110 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



STELLAR ENERGY SPECTRA 



As noted in last year's report, W. H. Hoover set up apparatus in the 

 100-incli t^lescoi^e building on Mount Wilson for measuring the dis- 

 tribution of energy in the spectra of the brighter stars. He observed 

 on August 31 and on September 21 with considerable success. 



The apparatus comprised 10 Christiansen filters adapted to select 

 narrov^ regions of spectra ranging from 0.345ju, in the ultraviolet to 

 1.030/1, in the infrared. It was particularly suitable for observing the 

 spectra of the blue and white stars which could not be well observed 

 with the arrangements used by Abbot in 1923 and 1928. Hoover em- 

 ployed a thermoelectric element made by L. B. Clark of this Institution 

 in connection with a highly sensitive reflecting galvanometer made by 

 Hoover himself to observe the heat in the spectral regions selected 

 by the filters. He calibrated the apparatus by observing the solar 

 spectrum, whose distribution is known with considerable accuracy. 



Owing to disturbances arising in the thermoelectric circuit, it was 

 impossible to use the galvanometer at more than one-third its available 

 sensitiveness. Yet Hoover read deflections as gi'eat as 20 millimeters 

 in the spectrum of Vega. 



Measurements were made in the spectra of five stars a Aurigae, 

 a Cygni, a Persei, a Aquilae, and a Lyrae, all but one of which agreed 

 very closely with those obtained for the same stars by Abbot in 1928, 

 except that Hoover's measurements extended to much shorter wave 

 lengths than could be reached by Abbot. However, the results are 

 still only provisional, and intended mainly to test whether the method 

 can be used with advantage when, about the year 1941, the 200-inch 

 telescope becomes available. 



Our conclusion is very favorable. It is believed that the disturbances 

 encountered in the thermoelectric circuit can be minimized; that the 

 full sensitiveness of the galvanometer can then be used ; that a diffrac- 

 tion grating can be ruled to throw 60 percent of the incident light 

 into one spectrum, and can be used with a simple optical system to 

 be more saving of light than the filters; that automatic photographic 

 registration will be practicable; and, in short, that continuous auto- 

 matically recorded stellar spectrum energy curves, probably at least 

 10 centimeters high at maximum, can be obtained for the brightest 

 stars when the 200-inch telescope becomes available. 



PERSONNEL 



Harlan H. Zodtner resigned from the service on October 31, 1938. 



Hugh B. Freeman, formerly with us, was retransferred to the work 



from the National Advisory Committee for Aeronautics on August 1, 



