The following web page were reproduced from a small 8-page A5-sized booklet that was issued by Sydney Observatory and written by the then N.S.W. Government Astronomer, Dr. Harley Wood. It is an adoption of “Elementary Astronomy for Service Use” produced in the early 1940’.
THE study of Double Stars is, I think, one of the most fascinating which astronomy gives us. The great number and variety of the objects already known, and the certainty that many new ones will be the reward of any diligent search for them, keep up the interest to such an extent that the observer needs no other incentive to his work. M. Flammarion, after an examination of the observations already made - and be it remember that this branch of astronomy may be said to have been originated by Sir William Herschel, about the year 1800 - find that there are 11,000 double and multiple stars catalogued. Of these, 819 give certain indication of relative movement ; of which 731 are double, 73 triple, 12 quadruples, 2 quintuples, 1 sextuple ; of these again, 518 seem to form orbital systems, and 316 are only united by celestial perspective. Observation further shows that the components of an orbital system may be separated by as much as 22″, and two stars separated by 15′ of arc may have a common proper motion. Again, Mr. Doberck, after a critical examination of double stars, considers that only the orbits of twenty-seven are known, and only seven are in the Southern Hemisphere. We know five stars whose period is under fifty years ; seven with periods of 50 to 100 years ; six between 100 and 200 years ; six between 200 and 350 years ; three over 400 years. If, in addition to these statistics, we bear in mind that the Southern Hemisphere is only partly explored, and that in the Northern Hemisphere, which has been examined over and over again with fine instruments, used by such observers as Sir William Herschel, Struve, and others, it has been recently shown that it is possible, with moderate or small telescopes and good eyes, to find many new and difficult objects, as Mr. Burnham has done, I think you will see there is justification for the opinion which I have just expressed, and that the observer, in watching for these changes, and then in the investigation of them to see whether they are due to the motion of one star around the other, to independent motions of the stars, or the annual motion of the earth, has his interest constantly maintained [l] and it is not lessened by the fact that he may go on thus for years making [2] observations which seem to prove that there is orbital motion, only to find in the end that the changes he sees are due independent motion, as I endeavoured to show you last year in reference to p Eridani, in the supposed orbit of which, as the observations accumulate, the ellipse had gradually to be increased, until in the end the most probable curve, if I may so express myself, was shown to be a straight line, or, in other words, the motion which was supposed to prove it a binary is found to be probably due to proper and not orbital motion at all. I may mention in passing, that if subsequent observation confirms this, the southern binaries referred by Mr. Doberck will be reduced to six.
Before proceeding to give you so the results of my own work on our southern double stars, it may be necessary to spend a few moments in describing the instruments and methods of observation. The first instrument with which the work was begun is a very fine 7¼-inch refractor by Merz, of 10 ft. 4in. focal length, and very fine defining power ; upon this is a position circle micrometer by the same maker, with 4½ in position circle, and means of a dark and bright wire illumination, and magnifying powers up to 580. For easy stars a power of 159, and more difficult ones 330 was used ; and the method of observing was, first to place the position wire so that it bisected both stars, and then to bisect each of the stars with one of the parallel wires. After which circle and micrometer were read. Hence two independent determinations of the angle, and two readings for the distance, the difference of which gives twice the angular difference between the stars. In reducing these, the two reading of the micrometer were in some cases compared with the coincidence of wires reading to get two measures ; at other times the difference between the readings divided by 2 was taken. The result was the same whichever way it was taken. As the latter involved the smaller amount of computation, it has generally be adopted. In many cases ten readings of the micrometer were taken, that is ten measures of angle and ten measures of distance ; but in the majority of cases only six have been taken. When the stars are very close, the method of setting the wires to the apparent distance of the stars has been frequently adopted, and found more satisfactory than the other method. In a few instances the distance has been obtained by placing one of the wires between the stars, and from its known diameter and its relation to the distances of the centres, estimate the distance.
The other telescope, used since 1874, is a very fine 11½-inch objective, 12½-ft focus, by Schröder, with position circle micrometer by the same maker. The illumination of wires (bright) is [3] obtained by four prisms placed near them, and the light from a small gas flame reflected into the side of the telescope. The magnifying powers are from 100 to 1,500, the power 800being used for all difficult objects. The same method of observation as used with the north dome, and has been used by Mr. Hargrave in measuring Herschel’s stars and verifying the positions of new stars.
