Astronomy Principles and Practice Fourth Edition.pdf

Black body radiation 215
A
d
Ω
Figure 15.3. The outflow of radiation from a surface has a range of direction. For radiation collected by an area
A at distance d from the surface, the received energy emanates from the solid angle = A/d 2 .
Extended
source
Ω
Figure 15.4. When directed to an extended source, the radiation received in the field of view of a telescope is
limited to some solid angle, .
15.5 Black body radiation
15.5.1 The basic behaviour
Laboratory experiments of the 19th century showed that the strengths of the various colours in a
continuous spectrum depended on the temperature of the radiating body. For example, a body glowing
at a ‘dull red heat’ obviously concentrates most of its energy into the red part of the spectrum and
exhibits very little energy in the blue wavelengths. If the temperature of the body is made to increase,
the wavelength at which the maximum of energy is radiated creeps further into the visible region of the
spectrum and the amount of energy radiated at the blue wavelengths increases. By making quantitative
measurements of the strength of the emitted energy at different wavelength positions, it is possible to
build up the energy–wavelength distribution curve or energy envelope for any source and this can be
done in the laboratory for a range of temperatures.