Line absorption spectrum and stars

[Clara Chung]

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?

 

[sophiecentaur]

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).

 

[mathman]

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.

 

[Drakkith]

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?

 

[sophiecentaur]

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?

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.

 

[davenn]

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.

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

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

Emission and absorption lines
When heated every element gives off light. When this light is decomposed using a prism it is found to be made up of a series of ``lines'', that is, the output from the prism is not a smooth spectrum of colours, but only a few of them show up. This set of colours is unique to each element and provides a unique fingerprint: if you know the colour lines which make up a beam of light (and you find this out using a prism), you can determine which elements were heated up in order to produce this light.
Similarly, when you shine white light through a cold gas of a given element, the gas blocks some colours; when the ``filtered'' light is decomposed using a prism the spectrum is not full but shows a series of black lines (corresponding to the colours blocked by the gas); see Fig. 8.3. For a given element the colours blocked when cold are exactly the same as the ones emitted when hot.

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

The Fraunhofer lines are typical spectral absorption lines. Absorption lines are dark lines, narrow regions of decreased intensity, that are the result of photons being absorbed as light passes from the source to the detector. In the Sun, Fraunhofer lines are a result of gas in the photosphere, the outer region of the sun. The photosphere gas is colder than the inner regions and absorbs light emitted from those regions.

my bolding ... the important part

Dave

 

[sophiecentaur]

"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.

 

[sophiecentaur]

not re-emission

The energy has to be re-emitted after a finite (short) time. It can't be stored up.

 

[davenn]

The energy has to be re-emitted after a finite (short) time. It can't be stored up.

yes, but it isn't the cause of the dark absorption line

 

[sophiecentaur]

yes, but it isn't the cause of the dark absorption line

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?