Caption: Grotrian diagram of He I (i.e., neutral helium).

Features:

1. The labeled atomic line wavelengths are in angstroms (Å).

2. Helium is a noble gas, and so has tightly bound electrons.

   Thus, exciting helium to even the lowest-energy excited states (i.e., energy levels of energy above the ground state) takes considerable energy supplied by either or both of heat energy and photon energy.

3. The diagram shows that even the 1st excited-state energy level is ∼ 19.75 electron-volts (eV) above the ground state.

   Note that 19.75 eV means that high occupation of this energy level in thermodynamic equilibrium conditions requires a temperature of >∼ 2*10**5 K.

   The argument is as follows. The fractional occupation of an excited state is of order the Boltzmann factor exp[-E/(kT)], where E is the energy above the ground state, T is temperature, and Boltzmann contant k = 8.617 3303(50)*10**(-5) eV/K ≅ 10**(-4) eV/K (see NIST: CODATA: Constants: Complete Listing). So for the Boltzmann factor to be >∼ exp(-1) one must have E/(kT) <∼ 1, and so T >∼ 2*10**5 K.

4. Most atoms have some much lower energy excited states than He I.

   Those excited states are, in fact, more abundantly occupied in stars with relatively low (stellar atmosphere) temperatures than the excited states of He I.

5. Typically, He I absorption lines are strong for B stars with temperatures ∼ 11000--25000 K (see UCL: The Classification of Stellar Spectra) where the relative occupation of the excited states is still << exp(-1), but evidentlly still big enough for relatively strong absorption lines.

   Very weak He I absorption lines can be found in lower temperature stars (e.g., the Sun) if you look in detail (see Wikipedia; Fraunhofer lines: Naming).

6. The most prominent He I absorption line in the visible band in stellar spectra is usually the He I 5876 Å line (which is labeled by 5875 in the figure).

7. The ultraviolet absorption lines arising from the ground state are unobservable by ground-based astronomy because of the strong opacity of the Earth's atmosphere in the ultraviolet blueward of ∼ 3000 Å (see Wikipedia: Earth's atmosphere: Absorption).

8. As you go to hotter stars than B2 stars, He I absorption lines get weaker. This is because as temperature increases, the helium becomes increasingly ionized i.e., becomes increasingly singly-ionized He II which exhibits He II spectral lines.

9. For O stars, He II absorption lines and/or emission lines are the dominant helium spectral lines (see UCL: The Classification of Stellar Spectra).

10. Actually, absorption lines of any kind of helium are only ever strong in stars because helium is so abundant---about 25 % by mass fraction of baryonic matter in the observable universe.

11. Note that non-thermal processes can lead to strong helium spectral lines (absorption lines or emission lines) when low temperature would otherwise preclude them.