208 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 28 



Spectroscopic work developed rapidly, and to-day more than 

 half of all astronomical observation is in this fascinating depart- 

 ment. From the spectrum one learns pi-actically all that he could 

 learn were the star in his own laboratory; even more, for there are 

 stars whose temperatures exceed any we know on the earth, and in 

 studying these fiery caldrons the astronomer finds many a strange 

 and often uncanny result. He discovers materials so vanishingly 

 thin that our lowest vacua are very high-pressure affairs by com- 

 parison ; materials so heavy that a cubic inch weighs a ton. He tells 

 us of the extraordinary velocities with which some of tlie celestial 

 bodies move, for example, the whirling nebula in Virgo. A spec- 

 trum exposed for 80 hours shows the nebula receding at a velocity 

 of 730 miles a second, while at the same time it is spinning around 

 with a velocity of ^05 miles a second at a ])()int 2 minutes of arc 

 from the center. 



To measure the total energy of the stars, a thermocouple or 

 bolometer is used, and readings are automatically recorded on 

 moving plates. 



The Michelson interferometer is the most recent instrument added 

 to the telescope, and with it we measure the diameter of the stars 

 A large steel beam cai*rying four mirrors is placed at the upper 

 end of the tube. The whole end of the tube is covered except for 

 two apertures underneath tlie central mirrors. The observer sits 

 at the eyepiece (see pi. 1) manij)ulating a small glass wedge until 

 dark lines appear across the star's image. As he gradually sepa- 

 rates the outer mirrors on the beam tliese black lines or fringes dis- 

 appear. Measuring the separation of the mirrors, he calculates the 

 angular size of the star, and knowing its distance he computes its 

 linear diameter. The first star thus measured was Betelgeuse, the 

 bright red star in the right shoulder of Orion. It proved to havs 

 a diameter comparable with that of the orbit of Mars, but by measures 

 of Mira and Antares has since been given third place in size. 



For solar work many telescopes are of small aperture and long 

 focal length. In the horizontal telescope the tube is laid along the 

 ground, so to speak, and sunlight is reflected into it all day by a 

 rotating mirror called a coelostat. In the 150-foot tower telescope on 

 Mount Wilson the tube stands vertically, however, and the light is 

 directed through it by mirrors at its upper end. The observer works 

 in a house at the foot of the tower, while below him is a pit 80 feet 

 deep in which the spectrograph is placed. The image of the sun 

 formed by this tower telescope is 17 inches in diameter, and the spec- 

 trum produced by its spectrograph is 20 feet or more long. The sun 

 is a vast globe of incandescent gases having a temperature at the 

 surface of about G,000°. Metals and other substances which are 

 solids on the earth appear there in the form of vapors, and vast 



