stars, or a miniature constellation, on the shoulder of _Taurus_. Hesiod
mentions them as "the seven virgins of Atlas born," and Milton calls
them "the seven Atlantic sisters." They are referred to in the Book of
Job. The Spaniards term them "the little nanny-goats;" and they are
sometimes called "the hen and chickens."
[Illustration: Fig. 400.]
[Illustration: Fig. 401.]
Usually only six stars in this cluster can be seen with the naked eye,
and this fact has given rise to the legend of the "lost Pleiad." On a
clear, moonless night, however, a good eye can discern seven or eight
stars, and some observers have distinguished as many as eleven. Fig. 400
shows the _Pleiades_ as they appear to the naked eye under the most
favorable circumstances. Fig. 401 shows this cluster as it appears in a
powerful telescope. With such an instrument more than five hundred stars
are visible.
344. _Cluster in the Sword-handle of Perseus._--This is a somewhat dense
double cluster. It is visible to the naked eye, appearing as a hazy
star. A line drawn from _Algenib_, or _Alpha_ of _Perseus_ (338), to
_Delta_ of _Cassiopeia_ (330), will pass through this cluster at about
two-thirds the distance from the former. This double cluster is one of
the most brilliant objects in the heavens, with a telescope of moderate
power.
[Illustration: Fig. 402.]
345. _Cluster of Hercules._--The celebrated globular cluster of
_Hercules_ can be seen only with a telescope of considerable power, and
to resolve it into distinct stars (as shown in Fig. 402) requires an
instrument of the very highest class.
[Illustration: Fig. 403.]
346. _Other Clusters._--Fig. 403 shows a magnificent globular cluster in
the constellation _Aquarius_. Herschel describes it as appearing like a
heap of sand, being composed of thousands of stars of the fifteenth
magnitude.
[Illustration: Fig. 404.]
Fig. 404 shows a cluster in the constellation _Toucan_, which Sir John
Herschel describes as a most glorious globular cluster, the stars of the
fourteenth magnitude being immensely numerous. There is a marked
condensation of light at the centre.
[Illustration: Fig. 405.]
[Illustration: Fig. 406.]
Fig. 405 shows a cluster in the _Centaur_, which, according to the same
astronomer, is beyond comparison the richest and largest object of the
kind in the heavens, the stars in it being literally innumerable. Fig.
406 shows a cluster in _Scorpio_, remarkable for the peculiar
arrangement of its component stars.
Star clusters are especially abundant in the region of the Milky-Way,
the law of their distribution being the reverse of that of the nebulæ.
Double and Multiple Stars.
347. _Double Stars._--The telescope shows that many stars which appear
single to the naked eye are really _double_, or composed of a pair of
stars lying side by side. There are several pairs of stars in the
heavens which lie so near together that they almost seem to touch when
seen with the naked eye.
[Illustration: Fig. 407.]
[Illustration: Fig. 408.]
Pairs of stars are not considered double unless the components are so
near together that they both appear in the field of view when examined
with a telescope. In the majority of the pairs classed as double stars
the distance between the components ranges from half a second to fifteen
seconds.
[Illustration: Fig. 409.]
_Epsilon Lyræ_ is a good example of a pair of stars that can barely be
separated with a good eye. Figs. 407 and 408 show this pair as it
appears in telescopes magnifying respectively four and fifteen times;
and Fig. 409 shows it as seen in a more powerful telescope, in which
each of the two components of the pair is seen to be a truly double
star.
[Illustration: Fig. 410.]
[Illustration: Fig. 411.]
348. _Multiple Stars._--When a star is resolved into more than two
components by a telescope, it is called a _multiple_ star. Fig. 410
shows a _triple_ star in _Pegasus_. Fig. 411 shows a quadruple star in
_Taurus_. Fig. 412 shows a _sextuple_ star, and Fig. 413 a _septuple_
star. Fig. 414 shows the celebrated septuple star in _Orion_, called
_Theta Orionis_, or the _trapezium_ of Orion.
349. _Optically Double and Multiple Stars._--Two or more stars which are
really very distant from each other, and which have no physical
connection whatever, may appear to be near together, because they happen
to lie in the same direction, one behind the other. Such accidental
combinations are called _optically_ double or multiple stars.
[Illustration: Fig. 412.]
[Illustration: Fig. 413.]
350. _Physically Double and Multiple Stars._--In the majority of cases
the components of double and multiple stars are in reality comparatively
near together, and are bound together by gravity into a physical system.
Such combinations are called _physically_ double and multiple stars. The
components of these systems all revolve around their common centre of
gravity. In many instances their orbits and periods of revolution have
been ascertained by observation and calculation. Fig. 415 shows the
orbit of one of the components of a double star in the constellation
_Hercules_.
[Illustration: Fig. 414.]
351. _Colors of Double and Multiple Stars._--The components of double
and multiple stars are often highly colored, and frequently the
components of the same system are of different colors. Sometimes one
star of a binary system is _white_, and the other _red_; and sometimes a
_white_ star is combined with a _blue_ one. Other colors found in
combination in these systems are _red_ and _blue_, _orange_ and _green_,
_blue_ and _green_, _yellow_ and _blue_, _yellow_ and _red_, etc.
[Illustration: Fig. 415.]
If these double and multiple stars are accompanied by planets, these
planets will sometimes have two or more suns in the sky at once. On
alternate days they may have suns of different colors, and perhaps on
the same day two suns of different colors. The effect of these changing
colored lights on the landscape must be very remarkable.
New and Variable Stars.
352. _Variable Stars._--There are many stars which undergo changes of
brilliancy, sometimes slight, but occasionally very marked. These
changes are in some cases apparently irregular, and in others
_periodic_. All such stars are said to be _variable_, though the term is
applied especially to those stars whose variability is _periodic_.
