Fraunhofer Lines: Absorption & Re-emission in Solar Atmosphere

[Anton Alice]

Hello people,

I was wondering, why the absence of certain frequencies in the sun spectrum (fraunhofer lines) is so strong? In all pictures I have seen so far these lines are very dark, so there is nothing reaching us at these frequencies. Why is that?

The absorption occurs in the solar atmosphere. But also, there must be a re-emission of these missing frequencies, and the re-emission occurs in all spatial directions equally. Therefore some of the light should be able to exit the solar atmosphere, because the probability of going back to the sun equals the probability of going the other way around.

 

[tech99]

Hello people,

I was wondering, why the absence of certain frequencies in the sun spectrum (fraunhofer lines) is so strong? In all pictures I have seen so far these lines are very dark, so there is nothing reaching us at these frequencies. Why is that?

The absorption occurs in the solar atmosphere. But also, there must be a re-emission of these missing frequencies, and the re-emission occurs in all spatial directions equally. Therefore some of the light should be able to exit the solar atmosphere, because the probability of going back to the sun equals the probability of going the other way around.

The Fraunhoffer lines occur when light passes through relatively cool gas surrounding the Sun. The gas is too cool to radiate those frequencies. You can see the same in the lab if you pass a sodium light through a flame into which a bit of salt has been placed. The Fraunhoffer lines are best seen when the light beam passes through the cool area above the incandescent flame.

 

[Anton Alice]

The temperature in the solar atmosphere is about 6000K, why is this low? Do you mean, that there are only few collisions occurring?

The gas is too cool to radiate those frequencies.

But at some point in time the gas in the solar should be saturated with exited atoms, and all the light should be able to pass through.

The exited state in a single, isolated Atom (no collisions) should still have an intrinsic and finite lifetime due to vacuum fluctuation.

 

[jtbell]

these lines are very dark, so there is nothing reaching us at these frequencies.

They only appear (to our eyes+brain) to be very dark, by contrast with the adjacent bright sections of the spectrum.