uly 8, 1886] NATURE 229 
and then die away. These stars have bright lines in their | change in its light at some time or other, but at all events those 
spectra. | stars in Class IV. have undergone sufficient change to find them- 
The second class gives us those bodies which, although form | selves recorded among suspected variables, while the change 
do not appear and disappear with any suddenness comparable to | actually has been so irregular that one has really practically not 
that, yet indicate that there is something very extraordinary going | known to what class to assign them ; and therefore they have a 
oninthem. They also belong to our fifth class of stellar spectra. | class of their own. 
They have bright lines as well as absorption-lines. ‘These bright The next class of variables I will, on Dr. Pickering’s authority, 
lines, however, only last for a short time ; but bright lines there are. | define-as eclipsed stars : that is to say, in this class the change of 
Next we get stars not so interesting from the large point of | light does not come from anything in the star itself, but from 
view, in which we get considerable changes in their luminosity | something that is happening outside it. What is happening you 
extending over very long periods, but their spectrum apparently | will see by and by. 
does not change to any great extent. At least, no change of the | Now with regard to our first class—the new stars. The ac- 
spectra of these stars has yet been recorded. | companying diagram will give an idea of what has been recorded 
After these, in Class IV. we get small irregular changes, and | withregard tothem. The information which the diagram affords 
in fact, Dr. Gould—and there is no greater authority than | will also give a pretty fair comparison between these variables and 
he—says that every star in the heavens undergoes some slight | the other classes. 
fic. 24.—Light curves of T Corone and Nova Cygni. 
In the year 1866 there was a star which had been chronicled | was that in its spectrum when it was most brightly shining we 
for many years asa star between the ninth and tenth magnitudes; | got the spectrum of incandescent hydrogen. We had, in fact, 
for this reason till 1866 its light curve is shown as a straight | the spectrum of the chromosphere of the sun. It was called “‘a 
line. But suddenly, at the beginning of May 1866, this star | world on fire.” But you know that even the sun is not a world 
suddenly burst up into a star of very nearly the first magnitude— | on fire. If it were, and if it were made of the best Welsh coal, 
between the first and the second. Many observations, as you | we are told that it would last only a few thousand years. But at 
may imagine, were made on it, 
and among them Dr. Huggins | all events, whatever happened, there was an immense quantity 
turned the spectroscope to it, and it was found that the difference | of hydrogen suddenly rendered incandescent, which radiated its 
between the star when it was between the first and second mag- | light to us. 
nitude, and when it was between the ninth and tenth magnitude, Almost as suddenly this star went down again, and by the 
al 
Fic. 25.—Cornu’s spectrum of Nova Cygni. 
end of the month it had become a ninth or tenth magnitude star, | sequent history. Its light curve, instead of going suddenly down 
and went about its ordinary business just asif the incident had | as the one in Corona did in 1866, goes down gently, and takes 
never happened to it. | nearly a year to get to the tenth magnitude. When it got to the 
Take the next star in 1876, ten years afterwards. It was | tenth magnitude what happened to it? It gave the spectrum of 
called a new star—Nova in Cygnus. The point about this one | a nebula. It had ceased to be a star. An interesting point is to 
is that it began suddenly as a star of between the third and the | inquire—unfortunately we shall never now know—whether or 
fourth magnitudes. It had had no former history. It had never | not that mass of matter did not exist as a nebula before 
been mapped. It did not visibly rise to the position of a third | 1876. . ’ es 
or fourth magnitude star from a lower level as the other one had I have stated that, following close upon the publication of Dr, 
done, but it burst out suddenly. Note the difference in its sub- | Vogel’s paper on the new star, another paper announced the fact 
