Environmental Engineering Reference
In-Depth Information
2.2 (a) Extraterrestrial and ground-level irradiance.
(b) Electromagnetic spectrum. From Smil (2002).
star's orbit prevents the Sun from an abrupt crossing of
the galactic plane that would stir up the Oort cometary
cloud. Moreover, the Sun, unlike most of the stars, is sol-
itary and hence much more likely to generate suitable
planetary orbits (Donnison and Mikulskis 1992). And
unlike the more massive stars with their very high rates
of thermonuclear reactions, the Sun has been sufficiently
long-lived to allow for billions of years of life's evolution.
The energy output of much smaller and long-lasting stars
could be too low, or it could be almost totally in infrared
(IR) and too variable.
In contrast, the solar spectrum ranges from wave-
lengths less than 0.1 nm (g rays) to more than 1 m (fig.
2.2), resembling radiation from a blackbody (an object
with a continuous radiation spectrum in all wavelengths
that absorbs all radiant energy reaching it) at about
5800 K. According to the Wien's displacement law,
l
max
¼
0
:
002898
T
ð
K
Þ
;
the maximum wavelength corresponding to that photo-
spheric temperature is almost exactly 500 nm, near the
lower limit of green light (491-575 nm). According to
Planck's law, which expresses the distribution of energy
emitted from a black body as a function of its wavelength
and temperature, the energy flux density emitted at this
maximum wavelength is about 2 W/m
2
nm.
