Most of the visible photons we receive from the Sun originate in the photosphere. One question you might ask is why we see continuum radiation at all. We have already seen how atoms can emit or absorb energy at particular wavelengths, producing spectral lines. However, we have not discussed the source of the emission and absorption of the continuum. It turns out that the continuum opacity in the Sun at optical wavelengths comes from the presence of H~ ions. An H~ ion is an H atom to which an extra electron has been added. As you might guess, this extra electron is held only very weakly to the atom. Very little energy is required to remove it again. H~ ions are present because there is so much hydrogen, and there are a large number of free electrons to collide with those atoms.
If we have an H~ ion and a photon (7) strikes it, the photon can be absorbed, and the electron set free:
The H~ ion is a bound system. The final state has an H atom and a free electron. We call this process a bound-free process. In such a process, the wavelength of the incoming photon is not restricted, as long as the photon has enough energy to remove the electron. The electron in the final state can have any kinetic energy, so a continuous range of photon energies is possible. This process then provides most of the continuum opacity of the photosphere. The continuum emission results from the inverse process.
We have said that the Sun is the one star that we can study in great detail. To do this, we try to observe the photosphere with the best resolution possible. When we observe the Sun, the light-gathering power of our telescope is not usually a problem. Therefore, we can try to spread the image out over as large an area as possible, making it easier to see detailed structure. We therefore want a telescope with a long focal length to give us a large image scale. The solar telescope shown in Fig. 6.6 provides this type of detailed picture.
Solar telescope on Kitt Peak (operated by NOAO).The telescope has a very long focal length, so that two can produce a large image and study the detailed appearance. Since the tube is so long, it is not reasonable to move it. Instead, the large mirror at the top (the objective) is moved to keep the sunlight directed down the tube. [NOAO/AURA/NSF]
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