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