How hot is the Sun?

Surely. We look at the Sun and it looks yellow. Yellow light has a wavelength of about 500 nm. The temperature of a blackbody emitting light with maximum intensity at this wavelength is

T = (0.0029 K * m)/wavelength
= [(29 x 10^-4)/(5 x 10^-7)] K
= 6 x 10³ K
= 6000 K

This calculation is a little crude. One can do the best job by looking at the whole curve of intensity vs. wavelength. The best answer for the surface temperature of the sun is 5800 K. However, 6000 K is good enough for us and is easier to remember.

It is remarkable that we can learn the temperature of an object that is 150 x 10⁶ km away from us. We needed

• a lot of information about the behavior of light and matter,
• a small clue: "the sun is yellow."

ASTR 122 course home page

Updated 1 October 2007