisions with high energy
particles (other atoms or electrons) and absorption of photons to occur
rapidly enough to overcome the tendency for all things to reside in the
ground state.
- So, for cooler stars, the electrons in most hydrogen atoms are in
the ground state. Therefore, if an absorption occurs, a
Lyman line is produced. Because Lyman lines correspond to
large transitions, the lines will fall in the ultraviolet portion
of the spectrum and thus will not appear in the spectra of the optical
portion of the spectrum. As a result, in cool stars one does not see
strong hydrogen lines, because not enough electrons reside in the n = 2
level.
- For warmer stars, the number of electrons which can be pushed into
the n = 2 level increases and consequently, the number of absorptions by
electron s in the n = 2 level increases (stronger Balmer lines are
produced). Because Balmer transitions are less energetic than
Lyman transitions, they fall in the optical portion of the
spectrum and thus appear in optical spectra.
- The strength of the hydrogen lines does not increase forever,
however, why? Well, once the stars start to get hot enough, the
collisions in their atmospheres can be become so energetic that they
will start to liberate the electrons, that is, the atoms start to become
ionized. Since lines are produced by electrons changing energy states,
if an atom has no electrons then it can produce no lines! Because of
this for the bery hottest stars, hydrogen are not seen.
The competition between excitation and ionization leads to the maximum
hydrogen line strength in stars with T roughly 8,000-10,000 K, A
stars.
Similar arguments can be advanced for all of the other lines seen in
stellar spectra.
