known as the _zodiacal light_ consists of a very faint luminosity, which
may be seen rising from the western horizon after twilight on any clear
winter or spring evening, also from the eastern horizon just before
daybreak in the summer or autumn. It extends out on each side of the
sun, and lies nearly in the plane of the ecliptic. It grows fainter the
farther it is from the sun, and can generally be traced to about ninety
degrees from that luminary, when it gradually fades away. In a very
clear, tropical atmosphere, it has been traced all the way across the
heavens from east to west, thus forming a complete ring. The general
appearance of this column of light, as seen in the morning, in the
latitude of Europe, is shown in Fig. 360.
[Illustration: Fig. 360.]
Taking all these appearances together, they indicate that it is due to a
lens-shaped appendage surrounding the sun, and extending a little beyond
the earth's orbit. It lies nearly in the plane of the ecliptic; but its
exact position is not easily determined. Fig. 361 shows the general form
and position of this solar appendage, as seen in the west.
[Illustration: Fig. 361.]
320. _The Visibility of the Zodiacal Light._--The reason why the
zodiacal light is more favorably seen in the evening during the winter
and spring than in the summer and fall is evident from Fig. 362, which
shows the position of the ecliptic and the zodiacal light with reference
to the western horizon at the time of sunset in March and in September.
It will be seen that in September the axis of the light forms a small
angle with the horizon, so that the phenomenon is visible only a short
time after sunset and low down where it is difficult to distinguish it
from the glimmer of the twilight; while in March, its axis being nearly
perpendicular to the horizon, the light may be observed for some hours
after sunset and well up in the sky. Fig. 363 gives the position of the
ecliptic and of the zodiacal light with reference to the eastern horizon
at the time of sunrise, and shows why the zodiacal light is seen to
better advantage in the morning during the summer and fall than during
the winter and spring. It will be observed that here the angle made by
the axis of the light with the horizon is small in March, while it is
large in September; the conditions represented in the preceding figure
being thus reversed.
[Illustration: Fig. 362.]
[Illustration: Fig. 363.]
321. _Nature of the Zodiacal Light._--Various attempts have been made to
explain the phenomena of the zodiacal light; but the most probable
theory is, that it is due to an immense number of meteors which are
revolving around the sun, and which lie mostly within the earth's orbit.
Each of these meteors reflects a sensible portion of sunlight, but is
far too small to be separately visible. All of these meteors together
would, by their combined reflection, produce a kind of pale, diffused
light.
III. THE STELLAR UNIVERSE.
I. GENERAL ASPECT OF THE HEAVENS.
322. _The Magnitude of the Stars._--The stars that are visible to the
naked eye are divided into six classes, according to their brightness.
The brightest stars are called stars of the _first magnitude_; the next
brightest, those of the _second magnitude_; and so on to the _sixth
magnitude_. The last magnitude includes the faintest stars that are
visible to the naked eye on the most favorable night. Stars which are
fainter than those of the sixth magnitude can be seen only with the
telescope, and are called _telescopic stars_. Telescopic stars are also
divided into magnitudes; the division extending to the _sixteenth_
magnitude, or the faintest stars that can be seen with the most powerful
telescopes.
The classification of stars according to magnitudes has reference only
to their brightness, and not at all to their actual size. A sixth
magnitude star may actually be larger than a first magnitude star; its
want of brilliancy being due to its greater distance, or to its inferior
luminosity, or to both of these causes.
None of the stars present any sensible disk, even in the most powerful
telescope: they all appear as mere points of light. The larger the
telescope, the greater is its power of revealing faint stars; not
because it makes these stars appear larger, but because of its greater
light-gathering power. This power increases with the size of the
object-glass of the telescope, which plays the part of a gigantic pupil
of the eye.
The classification of the stars into magnitudes is not made in
accordance with any very accurate estimate of their brightness. The
stars which are classed together in the same magnitude are far from
being equally bright.
The stars of each lower magnitude are about two-fifths as bright as
those of the magnitude above. The ratio of diminution is about a third
from the higher magnitude down to the fifth. Were the ratio two-fifths
exact, it would take about
2-1/2 stars of the 2d magnitude to make one of the 1st.
6 stars of the 3d magnitude to make one of the 1st.
16 stars of the 4th magnitude to make one of the 1st.
40 stars of the 5th magnitude to make one of the 1st.
100 stars of the 6th magnitude to make one of the 1st.
10,000 stars of the 11th magnitude to make one of the 1st.
1,000,000 stars of the 16th magnitude to make one of the 1st.
