Why does the Sun behave like a black body?

[Ken G]

I am still a little bit confused about the absorption lines. It seems that you are saying they are really due to the H minus ions in the photosphere. Is that correct?

No, you had it right before-- the continuum comes from H minus opacity, but H minus opacity is over all energies throughout the visible range, because all visible photons are capable of being absorbed by the H minus ion (which ionizes it back to neutral H), and all neutral H is capable of emitting any visible photon (if it can in the process grab an electron and make H minus). So that will not give any lines at all. The absorption lines come from the lower density gas above that, which is largely at cooler temperatures, and that tends to remove photons at the specific energies of the line transitions in that low density gas (a line transition involves electrons jumping between bound states in the atom).

Previously people had been saying that it was because the photosphere is relatively cooler.

There are several ways to get an absorption line, but the simplest and most important comes from the fact that the photosphere is not just a single sphere, it is more like a shell with finite width, and the gas in that shell gets cooler the farther out you go. So that means there is cooler gas overlying warmer gas, but this cooler gas only has an effect on the line frequencies, not the rest of the continuum because the overlying cool layer is transparent except at line frequencies. Perhaps what is confusing you is that the atoms responsible for the absorption lines are mixed in with the H minus atoms responsible for the continuum, but the lines are much more opaque, so are seen at higher and cooler altitudes.

Or are we saying that the photosphere is also mainly consisting of ions that really this hot surface is actually emitting the radiation that we 'see' and that this is mainly continuos due to ion / ion interactions in the photosphere.

The key process that generates the continuum that we call "sunlight" is when a free electron is grabbed into the single bound state that neutral H has for adding an electron and becoming H minus. Any process that can emit light has an inverse process that can absorb it, so when light hits the H minus and ionizes it, that is what makes the photosphere opaque and stops us from seeing even deeper and hotter layers.

 

[resurgance2001]

Thanks Ken - I think I finally got it! H minus ions in the hotter part of the photosphere produce the broad continuous spectrum, and then in upper, slightly cooler (are we able to say what temperature?) parts of the photosphere ordinary H atoms (and presumably some other elements) produce absorption lines. Yes? So obviously within the photosphere there is a fairly continuous temperature gradient and a mixture of H minus ions together with ordinary H atoms. Do we know why the corona is so hot? I seem to remember that the temperature of that soars again but when I first studied this twenty years ago I learned then that scientists were not sure of the reason for this. That of course could have changed since I first read about it. Cheers

 

[Ken G]

Thanks Ken - I think I finally got it! H minus ions in the hotter part of the photosphere produce the broad continuous spectrum, and then in upper, slightly cooler (are we able to say what temperature?) parts of the photosphere ordinary H atoms (and presumably some other elements) produce absorption lines. Yes? So obviously within the photosphere there is a fairly continuous temperature gradient and a mixture of H minus ions together with ordinary H atoms. Do we know why the corona is so hot? I seem to remember that the temperature of that soars again but when I first studied this twenty years ago I learned then that scientists were not sure of the reason for this. That of course could have changed since I first read about it. Cheers

Yes that's right. The continuum comes from a range of temperatures somewhere around 6000 K, and the temperature drops to around 5000 K at the minimum, though there's not much point in trying to be too precise because the mottled surface of the Sun shows you that the temperature varies a lot from place to place. You are right that the chromosphere gets hotter and the corona gets much much hotter (it gets to temperatures similar to those found in the center of the Sun where fusion occurs, but the corona is too low of a density to have much fusion). The reason the corona gets so hot is that it is so low density that any appreciable heating can easily make it very hot, though the exact nature of that heating is still not known in detail. It must be some form of "mechanical" heating, meaning it is not heated by sunlight. The players are the convection, which excites waves and eddies, and magnetic fields, which can twist up and store energy that can then be released in the coronal plasma. You can amaze your students with online pictures from things like the TRACE satellite.