528 



SCIENCE 



[N. S. Vol. XLV. No. 1169 



4. The so-called new stars, otherwise 

 known as temporary stars, afford interest- 

 ing evidence on this point. These are stars 

 which suddenlj' flash out at points where 

 previously no. stars were known to exist ; or, 

 in a few cases, where a faint existing star 

 has in a few days become immensely 

 brighter. Twenty-nine such stars have been 

 observed in the past three centuries, nine- 



'4^J^ 



Fig. 17. The D.-irk Holes, m (_,...il -?,aU," m 

 Sagittarius, photographed by Barnard with the 

 10-inch Bruce Telescope of the Yerkes Observatory. 



teen of them since 1886 when the photo- 

 graphic dry i^late was applied systemat- 

 ically to the mapping of the heavens, and 

 fifteen of the nineteen are to the credit of 

 the Harvard observers. This is an average 

 of one new star in less than two years in the 

 last three decades; and, as some of the 

 fainter new stars undoubtedly come and go 

 unseen, it is evident that they are by no 

 means rare objects. Now all of the tempo- 

 rary stars except five appeared in the 

 Milky "Way, and four of the five exceptions 

 are worthy of note. Two of the five ap- 

 peared in well-known nebulffi ; another was 

 located close to the edge of a spiral nebula, 

 and quite probably in a faint outlying part 

 of it ; a fourth was observed to have a nebu- 

 lous halo about it ; and the fifth was but 

 meagerly and imperfectly observed. Keep- 



ing the story as short as possible, a tempo- 

 rary star is seemingly best explained on the 

 theory that a dark or relatively dark star 

 traveling rapidly through space liaS' met 

 with resistance, such as a great nebula or 

 cloud of particles would afford. While 

 passing through the cloud the star is in 

 effect bombarded at high velocity by the re- 

 sisting materials. The surface strata be- 

 come heated, and the luminosity of the star 

 increases rapidly. The new star of Feb- 

 ruary; 1901, in Perseiis afforded interesting 

 testimom\ Wolf at Heidelberg photo- 

 graphed in August an irregular nebulous 

 object near the star. Ritchey's photograph 

 of September showed extensive areas of 

 nebulosity in all directions from the star. 

 In October Perrijie and Ritchey discov- 

 ered that the nebular structure had ap- 

 parentl.y moved outward from the star 

 (see Fig. 19). Going back to a March 

 photograph, taken for a different pur- 

 pose, Perrine found recorded upon it an 

 irregular ring of nebulositj^ closely sur- 

 rounding the star which was not visible 

 on later photographs. The region seemed 

 to be full of nebulosity not visible to us 

 under normal conditions. The rushing 

 of the dark star into and through this 

 resisting medium made the star the bright- 

 est one in the northern sky for several 

 days. The great wave of light going out 

 from the star when at this maximum 

 brightness traveled far enough in five weeks 

 to fall upon non-luminous materials and 

 made a ring of nebulosity visible by reflec- 

 tion. Continuing its progress, with a speed 

 of 186,000 miles per second, the wave of 

 light illuminated the material which Wolf 

 photographed far away from the star in Au- 

 gust, the material which Ritchey photo- 

 graphed still farther away in September, 

 and the still more distant materials which 

 Perrine and Ritchey photographed in Oc- 

 tober, November and in later mouths. We 



