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that the other spiral nebulae may be separate universes. Astro- 

 nomers divide nebulae into three classes. These are : 



(i) The irregular nebulae ol which the Great Nebula in 

 Orion is the most conspicuous example. This class comprises 

 nebulae of many varied shapes, whose names are often given 

 from a more or less striking resemblance to some terrestrial 

 object, such as the Dumbbell Nebula, the Crab Nebula, the North 

 America Nebula, the Keyhole Nebula, and many others. To 

 the same category belong the nebulous backgrounds, obviously 

 associated with stars, such as the nebulosity around the 

 Pleiades, and in the constellation of Taurus. These irregular 

 nebulae occur mainly in the neighbourhood of the galaxy, and 

 undoubtedly belong to our system. In many cases they show 

 undeniable connection with certain stars. 



(2) The second class is known as the Planetary or Gaseous 

 nebulae. They were first classified as such by Sir William 

 Herschel, but he did not originally recognise their nebulous 

 nature. " We can hardly suppose them to be nebulae," he says ; 

 " their light is so uniform as well as so vivid, their diameters so 

 small and well defined, as to make it almost improbable that 

 they should belong to that species of bodies." He considered 

 that they might be planets attached to distant suns, but recog- 

 nised that this supposition was untenable. Their spectra 

 present many analogies to the spectra of the Wolf-Rayet or 

 gaseous stars, and, like the latter, they occur almost exclusively 

 in the Milky Way. This apparent connection seemed to 

 indicate that nebulae passed, in the ordinary course of evolution, 

 into the gaseous or early-type stars. This hypothesis received 

 a rude shock when measurements of the line-of-sight velocities 

 of these objects became possible. If by evolution they passed 

 into early-type stars, their average radial velocity should be 

 very small, of the order of 5 or 6 km. per second. The first few 

 results obtained indicated that this was far from being the case, 

 and enough measurements have now been made to assert 

 definitely that, so far from their velocities being small, their 

 average velocity is considerably greater than that of the late 

 M-type stars— the average line-of-sight velocity of the latter 

 being about 17 km. per second, and that of the planetary nebulae 

 of the order of 40 km. per second. Several of them have been 

 found to possess extremely high velocities, even as great as 

 200 km. per second. How the planetary nebulae are to be 



