216 CARNEGIE INSTITUTION OF WASHINGTON. 



changes to yellow and then to white. A critical point is reached when 

 the increasing pressure carries the mass beyond the state of a perfect 

 gas and into the period of declining stellar life. Beyond this stage lie 

 the denser and smaller stars known as "dwarfs," of which our sun, 

 1.4 times as dense as water, is one. As the shrinkage goes on, the colors 

 that mark the giant state return, first yellow and then through deepen- 

 ing shades of red as the star approaches final extinction. 



Knowledge of the distances of these stars, and of their motions across 

 and parallel to the line of sight, has rendered possible the determina- 

 tion of their actual motions in space. It has thus been found that the 

 space velocities of the yellow and red stars, especially the giants, de- 

 pend upon their absolute luminosities, the fainter stars moving more 

 rapidly than the brighter ones. A decrease in brightness of one magni- 

 tude is found to correspond to an increase of about 3 km. per second 

 in the average velocity in space. While the bearing of this result is not 

 yet certain, it probably means that stars of small mass move more 

 rapidly, on the average, than those of large mass. Of the many other 

 important conclusions of this investigation given in the body of the 

 report, one of the most significant is that the stars with large veloci- 

 ties, most of which are of the dwarf class, are moving toward the center 

 of the galactic system. Thus this extensive mvestigation is leading 

 to conclusions of the most fundamental significance, which are rapidly 

 advancing our knowledge of the structure and evolution of the stellar 

 universe. 



In the last annual report the successful application of the inter- 

 ferometer, in the form due to Dr. Anderson, to the measurement with 

 extiaordinar}^ precision of the close double star Capella, was fully ex- 

 plained. Dr. Merrill has since continued the measurement of Capella 

 and other doubles, and Professor Aitken has undertaken to apply this 

 method to many of his closest pairs with the 36-inch Lick refractor. 

 Reference was also made to a special interfeiometer 20 feet in length, 

 built for use on the tube of the 100-inch telescope, which had given 

 sharp fringes when tested by Professor Michelson at its full aperture 

 on certain stars. With this instrument remarkable results have since 

 been obtained by Mr. Pease. 



On the night of December 13, 1920, when observing Betelgeuse, he 

 found that the sharpness ("visibility") of the fringes steadily decreased 

 as the interferometer mirrors were separated, until they finally disap- 

 peared completely when the mirrors were 10 feet apart. Assuming the 

 mean wave-length of the light of Betelgeuse to be X5750, the angular 

 diameter is at once found to be 0^047, corresponding to a linear 

 diameter of about 215,000,000 miles. ^ Since that time Mr. Pease has 

 found that the fringes of Arcturus and Antares disappear at mirror 



'These linear diameters may be considerably in error, chiefly because of uncertainty regard- 

 ing the exact value of the parallaxes of these stars. 