About 746 of Herschel’s stars have been remeasured, some of them many times over, and 350 new double stars have been found. The results are appended, representing in this small compass some 15,000 measures of angle and distance.
For the sake of completeness I have included, in the general list, a few stars mentioned in my paper read before the Society last year.
With regard to other matters affecting the observations, I may mention that both domes are made of thin brass, and the temperature is very nearly always the same as inside as it is outside ; hence the work could be begun as soon as the shutters were opened. It has been my practice to observe stars on the meridian, or very near to it, and always taking the R.A. by means of the instrument, the hour circle readings show the distance from the meridian when the observations were finished. In some instances bright stars have been measured during the day-time, but generally the aperture has been reduced to get rid of excessive light ofbright stars.
In entering the notes at the time of observation, a diagram showing the estimated distance and angle has been made almost in every case.
A few words about the list of 350 new double stars, which I cannot have the honor to present to the Society to-night. They cannot be said to be the fruits of a search for new stars, for except an evening now and then devoted to that work, and some time recently given to it at my request of Mr. Hargrave, my object has been to re-examine Sir John Herschel’s Cape list between 34o south and the pole. It would have been very easy to double the number even under these circumstances, if I had adopted the same limit of distance as Sir John Herschel ; but I was very anxious to avoid burdening the list in that way, and made my limit much smaller, and was always more anxious to record close pairs than wide ones. Nine of the new ones are under one second of arc* - several of the very difficult. Sixty-six are under five seconds, others under twenty-five seconds, and all of them are between the parallel 42o south and the pole, with one exception, which was found in the field of view with one of h.’s stars. As they are so far south [4] they are out of reach of northern observers, and, so far as I can learn from published lists, they are new ; but, hereafter, when the work of several double star observers in the southern hemisphere is published, it may be that some will find other lists.
Many of them are very close and otherwise interesting doubles ; and there is every probability, seeing that h. overlooked them when in close proximity to star he observed, that someof them will prove to be binaries.
Only a few of the have as yet be repeatedly measured, but of these several show signs of motion. One at 12h. 4m. -60o 21″ was found in April, 1873, and then the measured angle was 212.35o, in May, 1880, it was 209.55o; at first the distance was 4.33″, last year it was 3.87″, showing a change of 3o in angle and 0.46″ in distance. Another at 15 h . 50 m ., and dec - 65 o 37 ′ in July 31, 1872, angle 134o 12′ , dist., 2.43″ ; on July 21, 1880, angle 131o 19′, distance 1.91″, again showing a change of 3o and 0.52″. Another very difficult pair at 10h. 45m., dec. -58o 38′ found in March, 1874, angle 258.81o, distance 1.15″; this one I have mentioned in the paper on double stars last year, but place it here for completeness.
Another star at 13h 0m. 59.14o, found in April, 1880, gives some indication of increasing distance, which when first measured, was 0.33″, and when last seen was 0.70″.
Amongst the h. stars observed are all those which are or have been supposed to be in motion ; of these p or 6 Eridani is one of the most interesting ; orbits have several times been computed for it, and I found that my earlier observations require a considerable extension of the period, but the later ones demand more still ; in fact, a straight line accords better with all the observations made subsequently to Herschel’s than any ellipse, and it would appear that the changes are simply to proper motion ; of this I think there can be no doubt ; and it would appear from the meridian observations made at the Cape and Madras that it is the preceding star which is in motion, not the following one - h.’s and Dunlop’s observations will not plot into a straight line, but it must be remembered that Dunlop had a very imperfect telescope and only guessed the distance ; and h.’s angle does agree, the distance only being too little - a fault the possibility of which no one would have been more readily admitted than h. himself. (See diagram.)
Alpha Centauri. It will be seen from the observations that periastron did not take place as predicted, in March, 1875, but in March, 1878, three years after. A great many measures of this binary will be found in the catalogue. At present the distance is increasing rapidly. (See diagrams showing the plot of Sydney Observations.) [5]
Some of h,’s stars present considerable difficulty, and are probably in motion, as for the instance 4786, γ Lupi, an easy double in 1836 is now a single star, with the highest powers on the large equatorial. The motion is evidently slow, and it is remarkable that Herschel says of this star, “Cleanly divided with power of 480 and the black division well seen. well separated with power 800” ; and of π Lupi he says, “I do not think measures of this star will be got with this instrument.“ “Excessively difficult. It is closer than γ Lupi, for the discs are smaller, and yet are not so much divided.” Now I found &pi Lupi quite an easy object, and the mean of my measures make the distance 0.73″, while Herschel made it 0.67″, so that there has been no change in this star. But γ Lupi, which h. found so easy, I have examined a great many times and always fail to divide it even with the greatest powers on the large refractor.
