Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Line absorption spectrum and stars

  1. Oct 19, 2016 #1
    Why can't hydrogen gas on the stars be detected by using line emission spectrum of hydrogen. Why must we use line absorption spectrum to detect?
     
  2. jcsd
  3. Oct 19, 2016 #2

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The hydrogen atoms that are producing the light energy inside a star are under high pressure and temperature. What emerges is not Hydrogen Line spectra but Black Body Radiation (a continuum of frequencies from 'DC to Daylight' and beyond). The H atoms that absorb light are cold and under low density conditions. They are operating in the conditions that our early QM lectures describe so they will absorb specific frequencies (lines).
     
  4. Oct 19, 2016 #3

    mathman

    User Avatar
    Science Advisor
    Gold Member

    The radiation from stars is black body radiation passing through a relatively cooler surface layer. We see the absorption lines from the elements of the star surface.
     
  5. Oct 19, 2016 #4

    Drakkith

    User Avatar

    Staff: Mentor

    As a follow on question to the OP's, why do we see an absorption line at all? Why doesn't the hydrogen (and other elements) simply re-emit the same light a short time later, resulting in a continuous spectrum?
     
  6. Oct 19, 2016 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That was a question I asked , myself once. The light is absorbed from the direction of the source and then re-emitted in all directions so what you see is a dim line at the absorbed frequency.
     
  7. Oct 20, 2016 #6

    davenn

    User Avatar
    Science Advisor
    Gold Member

    almost :smile: it's purely an absorption thing, not re-emission

    http://physics.ucr.edu/~wudka/Physics7/Notes_www/node107.html

    from wiki
    https://en.wikipedia.org/wiki/Fraunhofer_lines

    my bolding .... the important part :smile:

    Dave
     
  8. Oct 20, 2016 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    "The photosphere gas is colder than the inner regions and absorbs light emitted from those regions."
    I could take exception to that way of putting things because of the implication that the process only occurs in "colder" regions. There is a flux of photons moving out from the centre of the star and that involves energy being passed from one atom (ion) to the next in a chain. This takes hundreds of thousands of years, from core to surface, aamof. See Wiki Link on energy transfer. In the condensed state, the frequency of the photons can be anything (black body radiation spectrum) and can change at each interaction. So you have absorption and emission steps throughout the journey. The only difference with what happens in the cooler / less dense atoms is that the frequencies are limited to the characteristic ones of the gas atoms.
     
  9. Oct 20, 2016 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The energy has to be re-emitted after a finite (short) time. It can't be stored up.
     
  10. Oct 20, 2016 #9

    davenn

    User Avatar
    Science Advisor
    Gold Member

    yes, but it isn't the cause of the dark absorption line
     
  11. Oct 20, 2016 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I don't see how you can say that. Photons are absorbed (light on the path from the star to you) and they are then re-emitted in all directions, so each gas atom is a point source, from which you (in one particular direction) get only
    A/4πD2 of the energy. (A is the area of your Objective and D is the distance from the star / gas)
    i.e. the energy is scattered and not just absorbed.
    In your model, what happens to the energy that's absorbed by the gas atoms?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Line absorption spectrum and stars
  1. Absorption spectrum (Replies: 4)

  2. Absorption line spectrum (Replies: 20)

Loading...