323. _The Number of the Stars._--The total number of stars in the
celestial sphere visible to the average naked eye is estimated, in round
numbers, at five thousand; but the number varies much with the
perfection and the training of the eye and with the atmospheric
conditions. For every star visible to the naked eye, there are thousands
too minute to be seen without telescopic aid. Fig. 364 shows a portion
of the constellation of the Twins as seen with the naked eye; and Fig.
365 shows the same region as seen in a powerful telescope.
[Illustration: Fig. 364.]
[Illustration: Fig. 365.]
Struve has estimated that the total number of stars visible with
Herschel's twenty-foot telescope was about twenty million. The number
that can be seen with the great telescopes of modern times has not been
carefully estimated, but is probably somewhere between thirty million
and fifty million.
The number of stars between the north pole and the circle thirty-five
degrees south of the equator is about as follows:--
Of the 1st magnitude about 14 stars.
Of the 2d magnitude about 48 stars.
Of the 3d magnitude about 152 stars.
Of the 4th magnitude about 313 stars.
Of the 5th magnitude about 854 stars.
Of the 6th magnitude about 2010 stars.
----
Total visible to naked eye 3391 stars.
The number of stars of the several magnitudes is approximately in
inverse proportion to that of their brightness, the ratio being a little
greater in the higher magnitudes, and probably a little less in the
lower ones.
324. _The Division of the Stars into Constellations._--A glance at the
heavens is sufficient to show that the stars are not distributed
uniformly over the sky. The larger ones especially are collected into
more or less irregular groups. The larger groups are called
_constellations_. At a very early period a mythological figure was
allotted to each constellation; and these figures were drawn in such a
way as to include the principal stars of each constellation. The heavens
thus became covered, as it were, with immense hieroglyphics.
There is no historic record of the time when these figures were formed,
or of the principle in accordance with which they were constructed. It
is probable that the imagination of the earlier peoples may, in many
instances, have discovered some fanciful resemblance in the
configuration of the stars to the forms depicted. The names are still
retained, although the figures no longer serve any astronomical purpose.
The constellation Hercules, for instance, no longer represents the
figure of a man among the stars, but a certain portion of the heavens
within which the ancients placed that figure. In star-maps intended for
school and popular use it is still customary to give these figures; but
they are not generally found on maps designed for astronomers.
325. _The Naming of the Stars._--The brighter stars have all proper
names, as _Sirius_, _Procyon_, _Arcturus_, _Capella_, _Aldebaran_, etc.
This method of designating the stars was adopted by the Arabs. Most of
these names have dropped entirely out of astronomical use, though many
are popularly retained. The brighter stars are now generally designated
by the letters of the Greek alphabet,--_alpha_, _beta_, _gamma_,
etc.,--to which is appended the genitive of the name of the
constellation, the first letter of the alphabet being used for the
brightest star, the second for the next brightest, and so on. Thus
_Aldebaran_ would be designated as _Alpha Tauri_. In speaking of the
stars of any one constellation, we simply designate them by the letters
of the Greek alphabet, without the addition of the name of the
constellation, which answers to a person's surname, while the Greek
letter answers to his Christian name. The names of the seven stars of
the "Dipper" are given in Fig. 366. When the letters of the Greek
alphabet are exhausted, those of the Roman alphabet are employed. The
fainter stars in a constellation are usually designated by some system
of numbers.
[Illustration: Fig. 366.]
326. _The Milky-Way, or Galaxy._--The Milky-Way is a faint luminous
band, of irregular outline, which surrounds the heavens with a great
circle, as shown in Fig. 367. Through a considerable portion of its
course it is divided into two branches, and there are various vacant
spaces at different points in this band; but at only one point in the
southern hemisphere is it entirely interrupted.
[Illustration: Fig. 367.]
The telescope shows that the Galaxy arises from the light of countless
stars too minute to be separately visible with the naked eye. The
telescopic stars, instead of being uniformly distributed over the
celestial sphere, are mostly condensed in the region of the Galaxy. They
are fewest in the regions most distant from this belt, and become
thicker as we approach it. The greater the telescopic power, the more
marked is the condensation. With the naked eye the condensation is
hardly noticeable; but with the aid of a very small telescope, we see a
decided thickening of the stars in and near the Galaxy, while the most
powerful telescopes show that a large majority of the stars lie actually
in the Galaxy. If all the stars visible with a twelve-inch telescope
were blotted out, we should find that the greater part of those
remaining were in the Galaxy.
[Illustration: Fig. 368.]
