A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.
Our last half-hour with the double stars, &c., must be a short one, as
we have already nearly filled the space allotted to these objects. The
observations now to be made are supposed to take place during the fourth
quarter of the year,--at ten o'clock on October 23rd; or at nine on
November 7th; or at eight on November 22nd; or at seven on December 6th;
or at hours intermediate to these on intermediate days.
We look first, as in former cases, for the Great Bear, now lying low
down towards the north. Towards the north-east, a few degrees easterly,
are the twin-stars Castor and Pollux, in a vertical position, Castor
uppermost. Above these, a little towards the right, we see the brilliant
Capella; and between Capella and the zenith is seen the festoon of
Perseus. Cassiopeia lies near the zenith, towards the north, and the
Milky Way extends from the eastern horizon across the zenith to the
western horizon. Low down in the east is Orion, half risen above
horizon. Turning to the south, we see high up above the horizon the
square of Pegasus. Low down towards the south-south-west is Fomalhaut,
pointed to by [beta] and [alpha] Pegasi. Towards the west, about
half-way between the zenith and the horizon, is the noble cross in
Cygnus; below which, towards the left, we see Altair, and his companions
[beta] and [gamma] Aquilæ: while towards the right we see the brilliant
Vega.
During this half-hour we shall not confine ourselves to any particular
region of the heavens, but sweep the most conveniently situated
constellations.
[Illustration: PLATE V.]
First, however, we should recommend the observer to try and get a good
view of the great nebula in Andromeda, which is _not_ conveniently
situated for observation, but is so high that after a little trouble the
observer may expect a more distinct view than in the previous quarter.
He will see [beta] Andromedæ towards the south-east, about 18° from the
zenith, [mu] and [nu] nearly in a line towards the zenith, and the
nebula about half-way between [beta] and the zenith.
With a similar object it will be well to take another view of the great
cluster in Perseus, about 18° from the zenith towards the
east-north-east (_see_ the pointers [gamma] and [delta] Cassiopeiæ in
Map 4, Frontispiece), the cluster being between [delta] Cassiopeiæ and
[alpha] Persei.
Not very far off is the wonderful variable Algol, now due east, and
about 58° above the horizon. The variability of this celebrated object
was doubtless discovered in very ancient times, since the name Al-gol,
or "the Demon" seems to point to a knowledge of the peculiarity of this
"slowly winking eye." To Goodricke, however, is due the rediscovery of
Algol's variability. The period of variation is 2d. 20h. 48m.; during
2h. 14m. Algol appears of the second magnitude; the remaining 6-3/4
hours are occupied by the gradual decline of the star to the fourth
magnitude, and its equally gradual return to the second. It will be
found easy to watch the variations of this singular object, though, of
course, many of the minima are attained in the daytime. The following
may help the observer:--
On October 8th, 1867, at about half-past eleven in the evening, I
noticed that Algol had reached its minimum of brilliancy. Hence the next
minimum was attained at about a quarter-past eight on the evening of
October 11th; the next at about five on the evening of October 14th,
and so on. Now, if this process be carried on, it will be found that the
next evening minimum occurred at about 10h. (_circiter_) on the evening
of October 31st, the next at about 11h. 30m. on the evening of November
20th. Thus at whatever hour any minimum occurs, another occurs _six
weeks and a day later_, at about the same hour. This would be exact
enough if the period of variation were _exactly_ 2d. 20m. 48s., but the
period is nearly a minute greater, and as there are fifteen periods in
six weeks and a day, it results that there is a difference of about 13m.
in the time at which the successive recurrences at nearly the same hour
take place. Hence we are able to draw up the two following Tables, which
will suffice to give all the minima conveniently observable during the
next two years. Starting from a minimum at about 11h. 45m. on November
20th, 1867, and noticing that the next 43-day period (with the 13m.
added) gives us an observation at midnight on January 2nd, 1868, and
that successive periods would make the hour later yet, we take the
minimum next after that of January 2nd, viz. that of January 5th, 1868,
8h. 48m., and taking 43-day periods (with 13m. added to each), we get
the series--
h. m.
Jan. 5, 1868, 8 45 P.M.
Feb. 17, ----, 8 58 ----
Mar. 31, ----, 9 11 ----
May 13, ----, 9 24 ----
June 25, ----, 9 37 ----
Aug. 7, ----, 9 50 ----
Sept. 19, ----, 10 3 ----
Nov. 1 ----, 10 16 ----
Dec. 14, ----, 10 29 ----
Jan. 26, 1869, 10 42 ----
Mar. 10, ----, 10 25 ----
Mar. 13, ----, 7 43 ----[7]
Apr. 25, ----, 7 56 ----
June 7, ----, 8 9 ----
July 20, ----, 8 22 ----
Sept. 1, ----, 8 35 ----
Oct. 14, ----, 8 48 ----
Nov. 26, ----, 9 1 ----
Jan. 8, 1870, 9 14 ----
Feb. 20, ----, 9 27 ----
From the minimum at about 10 P.M. on October 31st, 1867, we get in like
manner the series--
h. m.
Dec. 13, 1867, 10 13 P.M.
Jan. 25, 1868, 10 26 ----
Mar. 8, ----, 10 39 ----
Apr. 20, ----, 10 52 ----
June 2, ----, 11 5 ----
June 5, ----, 7 53 ----[8]
July 18, ----, 8 6 ----
Aug. 30, ----, 8 19 ----
Oct. 12, ----, 8 32 ----
Nov. 24, ----, 8 45 ----
Jan. 6, 1869, 8 58 ----
Feb. 18, ----, 9 11 ----
Apr. 2, ----, 9 24 ----
May 15, ----, 9 37 ----
June 27, ----, 9 50 ----
Aug. 9, ----, 10 3 ----
Sept. 21, ----, 10 16 ----
Nov. 3, ----, 10 29 ----
Dec. 16, ----, 10 42 ----
Jan. 28, 1870, 10 55 ----
From one or other of these tables every observable minimum can be
obtained. Thus, suppose the observer wants to look for a minimum during
the last fortnight in August, 1868. The first table gives him no
information, the latter gives him a minimum at 8h. 19m. P.M. on August
30; hence of course there is a minimum at 11h. 31m. P.M. on August 27;
and there are no other conveniently observable minima during the
fortnight in question.
The cause of the remarkable variation in this star's brilliancy has been
assigned by some astronomers to the presence of an opaque secondary,
which transits Algol at regular intervals; others have adopted the view
that Algol is a luminous secondary, revolving around an opaque primary.
Of these views the former seems the most natural and satisfactory. It
points to a secondary whose mass bears a far greater proportion to that
of the primary, than the mass even of Jupiter bears to the sun; the
shortness of the period is also remarkable. It may be noticed that
observation points to a gradual diminution in the period of Algol's
variation, and the diminution seems to be proceeding more and more
rapidly. Hence (assuming the existence of a dark secondary) we must
suppose that either it travels in a resisting medium which is gradually
destroying its motion, or that there are other dependent orbs whose
attractions affect the period of this secondary. In the latter case the
decrease in the period will attain a limit and be followed by an
increase.
However, interesting as the subject may be, it is a digression from
telescopic work, to which we now return.
Within the confines of the second map in Plate 4 is seen the fine star
[gamma] Andromedæ. At the hour of our observations it lies high up
towards E.S.E. It is seen as a double star with very moderate telescopic
power, the distance between the components being upwards of 10"; their
magnitudes 3 and 5-1/2, their colours orange and green. Perhaps there is
no more interesting double visible with low powers. The smaller star is
again double in first-class telescopes, the components being yellow and
blue according to some observers, but according to others, both green.
Below [gamma] Andromedæ lie the stars [beta] and [gamma] Triangulorum,
[gamma] a fine naked-eye triple (the companions being [delta] and [eta]
Triangulorum), a fine object with a very low power. To the right is
[alpha] Triangulorum, certainly less brilliant than [beta]. Below
[alpha] are the three stars [alpha], [beta], and [gamma] Arietis, the
first an unequal and difficult double, the companion being purple, and
only just visible (under favourable circumstances) with a good 3-inch
telescope; the last an easy double, interesting as being the first ever
discovered (by Hook, in 1664), the colours of components white and grey.
Immediately below [alpha] Arietis is the star [alpha] Ceti, towards the
right of which (a little lower) is Mira, a wonderful variable. This star
has a period of 331-1/3 days; during a fortnight it appears as a star of
the 2nd magnitude,--on each side of this fortnight there is a period of
three months during one of which the star is increasing, while during
the other it is diminishing in brightness: during the remaining five
months of the period the star is invisible to the naked eye. There are
many peculiarities and changes in the variation of this star, into which
space will not permit me to enter.
Immediately above Mira is the star [alpha] Piscium at the knot of the
Fishes' connecting band. This is a fine double, the distance between the
components being about 3-1/2", their magnitudes 5 and 6, their colours
pale green and blue (see Plate 5).
Close to [gamma] Aquarii (see Frontispiece, Map 4), above and to the
left of it, is the interesting double [zeta] Aquarii; the distance
between the components is about 3-1/2", their magnitudes 4 and 4-1/2,
both whitish yellow. The period of this binary seems to be about 750
years.
Turning next towards the south-west we see the second-magnitude star
[epsilon] Pegasi, some 40° above the horizon. This star is a wide but
not easy double, the secondary being only of the ninth magnitude; its
colour is lilac, that of the primary being yellow.
Towards the right of [epsilon] Pegasi and lower down are seen the three
fourth-magnitude stars which mark the constellation Equuleus. Of these
the lowest is [alpha], to the right of which lies [epsilon] Equulei, a
fifth-magnitude star, really triple, but seen as a double star with
ordinary telescopes (Plate 5). The distance between the components is
nearly 11", their colours white and blue, their magnitudes 5-1/2 and
7-1/2. The primary is a very close double, which appears, however, to be
opening out rather rapidly.
Immediately below Equuleus are the stars [alpha]^{1} and [alpha]^2
Capricorni, seen as a naked-eye double to the right of and above [beta].
Both [alpha]^1 and [alpha]^2 are yellow; [alpha]^2 is of the 3rd,
[alpha]^1 of the 4th magnitude; in a good telescope five stars are seen,
the other three being blue, ash-coloured, and lilac. The star [beta]
Capricorni is also a wide double, the components yellow and blue, with
many telescopic companions.
To the right of Equuleus, towards the west-south-west is the
constellation Delphinus. The upper left-hand star of the rhombus of
stars forming the head of the Delphinus is the star [gamma] Delphini, a
rather easy double (see Plate 5), the components being nearly 12" apart,
their magnitudes 4 and 7, their colours golden yellow and flushed grey.
Turn we next to the charming double Albireo, on the beak of Cygnus,
about 36° above the horizon towards the west. The components are 34-1/2"
apart, their magnitudes 3 and 6, their colours orange-yellow, and blue.
It has been supposed (perhaps on insufficient evidence) that this star
is merely an optical double. It must always be remembered that a certain
proportion of stars (amongst those separated by so considerable a
distance) _must_ be optically combined only.
The star [chi] Cygni is a wide double (variable) star. The components
are separated by nearly 26", their magnitudes 5 and 9, their colours
yellow and light blue. [chi] may be found by noticing that there is a
cluster of small stars in the middle of the triangle formed by the stars
[gamma], [delta], and [beta] Cygni (see Map 4, Frontispiece), and that
[chi] is the nearest star _of the cluster_ to [beta]. The star [phi]
Cygni, which is just above and very close to [beta] (Albireo), does not
belong to the cluster. [chi] is about half as far again from [phi] as
[phi] from Albireo. But as [chi] descends to the 11th magnitude at its
minimum the observer must not always expect to find it very easily. It
has been known to be invisible at the epoch when it should have been
most conspicuous. The period of this variable is 406 days.
The star 61 Cygni is an interesting one. So far as observation has yet
extended, it would seem to be the nearest to us of all stars visible in
the northern hemisphere. It is a fine double, the components nearly
equal (5-1/2 and 6), both yellow, and nearly 19" apart. The period of
this binary appears to be about 540 years. To find 61 Cygni note that
[epsilon] and [delta] Cygni form the diameter of a semicircle divided
into two quadrants by [alpha] Cygni (Arided). On this semicircle, on
either side of [alpha], lie the stars [nu] and [alpha] Cygni, [nu]
towards [epsilon]. Now a line from [alpha] to [nu] produced passes very
near to 61 Cygni at a distance from [nu] somewhat greater than half the
distance of [nu] from [alpha].
The star [mu] Cygni lies in a corner of the constellation, rather
farther from [zeta] than [zeta] from [epsilon] Cygni. A line from
[epsilon] to [zeta] produced meets [kappa] Pegasi, a fourth-magnitude
star; and [mu] Cygni, a fifth-magnitude star, lies close above [kappa]
Pegasi. The distance between the components is about 5-1/2", their
magnitudes 5 and 6, their colours white and pale blue.
The star [psi] Cygni may next be looked for, but for this a good map of
Cygnus will be wanted, as [psi] is not pointed to by any well-marked
stars. A line from [alpha], parallel to the line joining [gamma] and
[delta], and about one-third longer than that line, would about mark the
position of [psi] Cygni. The distance between the components of this
double is about 3-1/2", their magnitudes 5-1/2 and 8, their colours
white and lilac.
Lastly, the observer may turn to the stars [gamma]_{1} and [gamma]_{2}
Draconis towards the north-west about 40° above the horizon (they are
included in the second map of Plate 2). They form a wide double, having
equal (fifth-magnitude) components, both grey. (See Plate 5.)