THE NEW STAR WHICH FADED INTO STAR-MIST. 1 31 



directed to the heavens, it would appear only as a point of 

 light If it lost the best part of its lustre, it would ap- 

 pear only as a dull point of light Now the planetary 

 nebulae show discs, sometimes of considerable breadth. 

 Sir J. Herschel, to whom and to Sir W. Herschel we owe 

 the discovery and observation of nearly all these objects, 

 remarks that " the planetary nebulae have, as their name 

 imports, a near, in some instances a perfect, resemblance 

 to planets, presenting discs round, or slightly oval, in 

 some quite sharply terminated, in others a little hazy or 

 softened at the borders. ..." Among the most remark- 

 able may be specified one near the Cross, whose light is 

 about equal to that of a star just visible to the naked eye, 

 " its diameter about twelve seconds, its disc circular or very 

 slightly elliptic, and with a clear, sharp, well-defined outline, 

 having exactly the appearance of a planet, with the excep- 

 tion of its colour, which is a fine and full blue, verging 

 somewhat upon green." But the largest of these planetary 

 nebulae, not far from the southernmost of the two stars called 

 the Pointers, has a diameter of 2f minutes of arc, "which, 

 supposing it placed at a distance from us not greater than 

 that of the nearest known star of our northern heavens, 

 would imply a linear diameter seven times greater than that 

 of the orbit of Neptune." The actual volume of this 

 object, on this assumption, would exceed our sun's ten 

 million million times. No one supposes that this planetary 

 nebula, shining with a light indicative of gaseity, has a mass 

 exceeding our sun's in this enormous degree. It probably 

 has so small a mean density as not greatly to exceed, or 

 perhaps barely to equal, our sun in mass. Now though the 

 " new star " in Cygnus presented no measurable disc, and 

 still shines as a mere blue point in the largest telescope, yet 

 inasmuch as its spectrum associated it with the planetary and 

 gaseous nebulae, which we know to be much larger bodies 

 than the stars, it must be regarded, in its present condition, 

 as a planetary nebula, though a small one ; and since we 

 cannot for a moment imagine that the monstrous planetary 



