•
•No
discrete “surface” – just increase r, T, P, and “opacity”
–“Surface”
or photosphere defined by depth from which visible
photons can
escape.
•
•Opacity
depends on wavelength, so apparent “surface” will be at different depths for different
wavelengths
–High
opacity in absorption lines because these photons easily absorbed/emitted
•Won’t
see very far in at these wavelengths.
–Low
opacity in between absorption lines
•Can
see in deeper at these wavelengths.
–Eventually
r so high gas opaque at all wavelengths
(just as in solid)
–“surface” high = cool = dark in lines;
deep = hot =
bright between lines
–
•This gives us way to “probe” to different
depths in sun
•Same technique used by satellites to
“probe” temperatures in Earth’s atmosphere