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11

The broadening of emission lines is not due to something that is happening to each individual star, but rather something that affects the whole population. As stars in a galaxy get older, their orbits change relative to the orbits of other stars of the same age. Most relevant for the discussion here, the "velocity dispersion" of a stellar population ...

1

Work out how a plane wave propagates. If its intensity grows in the direction of propagation, then you need to switch the sign of the imaginary part! It depends on whether you define $e^{+i\,k\,z}$ or $e^{-i\,k\,z}$ as your diffraction operator in the direction of propagation. If it's $e^{+i\,k\,z}$, then a lossy dielectric always has a positive imaginary ...

2

The lamp does indeed contain molecular hydrogen but: the discharge dissociates (a small proportion of) the $H_2$ molecules to produce excited hydrogen atoms the emission from molecular $H_2$ is quite weak under these conditions, though it can can sometimes be observed as braod bands between the blue, green, and red lines hydrogen atom lines. So the ...

0

There are lots of emission and absorption lines – at different energy scales. Atomic spectra, vibrational spectra, rotational spectra, and so on. For each of them, one may discuss millions of molecules in principle (most of the large number are organic molecules). The amount of data to cover "everything" is clearly unrealistically huge. But any subset of ...

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