Emission nebulae
• The red glow is Hydrogen
Balmer    (H ) emission
• Could be from hot gas but –
– relative strength of emission
lines not always right
• Can also get fluorescence:
– UV photon from bright star
boosts electron to high level (or
ionizes it)
– Emission lines created as
electron cascades back down
through H energy levels
• The “horse” is a dark cloud in
front of the glowing gas.