138 CARNEGIE INSTITUTION 



problem of stellar evolution. Ever}^ star in the universe is a sun, 

 resembling in man]- respects the central body of our own S3\stem, 

 but each representing a certain degree of development, from the 

 nascent state, in close connection with the nebuloe, to the period of 

 decline, exemplified in the red stars. With the exception of our 

 own Sun, all of these stars are so distant from the earth that they 

 appear in the most powerful telescopes as mere points of light. 

 But with good atmospheric conditions a sharply defined image of 

 the Sun a meter in diameter can be obtained, thus permitting the 

 bright and dark spots upon its surface, and the flames at its edge, 

 to be studied in their most minute details. A knowledge of the 

 laws governing the variations of these phenomena is derived in part 

 from statistical studies of photographs. 



Solar Statistics. — The Roj^al Observatory at Greenwich, aided by 

 auxiliary stations in India and Mauritius, has maintained a daily 

 photographic registration of the visible phenomena of the solar sur- 

 face on a scale of about 20 centimeters to the Sun's diameter. The 

 data obtained from these plates, published annually in the Green- 

 wich Observations, are suSicient to provide for investigations of the 

 more conspicuous photospheric phenomena. These are supple- 

 mented by the ' ' Sun-spot numbers, ' ' derived from the visual ob- 

 servations of many amateur and professional astronomers, which 

 are published in the Astronomische Mittheihingen, established by 

 Wolf, and continued by Wolfer, his successor at the Zurich Obser- 

 vatory. The prominences are observed visually with small instru- 

 ments at Rome, Catania, Kalocsa, and Odessa, and the results, 

 published regularly in the Meviorie delta Societa degli Spcttroscopisti 

 Italiani and elsewhere, are suitable for a study of the distribution 

 of the prominences in heliocentric latitude and longitude through- 

 out the Sun-spot period. 



In addition to prominences and spots which can be seen at the 

 telescope, with or without the aid of the spectroscope, there are 

 other solar phenomena of which a dail)^ record is no less desirable. 

 Scattered irregularlj^ over the Sun's surface there are extensive areas 

 of very hot calcium vapor, some of which coincide with faculae. 

 Though invisible to the eye, these regions can be photographed 

 with the spectroheliograph, which records all phenomena character- 

 ized by brilliant emission of the K line of calcium. Thus the chro- 

 mosphere and prominences also appear on suitably exposed plates 

 taken with this instrument. A daily record of solar phenomena was 

 maintained with the spectroheliograph at the Kenwood Observatory,. 



