A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.
We now commence a series of observations suited to the third quarter of
the year, and to the following hours:--Ten o'clock on the 22nd of July;
nine on the 8th of August; eight on the 23rd of August; seven on the 8th
of October; and intermediate hours on days intermediate to these.
We look first for the Great Bear towards the north-west, and thence find
the Pole-star. Turning towards the north we see Capella and [beta]
Aurigæ low down and slightly towards the left of the exact north point.
The Milky Way crosses the horizon towards the north-north-east and
passes to the opposite point of the compass, attaining its highest point
above the horizon towards east-south-east. This part of the Milky Way is
well worth observing, being marked by singular variations of brilliancy.
Near Arided (the principal star of Cygnus, and now lying due east--some
twenty-five degrees from the zenith) there is seen a straight dark rift,
and near this space is another larger cavity, which has been termed the
northern Coal-sack. The space between [gamma], [delta], and [beta] Cygni
is covered by a large oval mass, exceedingly rich and brilliant. The
neighbouring branch, extending from [epsilon] Cygni, is far less
conspicuous here, but near Sagitta becomes brighter than the other,
which in this neighbourhood suddenly loses its brilliancy and fading
gradually beyond this point becomes invisible near [beta] Ophiuchi.
The continuous stream becomes patchy--in parts very brilliant--where it
crosses Aquila and Clypeus. In this neighbourhood the other stream
reappears, passing over a region very rich in stars. We see now the
greatest extent of the Milky Way, towards this part of its length, ever
visible in our latitudes--just as in spring we see its greatest extent
towards Monoceros and Argo.
[Illustration: PLATE IV.]
I may note here in passing that Sir John Herschel's delineation of the
northern portion of the Milky Way, though a great improvement on the
views given in former works, seems to require revision, and especially
as respects the very remarkable patches and streaks which characterise
the portion extending over Cepheus and Cygnus. It seems to me, also,
that the evidence on which it has been urged that the stars composing
the Milky Way are (on an average) comparable in magnitude to our own
sun, or to stars of the leading magnitudes, is imperfect. I believe, for
instance, that the brilliant oval of milky light in Cygnus comes from
stars intimately associated with the leading stars in that
constellation, and not far removed in space (proportionately) beyond
them. Of course, if this be the case, the stars, whose combined light
forms the patch of milky light, must be far smaller than the leading
brilliants of Cygnus. However, this is not the place to enter on
speculations of this sort; I return therefore to the business we have
more immediately in hand.
Towards the east is the square of Pegasus low down towards the horizon.
Towards the south is Scorpio, distinguished by the red and brilliant
Antares, and by a train of conspicuous stars. Towards the west is
Bootes, his leading brilliant--the ruddy Arcturus--lying somewhat nearer
the horizon than the zenith, and slightly south of west. Bootes as a
constellation is easily found if we remember that he is delineated as
chasing away the Greater Bear. Thus at present he is seen in a slightly
inclined position, his head (marked by the third-magnitude star [beta])
lying due west, some thirty degrees from the zenith. It has always
appeared to me, by the way, that Bootes originally had nobler
proportions than astronomers now assign to him. It is known that Canes
Venatici now occupy the place of an upraised arm of Bootes, and I
imagine that Corona Borealis, though undoubtedly a very ancient
constellation, occupies the place of his other arm. Giving to the
constellation the extent thus implied, it exhibits (better than most
constellations) the character assigned to it. One can readily picture to
oneself the figure of a Herdsman with upraised arms driving Ursa Major
before him. This view is confirmed, I think, by the fact that the Arabs
called this constellation the Vociferator.
Bootes contains many beautiful objects. Partly on this account, and
partly because this is a constellation with which the observer should be
specially familiar, a map of it is given in Plate 4.
Arcturus has a distant pale lilac companion, and is in other respects a
remarkable and interesting object. It is of a ruddy yellow colour.
Schmidt, indeed, considers that the star has changed colour of late
years, and that whereas it was once very red it is now a yellow star.
This opinion does not seem well grounded, however. The star _may_ have
been more ruddy once than now, though no other observer has noticed such
a peculiarity; but it is certainly not a pure yellow star at present (at
any rate as seen in our latitude). Owing probably to the difference of
colour between Vega, Capella and Arcturus, photometricians have not been
perfectly agreed as to the relative brilliancy of these objects. Some
consider Vega the most brilliant star in the northern heavens, while
others assign the superiority to Capella. The majority, however,
consider Arcturus the leading northern brilliant, and in the whole
heavens place three only before him, viz., Sirius, Canopus, and [alpha]
Centauri. Arcturus is remarkable in other respects. His proper motion is
very considerable, so great in fact that since the time of Ptolemy the
southerly motion (alone) of Arcturus has carried him over a space nearly
half as great again as the moon's apparent diameter. One might expect
that so brilliant a star, apparently travelling at a rate so great
compared with the average proper motions of the stars, must be
comparatively near to us. This, however, has not been found to be the
case. Arcturus is, indeed, one of the stars whose distance it has been
found possible to estimate roughly. But he is found to be some three
times as far from us as the small star 61 Cygni, and more than seven
times as far from us as [alpha] Centauri.
The star [delta] Bootis is a wide and unequal double, the smaller
component being only of the ninth magnitude.
Above Alkaid the last star in the tail of the Greater Bear, there will
be noticed three small stars. These are [theta], [iota], and [kappa]
Bootis, and are usually placed in star-maps near the upraised hand of
the Herdsman. The two which lie next to Alkaid, [iota] and [kappa], are
interesting doubles. The former is a wide double (see Plate 5), the
magnitudes of components 4 and 8, their colours yellow and white. The
larger star of this pair is itself double. The star [kappa] Bootis is
not so wide a double (see Plate 5), the magnitudes of the components 5
and 8, their colours white and faint blue--a beautiful object.
The star [xi] Bootis is an exceedingly interesting object. It is
double, the colours of the components being orange-yellow and ruddy
purple, their magnitudes 3-1/2 and 6-1/2. When this star was first
observed by Herschel in 1780 the position of the components was quite
different from that presented in Plate 5. They were also much closer,
being separated by a distance of less than 3-1/2 seconds. Since that
time the smaller component has traversed nearly a full quadrant, its
distance from its primary first increasing, till in 1831 the stars were
nearly 7-1/2 seconds apart, and thence slowly diminishing, so that at
present the stars are less than 5 seconds apart. The period usually
assigned to the revolution of this binary system is 117 years, and the
period of peri-astral passage is said to be 1779. It appears to me,
however, that the period should be about 108 years, the epoch of last
peri-astral passage 1777 and of next peri-astral passage, therefore,
1885. The angular motion of the secondary round the primary is now
rapidly increasing, and the distance between the components is rapidly
diminishing, so that in a few years a powerful telescope will be
required to separate the pair.
Not far from [xi] is [pi] Bootis, represented in Plate 5 as a somewhat
closer double, but in reality--now at any rate--a slightly wider pair,
since the distance between the components of [xi] has greatly diminished
of late. Both the components of [pi] are white, and their magnitudes are
3-1/2 and 6.
We shall next turn to an exceedingly beautiful and delicate object, the
double star [epsilon] Bootis, known also as Mirac and, on account of its
extreme beauty, called Pulcherrima by Admiral Smyth. The components of
this beautiful double are of the third and seventh magnitude, the
primary orange, the secondary sea-green. The distance separating the
components is about 3 seconds, perhaps more; it appears to have been
slowly increasing during the past ten or twelve years. Smyth assigns to
this system a period of revolution of 980 years, but there can be little
doubt that the true period is largely in excess of this estimate.
Observers in southern latitudes consider that the colours of the
components are yellow and blue, not orange and green as most of our
northern observers have estimated them.
A little beyond the lower left-hand corner of the map is the star
[delta] Serpentis, in the position shown in the Frontispiece, Map 3.
This is the star which at the hour and season depicted in Map 2 formed
the uppermost of a vertical row of stars, which has now assumed an
almost horizontal position. The components of [delta] Serpentis are
about 3-1/2 seconds apart, their magnitudes 3 and 5, both white.
The stars [theta]^{1} and [theta]^{2} Serpentis form a wide double, the
distance between the components being 21-1/2 seconds. They are nearly
equal in magnitude, the primary being 4-1/2, the secondary 5. Both are
yellow, the primary being of a paler yellow colour than the smaller
star. But the observer may not know where to look for [theta] Serpentis,
since it falls in a part of the constellation quite separated from that
part in which [delta] Serpentis lies. In fact [theta] lies on the
extreme easterly verge of the eastern half of the constellation. It is
to be looked for at about the same elevation as the brilliant Altair,
and (as to azimuth) about midway between Altair and the south.
The stars [alpha]^{1} and [alpha]^{2} Libræ form a wide double, perhaps
just separable by the naked eye in very favourable weather. The larger
component is of the third, the smaller of the sixth magnitude, the
former yellow the latter light grey.
The star [beta] Libræ is a beautiful light-green star to the naked eye;
in the telescope a wide double, pale emerald and light blue.
In Scorpio there are several very beautiful objects:--
The star Antares or Cor Scorpionis is one of the most beautiful of the
red stars. It has been termed the Sirius of red stars, a term better
merited perhaps by Aldebaran, save for this that, in our latitude,
Antares is, like Sirius, always seen as a brilliant "scintillator"
(because always low down), whereas Aldebaran rises high above the
horizon. Antares is a double star, its companion being a minute green
star. In southern latitudes the companion of Antares may be seen with a
good 4-inch, but in our latitudes a larger opening is wanted. Mr. Dawes
once saw the companion of Antares shining alone for seven seconds, the
primary being hidden by the moon. He found that the colour of the
secondary is not merely the effect of contrast, but that this small star
is really a green sun.
The star [beta] Scorpionis is a fine double, the components 13"·1 apart,
their magnitudes 2 and 5-1/2, colours white and lilac. It has been
supposed that this pair is only an optical double, but a long time must
elapse before a decisive opinion can be pronounced on such a point.
