September 1, 1900.] 



KNOWLEDGE. 



I'M) 



ASTRONOMY WITHOUT A TELESCOPE. 



By E. Walter Maunder, f.r.a.s. 



VIII.— FOUR VARIABLE STARS. 



It is 304 years since David Fabricius, one of the earliest 

 observers of sunspots. noticed that a star in the neck of 

 Cetus the Whale, which he had observed in August, 

 1596, to be of the ord magnitude, had disappeared by 

 October. This appeai'ed an observation of great im- 

 portance, since it seemed to show that the fixed stars arc 

 not all of them permanent, but that they might die out. 

 Seven years later Bayer recorded a 4th magnitude star 

 in precisely the same position as that which Fabricius 

 had noted to have disappeared. Here, however, the 

 matter rested for an entire generation, and it was not 

 until 1638 that Holwai'da detected the star again as of 

 the 3rd magnitude in December, but found it disappear 

 in the following summer to reappear again in the autumn. 

 This star, therefore, Omicron Ceti, which received from 

 Helvelius the name of Mira, the wonderful star, was 

 the first to become known as a periodic variable. 



The first stai% that is to say, in historic times. There is 

 another, more striking even than Mira, which it seems 

 likely w^as noted by the forgotten astronomers of Arabia 

 or the valley of the Euphrates centuries before even 

 Hipparchus and Ptolemy compiled their catalogues. This 

 is Beta in the constellation Perseus, described by 

 Ptolemy as the principal star in the head of the Gorgon 

 Medusa, which the hero is represented as carrying in his 

 hand. This star has come down to us from the Arabs 

 with the name Algol, the Demon Star, and it is at least 

 a probability that it owed this name to the fact that 

 though ordinarily of the 2ud magnitude it sinks down 

 almost to the 4th at perfectly regular intervals of two 

 days and twenty -one hours. 



The variability of Algol was discovered in modern 

 times by Montanari in 1669, and it was re-discovered by 

 Goodricke in 1782. The latter observer two years later 

 added two other variables to the list; Beta Lyrse with 

 a period of veiy nearly thirteen days, and Delta Cephei, 

 with one of five and a^-third days. At this date scarcely 

 more than a century ago these four stars were almost 

 the only variables known to us, and variables continued 

 to be rare objects until the middle of this century. 

 Now their number.^ have been added to so greatly that 

 the catalogue prepared by Prof. Chandler in 1896 com- 

 prises 400, the vai-iability of which is fairly well estab- 

 lished, and new members of the class are being 

 discovered every month. 



These four star's ai-e all well in view during the 

 September nights. Delta Cephei being a circumpolar 

 is necessarily always visible, and is now overhead at 

 midnight; Algol also is a circumpolar, but descends too 

 near to the northern horizon at its sub-polar transit for 

 observation then. At midnight on September 1, how- 

 ever, it is high up and almost due cast of the observer, 

 whilst Beta Lyr«e is at a similarly favourable elevation 

 in the west. Mira Ceti being slightly south of the 

 equator is only above the horizon about IH hours out 

 of the 24 ; it comes to the meridian at midnight towards 

 the end of October, consequently at midnight on Sep- 

 tember 1 it is low down in the south-east. 



Mira Ceti has been somewhat irregular in its period 

 of late, but should have just passed a maximum. Its 

 brightness at maximum varies through wide limits; 

 sometimes it scarcely exceed.s the 5th magnitude, some- 

 times it is distinctly brighter than the 2ud, but u.sually 



it ranks between the 3rd and the 4th. It is thus 

 always within the range of unassisted sight at maximum, 

 but it goes down far below that range at minimum, its 

 faintest light bringing it down practically to the 10th 

 magnitude. The " astronomer without a telescope," 

 therefore, can only watch it at its maxima, hut these 

 form for Mira Ceti the interesting pliase. The other 

 three stars are at all times well within the range of 

 vision. A telescope, therefore, is not needed for them, 

 and it is much better that it should not be used. 



The most striking star of the four with which to begin 

 is Algol. The student, avoiding all references to 

 Ephemerides, should look out at regular intervals and 

 compare the brightness of Algol with certain of the 

 neighbouring stars. Ordinarily Alpha Persei will be 

 distinctly but not very greatly brighter than our 

 variable, whilst Gamma, Delta, Espilon and Zcta will 

 bo distinctly fainter. At a little greater distance are 

 Alpha and Beta Arietis, the fonner slightly brighter, 

 the latter slightly fainter than Algol. Alpha and Beta 

 Trianguli arc at no great distance, and arc good com- 

 parison stars when Algol has begun to fade. 



It will not be long before the observer will find that 

 his stai- is undergoing a change, and that it no longer 

 nearly rivals Alpha Persei or Gamma Andromedre in 

 brightness. Directly this is noticed, systematic ob- 

 servation should be commenced. A star should be 

 chosen, reasonably near, distinctly brighter than the 

 variable, and a second star distinctly fainter. It is 

 usual among variable star observers to estimate these 

 differences in " steps," these " steps " corresponding 

 generally to about a tenth of magnitude, though pro- 

 bably the beginner will make his steps considerably 

 larger than this. The central principle, however, is that 

 two stars should be selected, one of which the observer 

 is clear to be fainter than the variable, and the other 

 brighter, and yet both of them pretty near the variable 

 in brightness. The student should further be careful 

 to record whether the difference between the variable 

 and the fainter star was equal to, greater than or less 

 than the difference between it and the brighter. An 

 observation therefore might run as follows : — ■ 



Sep. Id. Uh. 15m. 2 > a 3 < fe 

 where a and h are the two comparison stars This 

 would mean that at llh. 15m. the variable was noted 

 to be two " steps " brighter than a and three " steps " 

 fainter than h ; in other words that it is slightly nearer 

 an equality with a than with h. 



Of course there is no reason why the observer should 

 confine himself to two comparison stars. To begin with, 

 indeed, it is well that he should try more; bearing in 

 mind that the stars should be a.s nearly as possible at 

 the same altitude, as a marked difference in the height 

 above the horizon will have a considerable effect upon 

 the estimation. 



Having made one set of satisfactory observations, the 

 student should leave the star for a while — say for half- 

 an-hour — and then make an entirely fresh set of obser- 

 vations. If he should be fortunate enough to hit upon 

 the commencement of a minimum his second observation 

 will show him the star somewhat fainter than the first, 

 and the difference will become more marked at a third 

 observation. The entire period of decline and recovery 

 for Algol is nine hours, the light fading for 4| hours, 

 remaining constant for a few minutes, and then 

 gi-adually increasing again for another period of 4^ 

 hours. The light changes therefore at a most rapid 

 rate at about 23- hours before minimum or about the 