Another star of the same character it would seem is h. 4854, h. classes this as “very difficult to be verified.” On June 4, 1872, at the end of my evening’s work, I looked at it and divided it easily, with power 230 ; I only took one measure, making the angle 46o 25′, and the distance of 1.75″, being satisfied that in case, as in many others, that what was difficult in Herschel’s reflector was very easy in the Sydney refractor. By some chance I did not look at the star again until June 17, 1874, and to my surprise I could not divide it with any power. On July 16, 1880, I carefully examined it with the large telescope, and found only a round disc with all powers ; so that we have another interesting double in which the character of the motion has yet to be determined.
But it would take too long to go over all the cases or real or supposed change, and I have therefore collected them into a list, giving only bare measures, more details being found in the catalogue of stars measured. In all there are twenty-two stars which give satisfactory indications of motion or have at some time been supposed to be in motion.
I have added another list of seventeen interesting objects, being such double stars as have been found in the same field of view as stars which Herschel measured, and which therefore we are justified in assuming that he looked at without discovering their character. Of these no doubt some must be set down as too difficult for the reflector ; three, h. 3370, 4935, and 5078, have the large star double, but there are several instances in which the only satisfactory explanation is that the stars have changed since he looked at them. Perhaps the most striking case is 4909, where he was struck with the beauty of the group, and went on to describe it particularly, and he left out what is now one of the most striking features - a star within a pentagon, quite bright as three of the stars he mentions. His descriptions of such things are as a rule so accurate that I am convinced the additional star has [6] appeared since Herschel looked at the object. As another case I may mention h. 4890, which he is careful to say “is in a vacancy of the Milky Way, which here entirely of ground stars,” yet only 11s. follow 4890 ; in fact, where h. must have seen it visible, I find a beautiful double star, magnitudes 11 and 11, and in a field of 80s, diameter sixteen other stars, and the telescope (1871) used was 7½, while h. used the large reflector. There are others in the list of the same character, but I must pass on to notice changes in the magnitudes on some of the double stars. One is rather curious. In December, 1834, Herschel observed No. 3972 in his list, and called the magnitudes 8½and 8½ ; on March 7th, 1836, he looked again and called them 9½and 11 ; next night he verified this, making them 10 and 11. In 1834, he saw a 13 magnitude star which made an obtuse and nearly isosceles triangle with the other two, and he subsequently remarks it must have been ill seen (in 1836), as evident by the magnitudes assigned, and from the fact that the 13 magnitude was invisible. When in 1873 I examined this, the magnitudes were 10½ and 12, and I could not see the little star ; it would seem therefore that they must have been brighter in 1834. The night in 1873 was not favourable, but h. 4130 was examined at the same time, and the magnitude then assigned 8-10, or the same as h.’s.
There are many stars in the Cape list that cannot be found. Over some of these I have spent a good deal of time to see if they were examples of fading stars or change of position ; the result has not been satisfactory in most cases traced - they are evidently the results ofclerical errors ; for instance, entering a star with the wrong R.A. or declination. Probably most of those on the list of missing stars are of this character, but some may have been overlooked from changes in magnitudes or other causes. Perhaps the number (forty-six) is not surprising when we consider the difficulty of the work as carried on by Herschel.
I have not given much attention to colours, nor have I carefully compared my estimates of colour with h.’s, but one or two instances have been found of apparent decided change. h. 5193 he says the larger star is “very red”; I could not see any red in it in 1873 when I remeasured it.
In several cases I decided colours where h. mentions none ; these will be found in the list attached.
In conclusion, I may say that my object has been to remeasure all h.’s close double stars south of 34o south declination. In very many cases considerable differences between h.’s observations with the reflector and mine have been found ; but a complete list of them have not been made, because the reflector observations so often differ from those h. made with his equatorial that it did not appear to be worth while.
* In 2102 stars Herschel has only 25 of 1′ and under.
[AJ. Exactly 3000 words!]
[7]
h;’s No.
3407 h’s angle 78o 30′ to 80o 51′ ; probably not in motion.
λ Toucani
3423 h’s angle, 17o 15′ , dis. 5.78″ ; R., 0.8o dis. 5.34″.
κ Toucani
3453 h’s angle, 122o 6′. dis. 3.39″ ; R., 234o 41′, dis. 6.30″ ; all the later
p or 6 Eridani. observations plot into a straight line as if the the notion were proper, not binary.
3835 h.’s angle 343o, dis 4″ ; R. 2o 6′, dis. 2.90″ ; and 1882 R. 2.35″, angle
11o 39′.
3930 h.’s angle 72o 43’. dis. 3.49″ ; R., 77o 5′, dis. 2.82″; probably binary.
4025 Triple h.’s angles, 47o 6′ & 34o 8′, dis. 18″ & 40″; magnitudes 6-14 &
6-12 ; H. angles. 69o 53′ & 37o 40′.
4106 h. 139o 18′ ; R. 143o 8′.
4373 h.’s angle, 226o 1′, dis. 25″ ; H., 338o 18′, dis. 12.66″.
4507 h.’s angle, 227o 4′, dis. 20″ ; H., 180o 26′, dis. 12.94″
4521 h.’s angle, 191o 48′ to 122o 57′, dis. 4.86″ to 6.62″ R., 117o 27′, dis. 5.34″
motion improbable
4539 h.’s angle, 349o 47′ to 361o 58′, dis. 2/3”″ to 1″ ; R., 1880, 1o
γ Centauri 16′, dis. 1.39 ″ ; motion doubtful.
4645 h.’s angle, 231o 8′; H., 202o 11′, 1881.
4691 For complete measures, see list.
α Centauri
4728 h.’s angle, 108o 30′ to 115o 35′, dis. 0.67″ to 1.0″; R., 1880, 99o 18′,
&pi Lupi
dis. 0.90″. h. said this is closer than γ Lupi. R., with Sydney refractor, found this an easy object, while γ Lupi is too difficult to divide ; one or the other must have changed.
4728 h.’s angle, 90o 10′ to 104o 30′, dis. 2/3”″ to 1″; R., 1880, with power 1,200
γ Lupi seems to be elongated, angle 270o E.; but though I have often tried I never can divide it. See note to π Lupi.
4852 h.’s angle, 115o, dis. 6″ ; H., 145o 8″,
. dist.
14.66″
5014 h.’s angle, 69o, dis. 0.67″ ; R., 79o 18′, dis. 0.81″; but as h.’s angles range
from 61 o to 76o, it is probably not binary.
5041 h.’s angle, 260o 5′, dis. 2″ ; R., 1871, 253o 35′, dis. 3.27″; R., 1873, 259o 52′, dis. 3.11″ ; H., 1880, 264o 52′, dis. 1.82″.
5027 h’s. angle, 59o 2′; R., 1871, 84o 42′; H., 1881, 91o 52′; showing change of 25o in thirty-five years, and 7o in ten years.
5078 h.’s angle, 313o 4′; H., 212o 50′. Is this a clerical error, 313o for 213o in
Cape list ?
5084 h.’s angle, 37o 6′, dis. 1.23″; R. angle, 53o 8′, dis. 1.15 ″.
γ Cor. Aust
5246 h.’s angle, 216o 8′, dis. 1½″ ; R. angle, 129o, dis. 3.82″.
5258 h.’s angle, 307o 31′, dis. 3.65″; R. angle, 288o 50′ , dis. 4.68″.
θ Indi
[8]
(17 objects.)
h.’s No.
3370 Large star, double R. No.3. In the same field with this small triangle of stars - the preceding one of which is a close double not seen by h., R. No 11.
3499 Another double, precedes this only 15s, R. No.16.
3843 Another wide double in the field north following this, not seen by h.
3959 Another north, and only 12s following, R. No. 74
4077 Another double in the field with this , R. No. 82
4122 Another a little south and in the same field. R. No. 86
4645 Two pairin the field with this , R. Nos. 265 & 266.
4787 Two doubles in the field with this, R. Nos.265 & 266.
4835 Another double, 6s following this. R. Nos. 273.
4890 Another double in the field, 11s following, and 16 other stars. h. says. “This is a vacancy in the Milky Way, which is here entirely free of stars at its base.” R. No. 285
4909 h. says, “This is a very symmetrical little constellation, of two large and three 12 magnitude, forming a pentagon in a direction of right angles to the two stars at its base.” On 22 July, 1872, I found a small star very conspicuous within this pentagon, and adding much to its beauty. h. would certainly have mentioned it if sen. R. 289.
