4. Surface Temperature and Blackbodies
Surface temperature: the temperature of the visible disk of the Sun (photosphere)
blackbody: a perfect radiator of light that absorbs/re-emits all light incident upon it; its light output depends solely on its temperature & has the special shape of a Planck curve.
Stefan-Boltzmann Law for flux of a blackbody:
F= σTeff4
F= flux
σ = the Stefan-Boltzmann constant (5.67 x 10-8 T4 W/m2)
T = effective (surface) temperature
color provides a clue to surface temp.
- higher temp---> more energy
- more energy---> shorter wavelength, higher frequency

5. Planck Curve Planck curve Depends on temperature only
the higher the temperature, the farther to the left (shorter wavelength) the peak of the curve will be
The peak of a Planck curve yields the temp. of the stars
as temp. increases, object emits more intensely at ALL wavelengths

6. Wien’s Law
The Sun's blackbody spectrum peaks in the visible (yellow-green) part of the spectrum. This pertains to a temp. of about 5800 K due to Wien's Law:
λmax ≈ 3 x 10-3 m T
λmax = the wavelength where you receive the maximum flux-- shown by the peak of the Planck curve
Background Temp. of Universe: T = 2.7 K

7. Sunspots
Sunspots: dark areas on photosphere because they are cooler than surroundings.
Tendency is to form in groups (of even numbers)
form from pores between granules and then coalesce into a sunspot
make sets of magnetic poles
intense magnetic fields
Generally persist for 2-3 solar rotations (months)

8. Sunspots

9. Granules are “bubbles” with a size on the order of 2000 miles in diameter. The sunspot in this photograph is about the size of the earth.

11. Solar Magnetism Sunspots come and go, in anywhere from 1 - 100 days.
Typical groups last ~ 50 days
Sunspots come in pairs and are linked by magnetic field lines.

12. Solar Magnetism
Sunspots originate when magnetic field lines are distorted by Sun’s differential rotation

14. Sunspot Cycle
The Sun has an 11-year sunspot cycle, during which sunspot numbers rise, fall, and then rise again

15. Sunspot Cycle
This is really a 22-year cycle, because the spots switch polarities between the northern and southern hemispheres every 11 years
Maunder minimum: few, if any, sunspots (~1645 – 1715 C.E.)