The star [sigma] Scorpionis is a wider but much more difficult double,
the smaller component being below the 9th magnitude. The colour of the
primary (4) is white, that of the secondary maroon.
The star [xi] Scorpionis is a neat double, the components 7"·2 apart,
their magnitudes 4-1/2 and 7-1/2, their colours white and grey. This
star is really triple, a fifth-magnitude star lying close to the
primary.
In Ophiuchus, a constellation covering a wide space immediately above
Scorpio, there are several fine doubles. Among others--
39 Ophiuchi, distance between components 12"·1, their magnitudes 5-1/2
and 7-1/2, their colours orange and blue.
The star 70 Ophiuchi, a fourth-magnitude star on the right shoulder of
Ophiuchus, is a noted double. The distance between the components about
5-1/2", their magnitudes 4-1/2 and 7, the colours yellow and red. The
pair form a system whose period of revolution is about 95 years.
36 Ophiuchi (variable), distance 5"·2, magnitudes 4-1/2 and 6-1/2,
colours red and yellow.
[rho] Opiuchi, distance 4", colours yellow and blue, magnitudes 5 and 7.
Between [alpha] and [beta] Scorpionis the fine nebula 80 M may be looked
for. (Or more closely thus:--below [beta] is the wide Double [omega]^{1}
and [omega]^{2} Scorpionis; about as far to the right of Antares is the
star [sigma] Scorpionis, and immediately above this is the
fifth-magnitude star 19.) The nebula we seek lies between 19 and
[omega], nearer to 19 (about two-fifths of the way towards [omega]).
This nebula is described by Sir W. Herschel as "the richest and most
condensed mass of stars which the firmament offers to the contemplation
of astronomers."
There are two other objects conveniently situated for observation, which
the observer may now turn to. The first is the great cluster in the
sword-hand of Perseus (see Plate 4), now lying about 28° above the
horizon between N.E. and N.N.E. The stars [gamma] and [delta] Cassiopeiæ
(see Map 3 of Frontispiece) point towards this cluster, which is rather
farther from [delta] than [delta] from [gamma], and a little south of
the produced line from these stars. The cluster is well seen with the
naked eye, even in nearly full moonlight. In a telescope of moderate
power this cluster is a magnificent object, and no telescope has yet
revealed its full glory. The view in Plate 5 gives but the faintest
conception of the glories of [chi] Persei. Sir W. Herschel tried in
vain to gauge the depths of this cluster with his most powerful
telescope. He spoke of the most distant parts as sending light to us
which must have started 4000 or 5000 years ago. But it appears
improbable that the cluster has in reality so enormous a longitudinal
extension compared with its transverse section as this view would imply.
On the contrary, I think we may gather from the appearance of this
cluster, that stars are far less uniform in size than has been commonly
supposed, and that the mere irresolvability of a cluster is no proof of
excessive distance. It is unlikely that the faintest component of the
cluster is farther off than the brightest (a seventh-magnitude star) in
the proportion of more than about 20 to 19, while the ordinary estimate
of star magnitudes, applied by Herschel, gave a proportion of 20 or 30
to 1 at least. I can no more believe that the components of this cluster
are stars greatly varying in distance, but accidentally seen in nearly
the same direction, (or that they form an _enormously long system_
turned by accident directly towards the earth), than I could look on the
association of several thousand persons in the form of a procession as a
fortuitous arrangement.
Next there is the great nebula in Andromeda--known as "the
transcendantly beautiful queen of the nebulæ." It will not be difficult
to find this object. The stars [epsilon] and [delta] Cassiopeiæ (Map 3,
Frontispiece) point to the star [beta] Andromedæ. Almost in a vertical
line above this star are two fourth-magnitude stars [mu] and [gamma],
and close above [nu], a little to the right, is the object we
seek--visible to the naked eye as a faint misty spot. To tell the truth,
the transcendantly beautiful queen of the nebulæ is rather a
disappointing object in an ordinary telescope. There is seen a long
oval or lenticular spot of light, very bright near the centre,
especially with low powers. But there is a want of the interest
attaching to the strange figure of the Great Orion nebula. The Andromeda
nebula has been partially resolved by Lord Rosse's great reflector, and
(it is said) more satisfactorily by the great refractor of Harvard
College. In the spectroscope, Mr. Huggins informs us, the spectrum is
peculiar. Continuous from the blue to the orange, the light there
"appears to cease very abruptly;" there is no indication of gaseity.
Lastly, the observer may turn to the pair Mizar and Alcor, the former
the middle star in the Great Bear's tail, the latter 15' off. It seems
quite clear, by the way, that Alcor has increased in brilliancy of late,
since among the Arabians it was considered an evidence of very good
eyesight to detect Alcor, whereas this star may now be easily seen even
in nearly full moonlight. Mizar is a double star, and a fourth star is
seen in the same field of view with the others (see Plate 5). The
distance between Mizar and its companion is 14"·4; the magnitude of
Mizar 3, of the companion 5; their colours white and pale green,
respectively.