Fuly 22, 1886] 
NATORE 
281 
sphere, and during the last twenty or thirty years a considerable 
discussion has been going on among astronomers as to whether 
the surrounding nebula is or is not changing its position with 
regard to the star. Now what happens to the star? I may 
tell you that the curve is only a rough one. But still you see 
the point fairly enough. This is, that this star, which has a bright 
line spectrum like 8 Lyre, has a period not of thirteen days, but 
of seventy years. We find that the star, which is at first possibly 
below a sixth magnitude star, rises up to the first magnitude, but 
then goes down to the second, and soon. ‘The curve shows a 
period of seventy years, the curve being very irregular. 
The third and fourth classes, so far as we can see, resemble 
Fic. 27.—Light curve of 8 Lyre. 
our sun. The curves suggest that of the sun’s spot period, when 
we can make anything out of them at all. 
But when we come to another class, in which we get a large | 
light change in one period, there is one star, the history of which 
is so extraordinary that it is quite worth while to throw its light 
curve on the screen, It is called Mira, or the Marvellous. It 
visible as a star of the tenth magnitude, 
1 
is in the constellation of the Whale, and what happens to it in 
just a little less than a year is this. First it is of the second 
magnitude, and then in about eighty days it descends to the 
tenth magnitude, and then, so far as the observations have gone, 
it is invisible. In about another hundred days it againf'becomes 
It then increases its 
Fic. 
lizht to the second magnitude, and begins the story over again. 
But sometimes at the maximum its brilliancy is not quite constant. | 
That is to say, sometimes it goes nearer the first magnitude than 
the second. What happens to the light of the star below the 
tenth magnitude it is impossible to say. Whether it follows 
more nearly either of the dotted curves in the diagram is not | 
known. 
made, because it is very difficult to observe a star under those 
conditions. What one knows is that it remains invisible for about 
140 days or something like that, and then it begins its cycle over 
again. 
28.—Light curve of Mira. 
Below the tenth magnitude no observations have been 
The next diagram illustrates the last conditions of, variability, 
