I'm an amateur who watched the Cosmos episode on Cecilia Payne's contributions to stellar classification, which I liked. I've been going down a rabbit hole on this and and I have a question for clarification.

In the episode it says (unless I'm misunderstanding) that prior to her PhD thesis, astronomers thought that stars' spectral lines were due to their elemental composition, but that she showed that they were instead due to the ionization state of elements in the stars, which is ultimately due to temperature.

I'm a bit confused though because if you look at the Wikipedia article on stellar classification it says,

Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences.

To me, it seems like those two sentences contradict each other. The first seems to say that the line placements are due to the type of element and the line strength is due to its abundance. However the second sentence then says that the line strength is due to temperature (which Payne has connected to ionization state). How can this apparent contradiction be resolved? Are there some lines that we might attribute to chemical type/abundance and others that we might attribute to ionization state/temperature?

  • $\begingroup$ Probably because: Luminosity is proportional to the temperature (which is ionization) via the stefan-boltzmann law. So the temperature also controls the wavelength of light emitted in a form known as color temperature. Different elements have different emission wavelengths so once we find the temperature, we can find the the element because different elements have different emission lines . $\endgroup$
    – user47732
    Commented Feb 7, 2023 at 12:18
  • 1
    $\begingroup$ Line strengths can be affected by both abundance and temperature. The distribution of particles over various energy levels is described by Boltzmann's equation, which is dependent on temperature. So for a fixed abundance, changing the temperature leads to a different proportion of the particles at a particular energy level, and therefore a different number of transitions between levels. This manifests as a difference in the line strength. $\endgroup$
    – lucas
    Commented Feb 7, 2023 at 14:39

1 Answer 1


Both explanations are right. The strength of an absorption line does depend on the abundance of a chemical element in the photosphere. But it also does depend on the temperature of the photosphere.

The absorption depends on abundance because, simply put, the more absorbers there are, the more absorption you get.

However, the absorbers have to be in the right "state" to be effective as absorbers. For example there is an absorption line of lithium at 670.8 nm that is caused by photons being absorbed by neutral Li atoms in their ground state. The number of Li atoms in the ground state depends on the abundance of Li but also on the temperature. At low temperatures most of the Li will be in the ground state. At higher temperatures, most of the Li becomes ionised and cannot absorb photons at that wavelength, even though the abundance may be identical. At very low temperatures, the Li can form molecules and then also won't absorb at 670.8nm.


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