Luminosity of the Sun
luminosity = energy radiated per unit time
A convenient unit for measuring luminosity is watts (W).
The luminosity of a 100 W lightbulb is (approximately) 100 W if you
measure over all wavelengths. (Most of it is in the infrared; the part
of the luminosity in visible wavelenghts is less.)
How can we measure the Sun's luminosity?
- We can measure the intensity of sunlight at the Earth.
- This should include all wavelenghts.
- I ~ 1.4 x 103 W/m2.
- This is the rate at which energy crosses 1 m2 of
- All of the energy that leaves the sun per second must
cross a big sphere of radius 1 AU.
- So we should multiply I by the area of this sphere:
- area = 4 Pi R2.
- = 4 x 3.14 x (1.5 x 1011m)2.
- = 2.8 x 1023 m2.
- So the luminosity is
- Luminosity = (1.4 x 103 W/m2)
x (2.8 x 1023 m2).
- ~ 4 x 1026 W .
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA