William F. Denning.

Telescopic Work for Starlight Evenings online

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| | | is wider. |
|25.|70 Ophiuchi |18 0·0|+ 2 32 | 4 6 | 348·7 | 2·16 |1889·3| β. |
| | | Binary. Period nearly 88 years (Gore). Good |
| | | object for a 3-inch. Yellow and purple. |
|26.|ι Leonis |11 18·2|+11 8 | 4½ 7½ | 62·0 | 2·56 |1889·2| L. |
| | | Binary; but distance shows little variation since|
| | | 1839. Yellowish and blue. |
|27.|ε Boötis |14 40·2|+27 32 | 3 5½ | 328·1 | 2·88 |1885·4| T. |
| | | A very interesting object, and visible in a small|
| | | instrument. |
|28.|α Scorpii |16 22·7|-26 11 | 1 8 | 271·7 | 2·92 |1880·0| β. |
| | | This pair forms an atmospheric rather than an |
| | | optical test. |
|29.|γ Ceti | 2 37·6|+ 2 46 | 3 7 | 289·7 | 2·94 |1883·9| P. |
| | | A binary system. Test for a 2½-inch. |
| | | Yellow and blue. |
|30.|α Piscium | 1 56·3|+ 2 14 | 5 6 | 321·9 | 3·03 |1886·9| T. |
| | | A probable binary, but since 1831 not much change|
| | | in position or distance. |
|31.|ζ Aquarii |22 23·1|- 0 35 | 4 4 | 325·8 | 3·08 |1889·9| L. |
| | | A fine binary, with very long period. 1625 years |
| | | (Doberck). |
|32.|ε^1 Lyræ |18 40·7|+39 34 | 4½ 6½ | 15·3 | 3·24 |1877·4|Dob- |
| | | | | | | | |erck.|
|33.|ε^2 Lyræ |18 40·7|+39 30 | 5 5 | 137·6 | 2·45 |1877·4| Hall|
| | | These stars form a wide double (distance 3′ 27″),|
| | | just separable by the naked eye. A 2½-inch |
| | | shows a fine double-double. A 4-inch reveals |
| | | three faint stare between. |
|34.|ε Hydræ | 8 41·0|+ 6 49 | 4 7 | 226·5 | 3·16 |1889·1| β. |
| | | A new _comes_, Pos. 154°·4; Dist. 0″·26; Mag. 6, |
| | | 1889; 36-inch, power 3300! β. |
|35.|γ Leonis, A.B. |10 13·9|+20 24 | 2 4 | 114·6 | 3·51 |1889·3| β. |
| | | A fine binary. Period 407 years (Doberck). |
| | | Readily seen in a 3-inch. |
|36.|δ Serpentis |15 29·6|+10 55 | 3 5 | 189·9 | 3·52 |1886·6|Ball.|
| | | Probably binary. Fine object in small instruments.|
|37.|α Canis Maj. | 6 40·3|-16 34 | 1 10 | 359·7 | 4·19 |1890·3| β. |
| | | Brilliant binary. Period 58·5 years (Gore). |
| | | Colours white and yellow. |
|38.|α Herculis |17 9·6|+14 31 | 3 4½ | 114·5 | 4·58 |1885·5| T. |
| | | A splendid object. Orange and bluish green. |
|39.|ζ Cassiopeiæ | 0 42·4|+57 14 | 4 8 | 184·7 | 4·76 |1888·3| M. |
| | | Binary. Period 195 years (Gruber). Difficult |
| | | object for 2-1/4-inch (Johnson). |
|40.|γ Virginis |12 36·1|- 0 51 | 3 3 | 153·9 | 5·45 |1889·3| L. |
| | | Well-known binary. Period 182 years |
| | | (J. Herschel). Single in 1836. |
|41.|α Geminorum | 7 27·6|+32 8 | 2 3 | 229·4 | 5·68 |1889·2| L. |
| | | Very fine object. Binary; Period doubtful |
| | | (Mädler 232 years, Doberck 1001 years). |
|42.|π Boötis |14 35·6|+16 54 | 4 6 | 104·3 | 6·04 |1885·4| T. |
| | | This pair has exhibited little change in pos. or |
| | | dist. since 1781. |
|43.|α^2 Capricorni,|20 11·9|-12 53 | 3 15 | 149·7 | 6·30 |1879·7| β. |
| | A.B. | Good light-test for 6-inches. Companion double; |
| | | pos. 240°, dist. 1″·5. |
|44.|δ Geminorum | 7 13·5|+22 11 | 3½ 9 | 207·2 | 6·98 |1886·1| T. |
| | | Rather wide pair of unequal mags. Difficult with |
| | | small apertures. |
|45.|γ Arietis | 1 47·5|+18 45 | 4½ 5 | 178·3 | 8·78 |1886·9| T. |
| | | A fine, easy object. Discovered in 1664 by Hooke.|
|46.|ι Ursæ Maj. | 8 51·7|+48 28 | 3 12 | 356·7 | 9·56 |1883·4| E. |
| | | Well seen in a 4-inch, powers 80 and 130. |
| | | Good light-test. |
|47.|β Orionis | 5 9·3|- 8 20 | 1 9 | 202·0 | 9·61 |1887·2| T. |
| | | A fine object for small instruments. Visible in a|
| | | 2-inch refractor. |
|48.|γ^1 Andromedæ | 1 57·1|+41 48 | 3 6 | 62·6 |10·50 |1876·0|Hall.|
| | | A splendid pair, stationary in relative positions|
| | | (see no. 4). |
|49.|γ Delphini |20 41·6|+15 44 | 4 6 | 271·2 | 11·35|1879·7|Hall.|
| | | Estimates of the colour of this pair differ, and |
| | | change is inferred. |
|50.|σ Orionis, A.D.| 5 33·2|- 2 40 | 4 10½| 236·8 | 11·62|1875·2| |
| | | Multiple. Fine group here. Schröter saw 12 stars,|
| | | Struve 18. |
|51.|β Scorpii |15 59·0|-19 30 | 2 5½| 26·7 | 12·72|1879·7| β. |
| | | The brighter star is a close double; Pos. 87°, |
| | | Dist. 0″·73 (Burnham). |
|52.|ζ Ursæ Maj. |13 19·5| +55 30| 2 4 | 150·5 | 14·38|1886·2| T. |
| | | Fine object for small instruments. Other stars in|
| | | the field. |
|53.|α Centauri |14 32·1| -60 23| 1 2 | 202·9 | 17·12|1888·6| S. |
| | | A fine southern binary with Period of 80·3 years |
| | | (Elkin). |
|54.|α Ursæ Min. | 1 18·5| +88 43| 2 9 | 210·1 | 18·60| | |
| | | Good test for a 2-inch. Dawes saw it with |
| | | 1-3/10-inch, Ward with 1-1/4 inch. |
|55.|61 Cygni |21 2·0| +38 12| 5 6 | 121·0 | 20·58|1887·7| S. |
| | | Probably a binary of long period (782½ years, |
| | | Peters; 1159 years, Mann). |
|56.|33 Arietis | 2 34·3| +26 35| 5 8 | 0·3 | 29·76|1879·7| β. |
| | | A distant and easy pair in small instruments. |
|57.|β Cygni |19 26·3| +27 44| 3 7 | 55·1 | 34·32|1879·7| β. |
| | | A beautiful pair, colours golden yellow and |
| | | smalt blue. |
|58.|β Geminorum | 7 38·6| +28 18| 2 14 | 274·9 | 43·00|1877·9| β. |
| | | Disc. by Burnham, who also finds the companion |
| | | double; dist. 1″·4 (1879·2). |
|59.|α′ Capricorni |20 11·9| -12 53| | 219·7 | 44·55|1879·7| β. |
| | | α^1 and α^2 Capricorni (No. 43) form a naked-eye |
| | | double; Pos. 291°, Dist. 373″·4. |
|60.|α Canis Min. | 7 33·6| + 5 30| 1 14 | 317·3 | 44·62|1877·9| β. |
| | | Difficult object; just seen steadily by Dawes |
| | | with 8-1/4-inch refractor. |
|61.|β Lyræ, A.B. |18 46·0| +33 14| 3 7 | 148·9 | 45·20|1886·9| T. |
| | | There are three other faint and distant |
| | | components. | |
|62.|α Lyræ |18 33·2| +38 41| 1 11 | 156·1 | 48·00|1879·7| β. |
| | | Good light-test for a 3-inch. There are other |
| | | more distant companions. |
|63.|α Cassiopeiæ | 0 34·3| +55 56| 2 13½| 280·2 | 61·33|1879·7| β. |
| | | The 36-inch refractor shows a very faint _comes_;|
| | | Dist. 17″·5 (Burnham). |
|64.|α Canis Maj., | 6 40·3| -16 34| 1 13 | 114·9 | 71·39|1877·5|Hall.|
| | A.C. | This faint and distant companion to Sirius was |
| | | disc. by Marth. |
|65.|α Andromedæ | 0 2·7| +28 29| 2 11 | 271·6 | 71·60|1878·6| G. |
| | | A wide double, visible in a 3-inch, but _comes_ |
| | | very faint. |
|66.|α Tauri | 4 29·6| +16 17| 1 12 | 34·1 |114·96|1879·7| β. |
| | | Good light-test for a 3-inch. Very faint _comes_ |
| | | Pos. 109°; Dis. 30″·4 (Burnham). |
+——-+——————————————-+——————-+——————-+——————-+——————-+——————+——————+—————+

The determination of the angles of position and distance of double
stars forms a very important and extensive branch of work in connection
with sidereal astronomy. In cases where double stars preserve
stationary places relatively to each other, there is clearly no need
for frequent re-observation. But in those numerous instances where the
two components form a binary system it is desirable to obtain as many
measures as possible, so as either to verify the calculated orbit or
to furnish the materials for an orbit if one has not been computed
before. Dr. Doberck, whose name is well known in these researches,
mentioned, in 1882, that ample data for purposes of computation had
not been secured for more than thirty or forty binaries out of between
five and six hundred such systems that were probably known to exist.
Sir W. Herschel, in 1803, estimated the period of revolution of α
Geminorum as 342 yrs. 2 mths. and of γ Virginis as 1200 yrs. Orbits[51]
do not appear, however, to have been computed until 1827, when Savery
of Paris showed that the companion of ξ Ursæ Majoris was revolving in
an ellipse with a period of 58-1/4 years. The accomplished Encke also
turned his attention to this work, and adopted a more elaborate method;
and many others have pursued the subject with very interesting and
valuable results. On pp. 302-305 is a selected list of some of the most
noteworthy double and binary stars, arranged according to the distance
between the components.

In compiling the above list, I have used some of the latest measures
available, as most of these doubles are binary systems, and therefore
in motion, so that a few years effect a perceptible difference in the
angles of position and distance of the components. Some of the pairs
are closing up, others are opening, and thus it happens that a binary
star, divided with great difficulty to-day, may become an easy object
some years hence, and _vice versâ_. In fact, as telescopic tests they
are constantly varying.

Before leaving this part of the subject it may be interesting to refer
individually to a few brilliant examples of double stars.

α _Canis Majoris_ (_Sirius_). A red star according to ancient records,
but it is now intensely white. In 1844 Bessel inferred from certain
little irregularities in the proper motion of this star that it
consisted of a binary system with a period of about half a century[52].
Peters confirmed this idea in 1851, and it was observationally verified
eleven years afterwards. On Jan. 31, 1862, Alvan Clark, jun., while
testing a new 18½-inch refractor, discovered a very faint companion
10″ distant. Measures in the few subsequent years proved that the
position-angle was decreasing, while the distance showed a slight
extension. In 1872 it was about 11″·50, but since then the two stars
have been approaching each other, and Mr. Burnham’s measures in April
1890 gave the distance as only 4″·19. It is now, therefore, a very
difficult object, and only visible in large instruments. In England it
is never easy, owing to its southern position, and it has been little
observed, but it is satisfactory to note that the large refractors at
Washington, Princeton, and Chicago, U.S.A., have been often employed
on this object in recent years. Mann gives a period of 51·22 years for
this interesting binary, and places the time of periastron-passage as
1890·55. This differs from Gore’s orbit, quoted in the table.

β _Orionis_ (_Rigel_). A favourite test-object for small instruments.
The companion has been seen with only 1½-inch aperture by experienced
observers familiar with the object, and accustomed to its appearance
in larger telescopes. The beginner may, however, esteem himself
fortunate if he distinguishes the smaller star with 3 inches of
aperture. When he has done this he may afterwards succeed with 2½
inches only, and quite possibly with 2 inches. He can ascertain his
ability in this direction by inserting cardboard diaphragms of the
diameters referred to in the dew-cap of his telescope. This object
is not a binary; the component stars are fixed relatively to each
other, and merely form an optical double. The colours are pale yellow
and sapphire blue. Burnham thought the smaller star was elongated,
as though a very close double, but the 36-inch at Mount Hamilton has
disproved the idea.

α _Lyræ_ (_Vega_). Another well-known object, and one upon which
amateurs are constantly testing their means. The companion star is
extremely faint, and small instruments would have no chance with it
but for its comparatively wide distance from Vega. Were it much nearer
it would be obliterated in the glare. This is a more difficult pair
than that of Rigel, though certain lynx-eyed observers have glimpsed
the minute star with ridiculously small apertures. It is no mean feat,
however, to discern the star with a 3-inch telescope. Webb saw it
more easily with a power of 80 than with 144 on a 3-7/10-inch. There
are many other stars in the same field, though more distant than the
companion alluded to. With power 60 on my 10-inch reflector, I counted
eighteen stars in the field with Vega on Oct. 9, 1889, though the full
Moon was shining at the time. Several faint stars have been alleged to
exist much closer to Vega than the well-known _comes_; but these have
resisted the great American refractors, and it may be safely assumed
that they were ghosts produced by a faulty image.

