THE STARS IN ACTION — JOY 185 



scribe its orbit not much more than one-lialf the distance from the 

 center to the circumference. If its mass were proportionately large, 

 it would weigh twenty-five million times as much as the sun, and 

 according to Einstein's theory its own gravitation would be so great 

 that its light would be unable to leave its surface. The mass has not 

 been directly determined, but it is probably about five or ten times 

 that of the sun, instead of several million times. This low value 

 means that we have an exceedingly large body with comparatively 

 small mass, and the density comes out surprisingly low. It is, in fact, 

 on the average for the Avhole star, only one ten-thousandth that of our 

 atmosphere at sea level. The outer regions which we see have a much 

 greater tenuity, for a star's mass is greatly concentrated toward the 

 center. These regions are much nearer to a perfect vacuum than any- 

 thing that can be produced in the laboratory. As far as our experi- 

 ence goes these stars are vast aggregations of nothingness. The po- 

 etical idea of tripping lightly from one star to another would be 

 difficult of realization in such cases, for we might approach one of 

 these giant bodies of gas and, except for the inconvenience of a rather 

 torrid temperature of 2,000° or more, we might wander about indefi- 

 nitely inside the star and not even know that we had arrived. If we 

 wished to use an airplane, we should fall to the center of the star and 

 yet find no place whore it could support itself as it does in the air of 

 the earth. At the surface, gravity is very small on account of the 

 great distance from the center of the star. It amounts to only one- 

 tliousandth tliat on the earth. If one could get a solid footing there, 

 he could jump to a height of a mile. 



Mira and the other long-period variables are the coolest stars we 

 know. The temperatures of the regions which we see vary with the 

 brightness of the star from 1,G00° to 2,500° centrigrade, absolute. Its 

 red color is easily noted by the eye. A limit of stellar temperature 

 seems to be set at this point below which stars can not exist, or if it 

 it is possible to exist, they do not give out enough light to make their 

 presence known to us. The temperature is so low that molecules of 

 titanium oxide are able to form in the upper atmosphere. As shown 

 by the bands in the spectrum, these molecules absorb some of the 

 light from the interior and are partly responsible for the great light 

 changes observed. As the star grows colder this absorption increases 

 and so diminishes the light. The storing of the absorbed heat finally 

 raises the temperature, the compounds become partly dissociated, and 

 the star brightens. 



Radial-velocity measures also indicate that the star expands and 

 contracts during its period to the amount of 20 per cent of its radius, 

 or about 30,000,000 miles. The cause of this pulsation is unknown, 

 but it is sufficient to produce a distinct change in the effective tem- 

 perature and a corresponding light variation. 



