[Illustration:  Fig. 81.—­Constellation Lyra, showing place of the Ring Nebula.]

Mira, or the Wonderful, in the Whale (Fig. 68), is easily found when visible.  Align from Capella to the Pleiades, and as much farther, and four stars will be seen, situated thus: 

    *
  * * *

The right-hand one is Mira.  For half a month it shines as a star of the second magnitude.  Then for three months it fades away, and lost to sight; going down even to the eleventh magnitude.  But after five months its resurrection morning mes; and in three months more—­eleven months in all—­our Wonderful is in its full glory in the heavens.  It its period and brilliancy are also variable.  The star Megrez, d in the Great Bear, has been growing dim [Page 222] for a century.  In 1836 Betelguese was exceedingly variable, and continued so till 1840, when the changes became much less conspicuous.  Algol (Fig. 68) has been already referred to.  This slowly winking eye is of the second magnitude during 2d. 14h.  Then it dozes off toward sleep for 4h. 24m., when it is nearly invisible.  It wakes up during the same time; so that its period from maximum brilliancy to the same state again is 2d. 20h. 48m.  Its recognizable changes are within five or six hours.  As I write, March 25th, 1879, Algol gives its minimum light at 9h. 36m.  P.M.  It passes fifteen minima in 43d. 13m.  There will therefore be another minimum May 7th, at 9h. 49m.  Its future periods are easy to estimate.  Perhaps it has some dark body revolving about it at frightful speed, in a period of less than three days.  The period of its variability is growing shorter at an increasing rate.  If its variability is caused by a dark body revolving about it, the orbit of that body is contracting, and the huge satellite will soon, as celestial periods are reckoned, commence to graze the surface of the sun itself, rebound again and again, and at length plunge itself into the central fire.  Such an event would evolve heat enough to make Algol flame up into a star of the first magnitude, and perhaps out-blaze Sirius or Capella in our winter sky.

None of the causes for these changes we have been able to conjecture seem very satisfactory.  The stars may have opaque planets revolving about them, shutting off their light; they may rotate, and have unequally illuminated sides; they may revolve in very elliptical orbits, so as to greatly alter their distance from us; they may be so situated in regard to zones of meteorites as [Page 223] to call down periodically vast showers; but none or all of these suppositions apply to all cases, if they do to any.

Temporary, New, and Lost Stars.

Besides regular movements to right and left, up and down, to and from us—­changes in the intensity of illumination by changes of distance—­besides variations occurring at regular and ascertainable intervals, there are stars called temporary, shining awhile and then disappearing; new, coming to a definite brightness, and so remaining; and lost, those whose first appearance was not observed, but which have utterly disappeared.