The increase in the number of the stars of all magnitudes as we approach
the plane of the Milky-Way is shown in Fig. 368. The curve _acb_ shows
by its height the distribution of the stars above the ninth magnitude,
and the curve _ACB_ those of all magnitudes.
327. _Star-Clusters._--Besides this gradual and regular condensation
towards the Galaxy, occasional aggregations of stars into _clusters_ may
be seen. Some of these are visible to the naked eye, sometimes as
separate stars, like the "Seven Stars," or Pleiades, but more commonly
as patches of diffused light, the stars being too small to be seen
separately. The number visible in powerful telescopes is, however, much
greater. Sometimes hundreds or even thousands of stars are visible in
the field of view at once, and sometimes the number is so great that
they cannot be counted.
328. _Nebulæ._--Another class of objects which are found in the
celestial spaces are irregular masses of soft, cloudy light, known as
_nebulæ_. Many objects which look like nebulæ in small telescopes are
shown by more powerful instruments to be really star-clusters. But many
of these objects are not composed of stars at all, being immense masses
of gaseous matter.
[Illustration: Fig. 369.]
The general distribution of nebulæ is the reverse of that of the stars.
Nebulæ are thickest where stars are thinnest. While stars are most
numerous in the region of the Milky-Way, nebulæ are most abundant about
the poles of the Milky-Way. This condensation of nebulæ about the poles
of the Milky-Way is shown in Figs. 367 and 369, in which the points
represent, not stars, but nebulæ.
II. THE STARS.
The Constellations.
[Illustration: Fig. 370.]
[Illustration: Fig. 371.]
329. _The Great Bear._--The Great Bear, or _Ursa Major_, is one of the
circumpolar constellations (4), and contains one of the most familiar
_asterisms_, or groups of stars, in our sky; namely, the _Great Dipper_,
or _Charles's Wain_. The positions and names of the seven prominent
stars in it are shown in Fig. 370. The two stars Alpha and Beta are
called the _Pointers_. This asterism is sometimes called the _Butcher's
Cleaver_. The whole constellation is shown in Fig. 371. A rather faint
star marks the nose of the bear, and three equidistant pairs of faint
stars mark his feet.
330. _The Little Bear, Draco, and Cassiopeia._--These are all
circumpolar constellations. The most important star of the Little Bear,
or _Ursa Minor_, is _Polaris_, or the _Pole Star_. This star may be
found by drawing a line from Beta to Alpha of the Dipper, and prolonging
it as shown in Fig. 372. This explains why these stars are called the
_Pointers_. The Pole Star, with the six other chief stars of the Little
Bear, form an asterism called the _Little Dipper_. These six stars are
joined with Polaris by a dotted line in Fig. 372.
[Illustration: Fig. 372.]
The stars in a serpentine line between the two Dippers are the chief
stars of _Draco_, or the _Dragon_; the trapezium marking its head. Fig.
373 shows the constellations of Ursa Minor and Draco as usually figured.
[Illustration: Fig. 373.]
To find _Cassiopeia_, draw a line from Delta of the Dipper to Polaris,
and prolong it about an equal distance beyond, as shown in Fig. 372.
This line will pass near Alpha of Cassiopeia. The five principal stars
of this constellation form an irregular _W_, opening towards the pole.
Between Cassiopeia and Draco are five rather faint stars, which form an
irregular _K_. These are the principal stars of the constellation
_Cepheus_. These two constellations are shown in Fig. 374.
[Illustration: Fig. 374.]
[Illustration: Fig. 375.]
331. _The Lion, Berenice's Hair, and the Hunting-Dogs._--A line drawn
from Alpha to Beta of the Dipper, and prolonged as shown in Fig. 375,
will pass between the two stars _Denebola_ and _Regulus_ of _Leo_, or
the _Lion_. Regulus forms a _sickle_ with several other faint stars, and
marks the heart of the lion. Denebola is at the apex of a right-angled
triangle, which it forms with two other stars, and marks the end of the
lion's tail. This constellation is visible in the evening from February
to July, and is figured in Fig. 376.
[Illustration: Fig. 376.]
In a straight line between Denebola and Eta, at the end of the Great
Bear's tail, are, at about equal distances, the two small constellations
of _Coma Berenices_, or _Berenice's Hair_, and _Canes Venatici_, or the
_Hunting-Dogs_. These are shown in Fig. 377. The dogs are represented as
pursuing the bear, urged on by the huntsman _Boötes_.
[Illustration: Fig. 377.]
