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Re: stellar spectra



At 19:47 10/20/99 -0700, Fred Brace wrote:
Today we were talking about the red shift in class.I was explaining that
the hydrogen lines are shifted towards the red end of the spectrum if the
star is moving away from us. What bothers me is that the spectrum of the
sun is a continuous spectrum with dark absorption lines. How does one
observe the hydrogen lines to see if there is a red shift?

There is a reasonable idea of the relative speed of Sun vs Earth, and so
you would not expect very much shift. You can compare spectral lines
emanating from the Sun with those generated in the lab from a lamp for
example - but it is necessary to find a means of measuring very delicate
differences in wavelength.

You may recall my enthusing about an experimental protocol relying
on the comparison of particular Solar lines of sodium. In this case, a
particular line can be split (locally) by a magnetic field, and this is
indeed a way to introduce a very small change in the frequency of sodium
light for comparison. If the magnetic field affecting the sodium light
is varied so as to stay in coincidence with the solar line, (and I think
this is done by matching an emission line with an absorption line) the
result is a measure of very small Solar radial velocities which can
even be distinguished as between various parts of the solar disk.

I am fairly sure that I am greatly oversimplifying the method, but still
it bears some resemblance to the facts.

Sincerely,