α _Ursæ Minoris_ (_Polaris_). This double, from its constant visibility
in northern latitudes, from its unvarying brightness, and from the
relatively stationary positions of the stars composing it, forms
an excellent test for small instruments. But it is a comparatively
easy object, and ought to be seen in a 2-inch telescope. With this
aperture the primitive efforts of a young observer will probably be
disappointing. If possible he should first look at the pair through a
3-or 4-inch, and then he will know exactly what he may expect to see
with inferior means. A difficult object is often readily glimpsed in a
small telescope after the eye has become acquainted with it in a larger
one. Experience of this kind is very requisite, and it is by thus
educating the eye that observers are gradually enabled to reach objects
which appeared hopelessly beyond them at their first attempts. The
companion to Polaris, like that of Rigel and Vega, though situated in
nearly the same line of sight is not physically related to the larger
star, the contiguity of the objects being accidental. Some dubious
observations have been made of _comites_ nearer to Polaris than the one
to which we have been adverting; but Burnham does not see these, and we
are forced to conclude that they have no objective existence.

α _Scorpii_ (_Antares_). A fiery-red star, with a rather close, faint
companion. This object being in 26° of S. declination is rarely
seen with advantage in places with latitudes far north. Atmospheric
disturbance usually affects the image in such degree that the smaller
star is merged in the contortions of the larger. This pair is, however,
interesting from the circumstance that it is frequently liable to
occultation by the Moon. A night on which this double star can be
distinctly seen may be regarded as an exceptional one in point of
definition. It appears to have been discovered nearly half a century
ago by Grant and Mitchel.

_Variable Stars._—A proportion of the stars exhibit fluctuations in
their visible brightness. In most cases, however, the variation is
but slight, though there are instances in which the differences are
considerable. The fluctuations are periodical in nature and capable
of being exactly determined. But the character of the variation and
the period are very dissimilar in different stars. Some are of short
period, and their variations occupy a few days only; others, however,
are more gradual, and twelve months or more may represent the complete
cycle of their changes. These alterations of brightness generally
escape the notice of casual observers of the heavens. To them the
stars appear as constant in their relative magnitudes as they are in
their relative positions. But a close observer of the firmament, who
habitually watches and records the comparative lustre of the stars,
must soon discover numerous evidences of change. He will remark certain
stars which alternately grow bright and faint, and, in fact, display
a regular oscillation of brilliancy. In the case of a pair of stars
he may possibly notice that the superior lustre is emitted first by
one and then by the other. The observation of these variables dates
from a period anterior to the invention of the telescope. Nearly three
centuries ago Fabricius remarked that ο Ceti (Mira) suffered
a great diminution of light; for while it was of the 3rd mag. in Aug.
1596, it became invisible in the following autumn. It was re-observed
by Holwarda in 1639, and as he appears to have been the first to
estimate its period, some authors, including Argelander, have credited
him with the discovery. The star has a period of about 331·3 days. Its
variations are somewhat erratic, for at max. it is sometimes only 4th
mag., while at others it is as bright as 2nd mag., and its min. are
equally inconsistent.

β Persei (Algol) is another and perhaps the best known of all the
variable stars. Its changes are very rapid, for it passes through its
various gradations of brilliancy in less than three days. It was first
noticed by Montanari in 1669, though it was left for Goodricke in
1782 to ascertain its period. The normal mag. of the star is 2·2, and
it only shows distinct variation during the five hours which precede
and follow a minimum, when it declines to 3·7 mag. Its period is
shortening, for in 1782 it was 2^d 20^h 48^m 59^s·4, in 1842, 2^d 40^h
48^m 55^s·2, and at present Chandler finds it 2^d 20^h 48^m 51^s. As
to the causes which contribute to these variations, they are invested
in mystery. It has been conjectured that dark spots on the surfaces
of the stars may, by the effects of rotation, introduce the observed
alternations. Another surmise is that the temporary diminutions of
lustre are to be ascribed to the interposition of dark satellites, and
this theory seems tenable in regard to stars of the Algol type. It is
satisfactory to note that a large amount of systematic work is being
done in this important and delicate branch of research. Such stars as
are subject to variation have been classed as follows:—1. Temporary or
new stars; 2. Stars having long and pretty regular variation; 3. Stars
irregularly variable; 4. Stars varying in short periods; 5. Stars of
the type of Algol, which are liable to temporary diminutions of lustre.
On the preceding page is a list of the most noteworthy variable stars.