4935 Large star, double, R. No. 295.
5075 h. says nearest star is distant 15′; R. finds nearest one of a pretty triangle; seems strange h. did not say so.
5078 Large star is double, R. No.317.
5256 Another double in the field with this, No. 328; and another precedes 65s and 15′north.
5592 Another double precedes this 60s, R. 336
h.’s No.
3366
3393
3397
3530 Could not see either.
3577
3590
3648
3678
3719
3748 Same as 3641.
3790
3837
3851
3870
3872
3884
3895
3979
4030 And 4038 probably the same
4085
4130 And 4142 probably the same [9]
4390
4480
4504
4544
4596 h.’s decln. 65o 10′ pair at 64o 10′
4628 No companion.
4695
4707
4840 probably same 4836.
4854 Single star now.
4858
4906
4959
4975
4979 Looked for several times.
4996
4998
5006
5008
5103
5234
5267
5319
5385
5437
h.’s No.
3419 h. does not mention colour ; R. seesgreenish yellow and copper red.
3962 h. calls magnitudes 8 and 10 ; R. several times 8 and 8½.
3972 h. calls 8½ and 8½, 24 December, 1834, and 9½ and 11 on
7 March 1836, and 10 and 11, 8 March. 1836 ; R., 1873, 10½and 12 ;
h. makes the remark, “ 13 mag. star - makes an obtuse angle and nearly
isosceles triangle with these two ; it does not appear to have been noticed
in either of the other observations, in which the stars must have been still
seen, as is evidentby the magnitudes affixed”; but then R., 1873, made
them 10½ and 12 (?) variable.
3977 h. 8-9, now 9-13.
4104a h. white and blue ; R. yellow and blue.
4104b h. no colour ; R. yellow and white.
4539 h, examined this star four times, - on three it was badly defined ; I have
γ Centauri examined it three times, and twice it was badly defined. Can there be
any peculiarity about the star ?
4813 h. does not mention colour ; R sees yellow and blue.
4890 h. says, “This pair of 8 and 9 magnitudes is in a vacancy of the Milky
Way which is here entirely free of ground stars” ; and on 13 July, 1871,
when I looked at it with 7¼-inch telescope
I found a pretty double star of 10 and 11 magnitudes, only 11″
following h . ’s star , and in a field 80s. in diameter ; I counted twenty stars
from 9. mag, downwards, i.e., two double and sixteen other stars.
5193 h. says large star very red ; R. cannot see red at all.
5292 h. mag. 7½-10 ;R. 10-11 (?) change. [10]
h.s. No
3748 No star at 5h. 13m., but there is at 4h. 13m., and this star is
3641 numbered 3641
4030 Probably the same star.
4038
4130 Probably the same star.
4142
4272 R.A. 6m. too large. Should be 9h 48m.
4596 h.’s declination is 65o 7′; star is at 64o 16′.
4684 Cannot find a double here.
4836 Probably the same star.
4840
5132 Angle of position 160o in error.
5235 Probably the same star.
5245
5327 Angle of position 180o in error.
In the Cape catalogue, Sir John Herschel used “h” as the symbol for himself. I have adopted the same letter for him in the following catalogue. R. is used for myself, and H. for Mr. Hargrave.
A diagram of the positions of p Eridani is given, and another showing the Sydney observations of α Centauri, also a number of smaller ones showing interesting triple or multiple systems among those of the Cape catalogue which have been measured, and similar diagrams for some of the new ones.
The value of a revolution of the screw of each of the micrometers has been carefully determined in the usual way, i.e. by separating the wires ten or more revolutions and observing transits of circumpolar stars ; that of the 11½-inch is 18.006″, and that of the 7¼-inch is 21.670″.
It has been the custom always to make measures with the star as close to the meridian as possible, and the star’s hour angle at the time of observations were finished has been recorded in every case, and can be referred to if necessary. All the observations prior to July, 1874, were made with the 7½-inch, and all the observations since that date which are marked as H. have been made with the same instrument, and all those marked R. since the date given above were made with the 11½-inch.
The date given in each of the catalogues is the date of the observation ; to many of the new stars the date of discovery is added. A table showing the value of the decimal date is given at the end.
H.C. RUSSELL.
Sydney Observatory,
24 March, 1882.
