•
•No
discrete “surface” – just increase r, T, P, and “opacity”
–“Surface”
or photosphere defined by depth from which visible photons
can escape.
•This
would also be depth to which visible photons could penetrate if photons were somehow directed back into the sun.
•
•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