Question

Why is the Hα (Balmer) absorption line in the sun relatively weak, even though the sun has abundant hydrogen?

Answer 1

H-alpha (Balmer) absorption line in the sun is relatively weak​ because of the not enough temperature of the sun.

H-alpha is the line produced from exciting an electron from n=2 to n=3 level. And to be able to absorb H-alpha, you would need to have a hydrogen atom with its electron already in the n=2 excited state. Exciting n=1 to n=2 (Lyman-alpha) takes a lot of energy, much more than n=2 to n=3, and the Sun's photosphere is not hot enough for much of its hydrogen to be in the n=2 excited state, so H-alpha (Balmer) absorption line in the sun is relatively weak​.

{However some hotter stars have more hydrogen in the n=2 state, so it is then easy to do the (less energetic) n=2 to n=3 H-alpha absorption. That's why hotter stars like A stars have much stronger H-alpha lines.

But stars hotter than A stars (O,B) again have weak hydrogen absorption lines, because they are so hot most of their hydrogen is ionized, and again there is not much hydrgoen in the n=2 state to excite.}

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