Which levels will be occupied?
The higher the temperature, the higher the typical level
Collisions can knock electrons to higher levels,
if moving atoms have enough kinetic energy
At T ~
300 K (room T)
almost all H in ground state (n=1)
At T ~ 10,000 K many H are in first excited state (n=2)
At T ~ 15,000 K many H are ionized
Because you have highest n=2 population at ~10,000K
you also have highest Balmer line strength there.
This gives us another way to estimate temperatures of stars
From our text:
Horizons, by Seeds