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I heard in a documentary that the elements of the sun can be read out from the absorption lines. But I have also heard that an elements absorption lines and emission lies coincide. That means the sun can emit only the colors that its elements absorb. So if that is the case where does all the other colors (which are not in the absorption spectra) of the sunlight come from? And why aren't they being absorbed?
Thanks a lot.
A stars emission comes from thermal radiation, which will approximately be equal to black body radiation. However part of this spectrum will be absorbed by the outer layers of this star. It is true that absorption lines and emission lies coincide, so these frequencies will be re-emitted. But they are scattered in all directions, so also back into the star. So the further you move up in the outer layer, the more of these frequencies are "reflected" back into the star.
So if that is the case where does all the other colors (which are not in the absorption spectra) of the sunlight come from?
It is important to realize that the radiation of atoms, molecules, and any pieces of matter, although can have pronounced lines in the spectrum, always contains some small amount of radiation at all frequencies. That is, the spectrum always contains all the colors. The lines are distinctly visible sometimes only because certain colors are radiated in so much higher intensity that the other colors are hard to detect.
This happens when the radiation is produced by atoms or molecules that systematically radiate due to driving other than equilibrium radiation. For example, the light of gas discharge lamp has well visible emission lines (look on reflection of its light from CD or through a piece of glass). This is because the electric energy is being converted into light. It is stationary process, but nevertheless very different from thermodynamic equilibrium.
However, if the matter is close to thermal equilibrium with itself and with radiation, (like air in the oven, or as we believe, the gas in the surface layers of the Sun) no matter how weak is the radiation of those other colors, the intensity of every color will attain its equilibrium value given by the Planck function. This function is broad, has one maximum, but no sharp lines.
The atoms and ions will still radiate these other colors with low intensity, but the radiation present in space can maintain its high equilibrium value, because it gets absorbed by those same atoms with small efficiency as well.
And why aren't they being absorbed?
The absorption lines in the spectrum we observe on Earth are due to absorption by matter anywhere above the surface of the Sun, on the line Sun-Earth. So the spectrum of radiated radiation could very well be smooth without absorption lines initially, but as the radiation proceeds towards the Earth, it can encounter gas that is not in equilibrium with it anymore. The matter will absorb with highest efficiency at colors of the absorption lines, so the spectrum we observe is not nice smooth spectrum, but has holes in it.