332. _Boötes, Hercules, and the Northern Crown._--_Arcturus_, the
principal star of _Boötes_, may be found by drawing a line from Zeta to
Eta of the Dipper, and then prolonging it with a slight bend, as shown
in Fig. 378. Arcturus and Polaris form a large isosceles triangle with a
first-magnitude star called _Vega_. This triangle encloses at one corner
the principal stars of Boötes, and the head of the Dragon near the
opposite side. The side running from Arcturus to Vega passes through
_Corona Borealis_, or the _Northern Crown_, and the body of _Hercules_,
which is marked by a quadrilateral of four stars.
[Illustration: Fig. 378.]
_Boötes_, who is often represented as a husbandman, _Corona Borealis_,
and _Hercules_, are delineated in Fig. 379. These constellations are
visible in the evening from May to September.
[Illustration: Fig. 379.]
[Illustration: Fig. 380.]
333. _The Lyre, the Swan, the Eagle, and the Dolphin._--_Altair_, the
principal star of _Aquila_, or the _Eagle_, lies on the opposite side of
the Milky-Way from Vega. Altair is a first-magnitude star, and has a
faint star on each side of it, as shown in Fig. 380. Vega, also of the
first magnitude, is the principal star of _Lyra_, or the _Lyre_. Between
these two stars, and a little farther to the north, are several stars
arranged in the form of an immense cross. The bright star at the head of
this cross is called _Deneb_. The cross lies in the Milky-Way, and
contains the chief stars of the constellation _Cygnus_, or the _Swan_. A
little to the north of Altair are four stars in the form of a diamond.
This asterism is popularly known as _Job's Coffin_. These four stars are
the chief stars of _Delphinus_, or the _Dolphin_. These four
constellations are shown together in Fig. 381. The _Swan_ is visible
from June to December, in the evening.
[Illustration: Fig. 381.]
334. _Virgo._--A line drawn from Alpha to Gamma of the Dipper, and
prolonged with a slight bend at Gamma, will reach to a first-magnitude
star called _Spica_ (Fig. 382). This is the chief star of the
constellation _Virgo_, or the _Virgin_, and forms a large isosceles
triangle with _Arcturus_ and _Denebola_.
[Illustration: Fig. 382.]
_Virgo_ is represented in Fig. 383. To the right of this constellation,
as shown in the figure, there are four stars which form a trapezium, and
mark the constellation _Corvus_, or the _Crow_. This bird is represented
as standing on the body of _Hydra_, or the _Water-Snake_. _Virgo_ is
visible in the evening, from April to August.
[Illustration: Fig. 383.]
[Illustration: Fig. 384.]
[Illustration: Fig. 385.]
335. _The Twins._--A line drawn from Delta to Beta of the Dipper, and
prolonged as shown in Fig. 384, passes between two bright stars called
_Castor_ and _Pollux_. The latter of these is usually reckoned as a
first-magnitude star. These are the principal stars of the constellation
_Gemini_, or the _Twins_, which is shown in Fig. 385. The constellation
_Canis Minor_, or the _Little Dog_, is shown in the lower part of the
figure. There are two conspicuous stars in this constellation, the
brightest of which is of the first magnitude, and called _Procyon_.
The region to which we have now been brought is the richest of the
northern sky, containing no less than seven first-magnitude stars. These
are _Sirius_, _Procyon_, _Pollux_, _Capella_, _Aldebaran_, _Betelgeuse_,
and _Rigel_. They are shown in Fig. 386.
[Illustration: Fig. 386.]
_Betelgeuse_ and _Rigel_ are in the constellation _Orion_, being about
equally distant to the north and south from the three stars forming the
_belt_ of Orion. Betelgeuse is a red star. _Sirius_ is the brightest
star in the heavens, and belongs to the constellation _Canis Major_, or
the _Great Dog_. It lies to the east of the belt of Orion. _Aldebaran_
lies at about the same distance to the west of the belt. It is a red
star, and belongs to the constellation _Taurus_, or the _Bull_.
_Capella_ is in the constellation _Auriga_, or the _Wagoner_. These
stars are visible in the evening, from about December to April.
336. _Orion and his Dogs, and Taurus._--_Orion_ and his _Dogs_ are shown
in Fig. 387, and _Orion_ and _Taurus_ in Fig. 388. _Aldebaran_ marks one
of the eyes of the bull, and is often called the _Bull's Eye_. The
irregular _V_ in the face of the bull is called the _Hyades_, and the
cluster on the shoulder the _Pleiades_.
[Illustration: Fig. 387.]
[Illustration: Fig. 388.]
[Illustration: Fig. 389.]
