H-alpha

This view of the sky comes from the Balmer H-alpha emission line - light emitted by electrons in the n=3 excited state falling down to the n=2 state.  Stars themselves emit strongly in H-alpha, which is why they’re so bright here.

HII regions are also bright since not all of the gas is ionized at any one instant.  Recombination takes place, with electrons being captured into different states, including n=3, which can then cascade down and emit the light seen here.

Click to see the sky in different wavelengths

WWT

Explore the sky to see different HII regions

Interactive answers