# Is there molecular hydrogen in the Sun's atmosphere? If so, how much, and how was that first determined and measured?

I don't have a license to practice chemistry, but I'll convert Wikipedia's bond dissociation energy of 435.7 kJ/mol to eV by dividing it by $$C / N_A$$ where $$C$$ is 1 Coulomb and $$N_A$$ is Avogadro's number, and I get about 4.5 eV which is pretty big.

Dividing that by the Boltzmann constant $$k_B$$ gives about 52,000 Kelvin, which is simultaneously hot compared to the Sun's photosphere's temperature of about ~5,800 Kelvin, and not hot compared to the Sun's corona temperature of roughly 5,000,000 Kelvin.

This suggest to me that since the Sun is mostly hydrogen, it makes sense to ask

Question: Is there molecular hydrogen in the Sun's atmosphere? If so, how much, and how was that first determined and measured?

• This can be calculated theoretically: en.wikipedia.org/wiki/… Not all parts of the sun are likely to be accessible spectroscopically, and the concentrations may be too small to be easily measurable.
– user15381
Jul 26 at 22:32

According to Formation of the UV Spectrum of Molecular Hydrogen in the Sun (S. A. Jaeggli et al. 2018 ApJ 855 134, also here) molecular hydrogen in the sun was first spectroscopically discovered in 1977. The model calculation in this reference give a ratio molecular/atomic hydrogen of around $$10^{-5}$$ at a height of about 650 km (where the $$H_2$$ emissions have a maximum). Temperature at this height is taken as 4220K Figure 7. The normalized line contribution as a function of height for the 100 brightest $$H_2$$ lines between 1205 and 1550 °A from the FALC×1 calculation. In general the lines originate in a 150 km thick region centered at 650 km. Red lines originate slightly deeper than blue lines in agreement with the wavelength dependence of opacity, but some lines originate higher due to their excitation source

• This ratio is clearly going to be different in different parts of the sun, for the reasons stated in the question.
– user15381
Jul 26 at 22:27
• @BenCrowell This figure refers to height of 650 km (where the H2 emissions have a maximum) See my edited answer Jul 27 at 7:28
• @uhoh See my edited answer Jul 27 at 17:58
• The paper is fascinating, excellent source! Since the original publication is open access I've added a direct link to it as well.
– uhoh
Jul 28 at 0:47