337. _The Wagoner._--The constellation _Auriga_, or the _Wagoner_
(sometimes called the _Charioteer_), is shown in Fig. 389. _Capella_
marks the _Goat_, which he is represented as carrying on his back, and
the little right-angled triangle of stars near it the _Kids_. The five
chief stars of this constellation form a large, irregular pentagon.
Gamma of _Auriga_ is also Beta of _Taurus_, and marks one of the horns
of the _Bull_.
[Illustration: Fig. 390.]
338. _Pegasus, Andromeda, and Perseus._--A line drawn from Polaris near
to Beta of _Cassiopeia_ will lead to a bright second-magnitude star at
one corner of a large square (Fig. 390). Alpha belongs both to the
_Square of Pegasus_ and to _Andromeda_. Beta and Gamma, which are
connected with Alpha in the figure by a dotted line, also belong to
Andromeda. _Algol_, which forms, with the last-named stars and with the
_Square of Pegasus_, an asterism similar in configuration to the _Great
Dipper_, belongs to _Perseus_. _Algenib_, which is reached by bending
the line at Gamma in the opposite direction, is the principal star of
_Perseus_.
[Illustration: Fig. 391.]
[Illustration: Fig. 392.]
[Illustration: Fig. 393.]
_Pegasus_ is shown in Fig. 391, and _Andromeda_ in Fig. 392. _Cetus_,
the _Whale_, or the _Sea Monster_, shown in Fig. 393, belongs to the
same mythological group of constellations.
[Illustration: Fig. 394.]
339. _Scorpio, Sagittarius, and Ophiuchus._--During the summer months a
brilliant constellation is visible, called _Scorpio_, or the _Scorpion_.
The configuration of the chief stars of this constellation is shown in
Fig. 394. They bear some resemblance to a boy's kite. The brightest star
is of the first magnitude, and called _Antares_ (from _anti_, instead
of, and _Ares_, the Greek name of Mars), because it rivals Mars in
redness. The stars in the tail of the Scorpion are visible in our
latitude only under very favorable circumstances. This constellation is
shown in Fig. 395, together with _Sagittarius_ and _Ophiuchus_.
_Sagittarius_, or the _Archer_, is to the east of _Scorpio_. It contains
no bright stars, but is easily recognized from the fact that five of its
principal stars form the outline of an inverted dipper, which, from the
fact of its being partly in the Milky-Way, is often called the _Milk
Dipper_.
[Illustration: Fig. 395.]
_Ophiuchus_, or the _Serpent-Bearer_, is a large constellation, filling
all the space between the head of _Hercules_ and _Scorpio_. It is
difficult to trace, since it contains no very brilliant stars. This
constellation and _Libra_, or the _Balances_, which is the zodiacal
constellation to the west of Scorpio, are shown in Fig. 396.
[Illustration: Fig. 396.]
[Illustration: Fig. 397.]
340. _Capricornus, Aquarius, and the Southern Fish._--The two zodiacal
constellations to the east of Sagittarius are _Capricornus_ and
_Aquarius_. _Capricornus_ contains three pairs of small stars, which
mark the head, the tail, and the knees of the animal.
_Aquarius_ is marked by no conspicuous stars. An irregular line of
minute stars marks the course of the stream of water which flows from
the Water-Bearer's Urn into the mouth of the _Southern Fish_. This mouth
is marked by the first-magnitude star _Fomalhaut_. These constellations
are shown in Fig. 397.
[Illustration: Fig. 398.]
341. _Pisces and Aries._--The remaining zodiacal constellations are
_Pisces_, or the _Fishes_, _Aries_, or the _Ram_ (Fig. 398), and
_Cancer_, or the _Crab_.
The _Fishes_ lie under _Pegasus_ and _Andromeda_, but contain no bright
stars. _Aries_ (between _Pisces_ and _Taurus_) is marked by a pair of
stars on the head,--one of the second, and one of the third magnitude.
_Cancer_ (between _Leo_ and _Gemini_) has no bright stars, but contains
a remarkable cluster of small stars called _Præsepe_, or the _Beehive_.
Clusters.
342. _The Hyades._--The _Hyades_ are a very open cluster in the face of
_Taurus_ (334). The three brightest stars of this cluster form a letter
_V_, the point of the _V_ being on the nose, and the open ends at the
eyes. This cluster is shown in Fig. 399. The name, according to the most
probable etymology, means _rainy_; and they are said to have been so
called because their rising was associated with wet weather. They were
usually considered the daughters of Atlas, and sisters of the Pleiades,
though sometimes referred to as the nurses of Bacchus.
[Illustration: Fig. 399.]