AQA A-Level Physics/Spectral Classes

Key Definitions

Balmer Series: A set of lines in the Hydrogen Spectrum

Spectral Classes

Stars can be classified into spectral classes based on their temperature and the absorption spectra that they produce. The following table summarises the properties of each class and was taken directly from the AQA specification.

Spectral Class	Colour	Temperature /K	Prominent Absorption Lines
O	blue	25,000 - 50,000	He+, He, H
B	blue	11,000 - 25,000	He, H
A	blue-white	7500 - 11000	H, ionized metals
F	white	6000 - 7500	Ionized metals
G	yellow-white	5000 - 6000	Ionized and neutral metals
K	orange	3500 - 5000	Neutral metals
M	red	< 3500	Neutral atoms, TiO (titanium dioxide)

Hydrogen Balmer Lines

Different elements in stars absorb different wavelengths of light. For example, if white light was shone through a cloud of pure hydrogen, all light would make it through except a few particular wavelengths. This is because the hydrogen has absorbed it, subsequently exciting the electrons around the hydrogen atoms.

Figure 1: Continuous spectrum of white light

The same effect occurs in stars. Each element in the star absorbs certain wavelengths of light, resulting in an absorption spectrum to a viewer one earth.It would look something similar to Figure 1, just with black bands over the absorbed wavelengths.

Hydrogen Balmer lines are a series of lines in the Hydrogen spectrum. By analysing these lines, we can narrow down our estimate of the temperature of the star. These lines only occur at a certain temperature, when the electrons of the hydrogen are in the $n=2$ state.

When star temperature is too hot:

Going back to our hydrogen cloud example, if the cloud was too hot, the hydrogen atoms would collide with each other with enough force to free their electrons. When an atom loses electrons, it becomes ionized, so the result of this is an ionized cloud of hydrogen. Due to the lack of electrons, no white light can be absorbed, so the hydrogen balmer lines are not visible; no light is being absorbed.

In this instance, the electrons are of course not in the $n=2$ level, as they're not even part of the atom at all.

When star temperature is too cold:

If the cloud were too cold, the light incident on the hydrogen atoms would not have enough energy to excite the electrons, meaning it would not be absorbed. For a cloud of pure hydrogen, this would give exactly the same effect as if the cloud were ionized.

Is the star too hot or too cold?

Given that the star being too hot or too cold have exactly the same effect to an oberver on earth looking at balmier lines, how can we tell what the temperature of the star is.

Fortunately, a star contains many elements, which all have different absorption patterns. We can analyse multiple elements to more accurately determine the temperature of the star.

We know that the strongest balmer lines occur in a star of approximately 9000K. As the temperature of the star gets closer to 9000K, the hydrogen balmer lines get stronger. As a result, we can use the intensity of the balmer lines to estimate a stars temperature.