The atmosphere of the Sun has three layers. The photosphere ("sphere of light")
is what you see in an ordinary white-light photograph of the Sun. You can see
about 400 km into the photosphere, which is so small compared to the Sun's
radius of 696,000 km that the photosphere appears to be a sharp, well-defined
surface. The temperature decrases as you go up in the photosphere, from 5800 K
to 4400 K. The cool gas contains primarily hydrogen in low-lying states, which
is why it produces absorption lines.
The absorption lines of hydrogen are relatively weak in the Sun's spectrum
because the photosphere is rather cool. Most H atoms are in their ground state,
where they can absorb only ultraviolet photons (it takes a 121.6-nm photon to
excite the H atom from its ground state to the 1st excited state). To absorb
visible-light photons, the H atoms must be in the 1st excited state (recall that
the Balmer series connects the first excited state with higher states). At the
photosphere's temperature, the fraction of H atoms in the first excited state is
low, and so the Balmer absorption lines are weak. These lines are stronger in
hotter stars (like the Pleiades or in Rigel, the bright blue star in Orion) and
nearly absent in cooler stars (like Betelgeuse, the bright red star in Orion).
Despite being a thin gas (density about 10^-4 kg/m^3), the photosphere is
surprisingly opaque to visible light. (If it were not so opaque, we could see
into the Sun's interior to a depth of hundreds of thousands of kilometers,
rather than the paltry 500 km that we can see down into the photosphere.) The
reason is that the photosphere's density and temperature are just right to
permit some of its hydrogen atoms to acquire an extra electron, becoming
negative hydrogen ions (H-). This extra electron is rather loosely attached to
the ion: it can be dislodged by absorbing a low-energy photon of infrared light.
An H- ion will also absorb a photon of any visible wavelength, since visible
photons have higher energy than infrared photons and can also eject the ion's
extra electron. There are enough H- ions in the photosphere to effectively
absorb all colors of visible light, which is what makes the photosphere so
opaque.
The presence of negative hydrogen ions also helps to explain the character of
the photosphere's spectrum. A blackbody spectrum is normally produced by a dense
object, not a very thin gas like the photosphere. But because negative hydrogen
ions make the photosphere as opaque as a dense object, the photosphere's
spectrum is very similar to that of a blackbody.
The chromosphere lies above the photosphere. It is very thin (about 10^-8
kg/m^3), but curiously its temperature *increases* as you go up in altitude; the
temperature rises from 4400 K to nearly 25,000 K over a vertical distance of
2000 km. (The base of the chromosphere is defined to be the height at which the
temperature begins to increase.) As a result, the chromosphere has an emission
spectrum. Since the chromosphere is so thin, the net spectrum we see from the
Sun is still an absorption spectrum dominated by the photosphere. You can see
the chromosphere during a solar eclipse, when the Sun's disk and photosphere are
covered by the Moon; the chromosphere has a characteristic red glow dominated by
the 656.3-nm Balmer emission line of hydrogen. This color gives the chromosphere
("sphere of color") its name. You can also see the chromosphere using a
conventional telescope with an H-alpha filter attached (which is transparent
only to wavelengths within a narrow range around 656.3 nm).
The corona ("fine Mexican beer") lies beyond the chromosphere. It is at even
higer temperatures than the chromosphere, approaching 2 x 10^6 K. (There is a
very sharp temperature increase at the top of the chromosphere, which defines
the base of the corona.) It extends from the top of the chromosphere out to a
distance of several million kilometers, where it gradually becomes the solar
wind. The total amount of visible light emitted by the solar corona is
comparable to the brightness of the Moon when it is full Q that is, the corona
is only about 10^-6 as bright as the photosphere. Hence the corona can be viewed
only when the light from the photosphere is blocked out, either during a total
eclipse or when a specially designed telescope called a coronagraph is used.
Because it is very hot and very thin, the corona is highly ionized. Collisions
between atoms can knock off many electrons from each atom, but the density is so
low that the ions can't locate the electrons once they're lost. This gives the
corona a very odd emission spectrum. For example, a strong green line at 530.3
nm is caused by iron atoms that have lost 13 of their 26 electrons!
The curious temperature profile of the solar atmosphere has been a long-standing
conundrum --- why should the temperature increase in the chromosphere and
corona? Recent observations have provided some hints. For more details, see