The Photosphere -The “visible” surface of the
Thin layer of gas (less than 500km deep) from
which we receive the majority of the Sun’s light.
Average surface temperature ~ 6000K
The photosphere is the layer dense enough to
emit plenty of light but not so dense that light can
not escape. How dense? 3400 times less dense as
the air that we breath!
Spectra? Gives off absorption spectra?!
Deeper layers are dense
enough to radiate like a
absorbs some of
The observer sees a
• Granulation – The photosphere is mottled by a
pattern of bright cells with dark edges.
o Convection rising and lowering gasses (hot/cold).
How do we know?? Doppler Effect is evident in
the spectral features of the gas!
Bright, Hot rising gas
Darker, cooler sinking gas
• The Chromosphere - “The Color Sphere.” The
chromosphere is an irregular layer above the
photosphere where the temperature rises from
6000°K to about 20,000°K. At these higher
temperatures hydrogen emits light that gives off a
reddish color (H-alpha emission). This colorful
emission can be seen in prominences that project
above the limb of the sun during total solar
eclipses. This is what gives the chromosphere its
• above the photosphere about
• nearly invisible (why?) – only
seen during a solar eclipse.
• Emission Spectra – It produces an
good emission spectra because it is
a low density hot glowing gas.
• Much hotter than the photosphere!
Temperature ~ 104 – 105K
(Image: Courtesy of
Spicules – flame-like
structures 100-1000m in
diameter extending up to
10,000km above the
photosphere for short
durations of time (5
minutes or so). The
spicules seem to appear
in the photosphere.
The Corona - “The Crown”
• Above the chromosphere.
• Also, only seen during eclipses
• *can* extend as far as 12 times the Sun’s radius from the
• Very low density!! BUT very hot ~ 1,000,000K average
temperature (varies from about 500,000K to nearly 2,000,000K)
• Continuous Spectra
• Why so hot??? Maybe the interaction with the Sun’s rotation
and magnetic field. (Joe Physics – “friction?”)
• The outer corona is so hot, Sun’s gravity can not hold onto the
gas. A high velocity of gas atoms streams out from the Sun in
all directions (mostly H, He, H+, and e- particles)
Image: Courtesy of Fred Espenak
The Solar Wind
• A continuous flow of particles Blows past the earth at
nearly 300-800km/sec (1000km/sec gusts)
• Bathes the Earth in a hot stellar wind!
• Sun is losing mass!! Yikes? 10-13 Msun/year (1013years to
lose 1 solar mass) [Some stars lose mass at much faster
• Solar wind – a few million particles per 1m3 at 1AU.
• Wind seems stronger around Coronal Holes: Areas
where the coronal gases have been blown away – regions
of high solar activity.
A prototypical prominence structure. The curved arch of
solar material wraps around the magnetic field stretching
between sunspots above the photosphere into higher layers.
In chaotic areas these structures can be associated with
A “Solar Flare” can be a truly violent explosion from the solar
surface reaching very far out from the Sun. Flares are much
more violent than prominences. Flares often last from minutes
to hours. A large flare can release the equivalent of nearly 2
billion megatons of TNT.
(Photo: Courtesy of Dick Hutchinson)
Figure: (Beautiful aurora display. Image: Courtesy of University Alaska - Fairbanks.)