286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
galaxies, the observation of which is so important for the understand- 
ing of the structure of the galaxy and of the universe beyond. 
The turbulence of the atmosphere is not a serious factor at heights 
greater than about 50,000 or 60,000 feet. Hence, its disadvantages 
can be overcome by observations from high-altitude balloons, as has 
been beautifully demonstrated recently by A. Dollfus in France and 
by M. Schwarzschild in the United States. But the airglow and the 
absorption of ultraviolet radiation occur at very much higher altitudes 
which can only be traversed by rockets. 
107 ancstroms 
| 10 «10% 10° {0% microns 
\ 10)" 1022" 102 “10% 10° CENTIMETERS 
Ficure 1.—Regions of the radiation spectrum blacked out by the earth’s atmosphere 
(cross-hatched areas). Wavelengths are shown in angstroms, microns, and centimeters. 
The problem posed by the absorption of radiation in the atmosphere 
is illustrated in figure 1. The astronomer investigates the physical 
nature of objects in the universe by analyzing the electromagnetic 
radiations which they emit. The sun, the stars, and the nebulae are 
aggregations of gas in which the temperatures range from a few 
thousand to a few millions of degrees. Gases at such high tempera- 
ture will emit radiation of all wavelengths, which, for X-rays, are 
as short as a few billionths of a centimeter, or for radio waves, as 
long as tens and hundreds of meters. With the aid of optical and 
radio spectroscopes, the astronomer sorts out the radiations of differ- 
ent wavelengths and arranges them in a spectrum in order of increas- 
ing wavelength. The measurement of intensity at each wavelength 
provides the basic observational data which are required for the 
physical investigation of the sun and the stars. Ordinarily, the 
wavelengths with which we are concerned are so small that special 
units of length must be introduced. For example, the wavelength 
of visible and ultraviolet radiation is usually given in angstrom units, 
1 angstrom being equal to 1 hundred-millionth of a centimeter, 
whereas for infrared radiation, the micron, or 1 ten-thousandth of 
a centimeter, is usually employed as a unit of wavelength. Thus, 
X-rays are included in the range from about one-tenth to 100 A., 
ultraviolet radiation from 100 to 4,000 A., visible radiation from 
4,000 to 8,000 A., infrared from 1 micron to 1 millimeter, and radio 
waves from 1 millimeter on up to hundreds of meters. 
The cross-hatched areas denote those regions of the spectrum to 
which the earth’s atmosphere is completely opaque from the ground. 
