Quote:
Originally Posted by malclocke
After thinking about it I realised I'm naively thinking about it in terms of absorption where n=2 is the starting point for all the balmer transitions.
For emission conditions may be favouring n=3, and not the levels for the other balmer lines. We will see the 3 -> 2 transitions (ha) but the 3 -> 1 transition is in the Lyman series (UV).
So it's not such a mystery why the lines are so imbalanced.
Hope that makes sense :/
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Malc, there's a good bit on the causes of relative changes in Ha/Hb (the "Balmer decrement") in the Keith Robinson book. As you say it's often to do with the relative optical thickness of the ejecta (in this case) to photons at the energy level to produce Lyman vs Balmer transitions.
There's also something going on here to do with a recombination front, which I think I just about understand :-) Along the lines of - recombination takes place, a 3->2 transition happens emitting an Ha photon, but then the Hydrogen immediately absorbs another photon of the same energy level bumping it back up to 3, because of the density of the Ha at this point in the nova evolution (unlike a nebula, for instance, where the much less dense Hydrogen will be more likely to continue to cascade down to 1).
Something like that.