In the light that we get from stars we discern certain lines belonging to the most abundant elements in those stars. Assume that we would pass the light from a star through a prism in order to disperse the spectrum, then isolate the hydrogen line(s).
Which one of the hydrogen lines is the most abundant in stars? And what are its properties: is the light in that line coherent light, or is it thermal light? Is it polarized?
This would depend on the type of star. In hot stars the hydrogen in the stellar envelope the hydrogen is fully ionized. In cold star it's not. Also, stars like the sun have a hot, low density corona where the hydrogen is ionized.
Thanks, Allard, but what about the other properties? In these lines, the emission is what we call "coherent light", or it is what we call "thermal light"? To put it in simple words, the coherent light is similar with the light from a laser. To the contrary, the thermal light is non-coherent.
Sofia,
After travelling umpteen light years, no light souce will retain temporal or spatial coherence. The light is thermalized.
And any polarization occurs as a result of passing through clouds of asymmetric particles (for instance), or media with magnetic fields.
If you can, have a look at 'Polarization in Spectral Lines' by Degl'Innocenti and Landolfi.
It is not a simple topic.
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