_List of Variable Stars._

+——————————————+——————————————————-+——————————————+——————————————————-+
| Name of Star.| Position, 1890. | Mags. | Period. |
| +————————-+————————-+ | |
| | R.A. | Dec. | | |
+——————————————+——————————————————-+——————————————+——————————————————-+
| | h m | ° ′ | | |
| μ Cephei | 0 52.5 | +81 17 | 7.2 9.4 | 2^d 11^h 50^m |
| ο Ceti | 2 13.8 | - 3 29 | 2 0 | 331-1/3 days |
| β Persei | 3 1.0 | +40 32 | 2.2 3.7 | 2^d 20^h 49^m |
| λ Tauri | 3 54·6 | +12 11 | 3.4 4.2 | 3^d 22^h 52^m |
| U Orionis | 5 49·3 | +20 9 | 6 12½ | |
| ζ Geminorum | 6 8.2 | +22 32 | 3.2 4·2 | 135-151 days |
| ζ Geminorum | 6 57.6 | +20 44 | 3.7 4·5 | 10^d 3^h 43^m |
| L_{2} Puppis | 7 10·2 | -44 28 | 3.5 6.3 | 136 days |
| R Canis Maj. | 7 14.5 | -16 11 | 6.2 6.8 | 1^d 3^h 16^m |
| U Geminorum | 7 48.6 | +22 18 | 9 14 | 71-126 days |
| S Cancri | 8 37.7 | +19 26 | 8.2 11·7 | 9^d 11^h 38^m |
| ζ Argûs | 10 40.8 | -59 6 | 1 6 | 46 or 67 yrs.? |
| R Hydræ | 13 23.7 | -22 43 | 4 10 | 436 days |
| δ Libræ | 14 55.1 | -8 5 | 4.9 6·1 | 2^d 7^h 51^m |
| U Coronæ | 15 13.7 | +32 3 | 7.6 8·8 | 3^d 10^h 51^m |
| α Herculis | 17 9.6 | +14 31 | 3.1 3.9 | 88^d 12^h (irreg.)|
| U Ophiuchi | 17 11.0 | + 1 20 | 6 6.7 | 0^d 20^h 8^m |
| β Lyræ | 18 46.0 | +33 14 | 3.5 4.5 | 12^d 21^h 47^m |
| χ Cygni | 19 46·3 | +32 38 | 4-6.5 13 | 406 days |
| ζ Aquilæ | 19 46.9 | + 0 44 | 3.6 4.7 | 7^d 4^h 14^m |
| Y Cygni | 20 47.7 | +34 15 | 7.1 7.9 | 1^d 11^h 57^m |
| μ Cephei | 21 40.1 | +58 16 | 3.6 4·8 | 432 days? |
| δ Cephei | 22 25.1 | +57 51 | 3.7 4.8 | 5^d 8^h 48^m |
+——————————————+——————————————————-+——————————————+——————————————————-+
+——————————————+——————————————————-+
| Name of Star.| Observer. |
| | |
+——————————————+——————————————————-+
| | |
| μ Cephei | Ceraski, 1880. |
| ο Ceti | Fabricius, 1596. |
| β Persei | Montanari, 1669. |
| λ Tauri | Baxendell, 1848. |
| U Orionis | Gore, 1885. |
| ζ Geminorum | Schmidt, 1865. |
| ζ Geminorum | Schmidt, 1847. |
| L_{2} Puppis | Gould, 1872. |
| R Canis Maj. | Sawyer, 1887. |
| U Geminorum | Hind, 1855. |
| S Cancri | Hind, 1848. |
| ζ Argûs | Burchell, 1827. |
| R Hydræ | Maraldi, 1704. |
| δ Libræ | Schmidt, 1859. |
| U Coronæ | Winnecke, 1869. |
| α Herculis | W. Herschel, 1795.|
| U Ophiuchi | Sawyer, 1881. |
| β Lyræ | Goodricke, 1784. |
| χ Cygni | Kirch, 1686. |
| ζ Aquilæ | Pigott, 1784. |
| Y Cygni | Chandler, 1886. |
| μ Cephei | Hind, 1848. |
| δ Cephei | Goodricke, 1784. |
+——————————————+——————————————————-+



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 26 of 32)