OUR REVOLVING "ISLAND UNIVERSE" — SKILLING 127 



made of gas and dust clouds mixed. But both the spectroscope and 

 the modern telescope give evidence that many others are, as Herschel 

 suspected, composed of stars. In addition, more recent knowledge of 

 their distances shows that gaseous nebulae or those composed of il- 

 luminated dust clouds are within our own stellar system whereas those 

 that are composed of stars are outside of that system. Hubble calls 

 them "extragalactic nebulae" to distinguish them from true nebulae. 

 Shapley, of the Harvard College Observatory, calls them simply "gal- 

 axies." This word indicates their likeness to the stellar system but 

 does not distinguish them from it as the prefix "extra" does. 



The name by which these outside galaxies are popularly known is 

 "spiral nebulae." Both parts of this name are in error for some do 

 not have the spiral form and none of them are mere clouds, which 

 "nebulae" implies. Nevertheless, a spiral form is the most striking 

 characteristic of those that do possess it. The name "island universes," 

 pretty well suggests their nature, but it violates the usual meaning of 

 the word "universe." Thus all names of these outside star systems seem 

 to be open to criticism. 



But what evidence exists to support Lindblad's theory that our 

 galaxy is one of these spiral nebulae ? First, there is the evidence de- 

 duced from their distances. 



The spiral nebulae lie at such great distances that the surveyor's 

 parallax method used for nearer stars fails utterly to give any idea 

 of their location. However, their distances had to be known, at least 

 approximately, before it could be definitely decided whether they are 

 separate systems beyond our stars or only outlying parts of our galaxy. 

 Happily an exceedingly powerful indirect means of measurement was 

 discovered by which Hubble, of the Mount Wilson Observatory, learned 

 that the distance to a spiral in the constellation Triangulum is about 

 three-quarters of a million light years. Another still more conspic- 

 uous one in Andromeda was found to be at practically the same 

 distance. 



The method employed by Hubble makes use of a certain kind of star 

 known as a Cepheid variable, a number of which had been discovered 

 in the spirals. These stars are exceptionally bright intrinsically and 

 can be seen at a great distance. As far away as these variable stars 

 can be clearly seen their distances can be calculated, for their period 

 of change from maximum brightness to maximum again is a key to 

 their real brightness, and this, in turn, is a key to their distance. 



A curve can be drawn (from table 1, for example) showing the 

 relation between period and brightness, and from the curve the actual 

 luminosity of a star of any period can be f ound. Then by simply com- 

 paring this real brightness with the brightness that the star seems to 

 have, as measured with a photometer, the star's distance can be found. 



