79 



TABLE 4-2. The Electromagnetic Spectrum — Absorption 



* Estimates of the internal temperature of the sun go as high as a million degrees K. Spectroscopic meas- 

 urements give the temperature of the incandescent gases surrounding the sun to be about 6000° K. A black 

 body at 6000°K radiates some energy at nearly all wavelengths, but the maximum energy is radiated at 

 about 5000 A, right in the middle of the range of wavelengths visible to man. This is no coincidence, of 

 course, for man's senses are adapted to his environment. 



After absorption of the damaging short-wavelength ionizing radiation by the upper atmosphere, the total 

 energy reaching the surface of the earth on a clear day is ~ 1.25 cal/min cm 1 . However, above the a 

 phere space travelers will have to be protected against the small amounts of ionizing radiation which extend 

 right down to wavelengths in the X-ray region. The most prominent of these is the strong emission of excited 

 hydrogen atoms, the"Lyman-alpha" line, at a wavelength of 1215 A. 



