Balmer spectroscopic lines from plasma?

In stars, the Balmer lines are usually seen in absorption, and they are "strongest" in stars with a surface temperature of about 10,000 kelvins (spectral type A). Balmer series|Role in astronomy

At 10,000 K the hydrogen is a plasma, I suppose. How can we talk about Balmer lines if the electrons are not bonded anymore to the nucleus?

The Balmer series is characterized by the electron transitioning from n ≥ 3 to n = 2, where n refers to the radial quantum number or principal quantum number of the electron. Balmer series | Overview.

Balmer absorption arises from the $$n=2$$ level. To have a significant occupation of the $$n=2$$ level it has to be quite hot, because of the 10.2 eV gap between the ground state and $$n=2$$. The fraction of atoms in the $$n=2$$ level just grows with temperature.
The peak number of atoms in the $$n=2$$ level is at around 10,000 K. At higher temperatures, too many atoms are ionised, at lower temperatures the occupation of the $$n=2$$ level is lower. This graph (from http://spiff.rit.edu/classes/phys440/lectures/saha/) might help. As to why ionisation occurs at lower temperatures than significant occupation of the $$n=2$$ level, this is because if there are collisions and photons capable of putting a hydrogen atom in the $$n=2$$ level then these are also easily capable of ionising an atom that is in the $$n=2$$ level. The peak temperature for Balmer absorption is telling us what the pressure is in the stellar atmosphere.
• The ratio of atoms in the first excited state to the ground state is always <1 in a gas in thermal equilibrium. At 1000K almost all the atoms would be in the ground state; there would therefore be no ionisation or Balmer lines. The term "plasma" could be used as soon as there is any ionisation. To "glow" in visible light requires atoms in the $n=3+$ states and or recombination radiation and would begin at ~7000K for that process. In an impure gas, then electrons from sodium and potassium atoms can provide electrons that form a recombination continuum with hydrogen atoms at lower temperatures Nov 24 '20 at 12:46