Question

Hydrogen is the most common element in the universe, and makes up ∼ 75% of the mass in almost all stars. Yet only stars with temperatures Teff ≃ 104 K exhibit prominent spectral lines of hydrogen in visible light. (a) Sketch a diagram of the energy levels of hydrogen, labeling each level by the quantum number n. Then identify the transitions which emit or absorb visible photons. (b) Visible hydrogen lines are weak in stars with surface temperatures Teff < 104 K. Explain this fact. The diagram you drew in part (a) may be helpful. (c) Hydrogen lines are also weak in stars with surface temperatures Teff > 104 K. Explain this fact. (d) To ionize a hydrogen atom takes an energy χ = 13.6 eV = 2.18 × 10−18 kg m2 s−2. At what temperature is the kinetic energy of a typical particle comparable to this energy?

Answer 1