[Illustration: Fig. 416.]
353. _Algol._--_Algol_, a star of _Perseus_, whose position is shown in
Fig. 416, is a remarkable variable star of a short period. Usually it
shines as a faint second-magnitude star; but at intervals of a little
less than three days it fades to the fourth magnitude for a few hours,
and then regains its former brightness. These changes were first noticed
some two centuries ago, but it was not till 1782 that they were
accurately observed. The period is now known to be two days, twenty
hours, forty-nine minutes. It takes about four hours and a half to fade
away, and four hours more to recover its brilliancy. Near the beginning
and end of the variations, the change is very slow, so that there are
not more than five or six hours during which an ordinary observer would
see that the star was less bright than usual.
This variation of light was at first explained by supposing that a large
dark planet was revolving round Algol, and passed over its face at every
revolution, thus cutting off a portion of its light; but there are small
irregularities in the variation, which this theory does not account for.
354. _Mira._--Another remarkable variable star is _Omicron Ceti_, or
_Mira_ (that is, the _wonderful_ star). It is generally invisible to the
naked eye; but at intervals of about eleven months it shines forth as a
star of the second or third magnitude. It is about forty days from the
time it becomes visible until it attains its greatest brightness, and is
then about two months in fading to invisibility; so that its increase of
brilliancy is more rapid than its waning. Its period is quite irregular,
ranging from ten to twelve months; so that the times of its appearance
cannot be predicted with certainty. Its maximum brightness is also
variable, being sometimes of the second magnitude, and at others only of
the third or fourth.
[Illustration: Fig. 417.]
355. _Eta Argus._--Perhaps the most extraordinary variable star in the
heavens is _Eta Argus_, in the constellation _Argo_, or the _Ship_, in
the southern hemisphere (Fig. 417). The first careful observations of
its variability were made by Sir John Herschel while at the Cape of Good
Hope. He says, "It was on the 16th of December, 1837, that, resuming the
photometrical comparisons, my astonishment was excited by the appearance
of a new candidate for distinction among the very brightest stars of the
first magnitude in a part of the heavens where, being perfectly familiar
with it, I was certain that no such brilliant object had before been
seen. After a momentary hesitation, the natural consequence of a
phenomenon so utterly unexpected, and referring to a map for its
configuration with other conspicuous stars in the neighborhood, I became
satisfied of its identity with my old acquaintance, _Eta Argus_. Its
light was, however, nearly tripled. While yet low, it equalled Rigel,
and, when it attained some altitude, was decidedly greater." It
continued to increase until Jan. 2, 1838, then faded a little till April
following, though it was still as bright as Aldebaran. In 1842 and 1843
it blazed up brighter than ever, and in March of the latter year was
second only to _Sirius_. During the twenty-five years following it
slowly but steadily diminished. In 1867 it was barely visible to the
naked eye; and the next year it vanished entirely from the unassisted
view, and has not yet begun to recover its brightness. The curve in Fig.
418 shows the change in brightness of this remarkable star. The numbers
at the bottom show the years of the century, and those at the side the
brightness of the star.
[Illustration: Fig. 418.]
356. _New Stars._--In several cases stars have suddenly appeared, and
even become very brilliant; then, after a longer or shorter time, they
have faded away and disappeared. Such stars are called _new_ or
_temporary_ stars. For a time it was supposed that such stars were
actually new. They are now, however, classified by astronomers among the
variable stars, their changes being of a very irregular and fitful
character. There is scarcely a doubt that they were all in the heavens
as very small stars before they blazed forth in so extraordinary a
manner, and that they are in the same places still. There is a wide
difference between these irregular variations, or the breaking-forth of
light on a single occasion in the course of centuries, and the regular
and periodic changes in the case of a star like _Algol_; but a long
series of careful observation has resulted in the discovery of stars of
nearly every degree of irregularity between these two extremes. Some of
them change gradually from one magnitude to another, in the course of
years, without seeming to follow any law whatever; while in others some
slight tendency to regularity can be traced. _Eta Argus_ may be regarded
as a connecting link between new and variable stars.
357. _Tycho Brahe's Star._--An apparently new star suddenly appeared in
_Cassiopeia_ in 1572. It was first seen by Tycho Brahe, and is therefore
associated with his name. Its position in the constellation is shown in
Fig. 419. It was first seen on Nov. 11, when it had already attained the
first magnitude. It became rapidly brighter, soon rivalling Venus in
splendor, so that good eyes could discern it in full daylight. In
December it began to wane, and gradually faded until the following May,
when it disappeared entirely.
[Illustration: Fig. 419.]
A star showed itself in the same part of the heavens in 945 and in 1264.
If these were three appearances of the same star, it must be reckoned as
a periodic star with a period of a little more than three hundred years.
358. _Kepler's Star._--In 1604 a new star was seen in the constellation
_Ophiuchus_. It was first noticed in October of that year, when it was
of the first magnitude. In the following winter it began to fade, but
remained visible during the whole year 1605. Early in 1606 it
disappeared entirely. A very full history of this star was written by
Kepler.
One of the most remarkable things about this star was its brilliant
scintillation. According to Kepler, it displayed all the colors of the
rainbow, or of a diamond cut with multiple facets, and exposed to the
rays of the sun. It is thought that this star also appeared in 393, 798,
and 1203; if so, it is a variable star with a period of a little over
four hundred years.
359. _New Star of 1866._--The most striking case of this kind in recent
times was in May, 1866, when a star of the second magnitude suddenly
appeared in _Corona Borealis_. On the 11th and 12th of that month it was
observed independently by at least five observers in Europe and America.
The fact that none of these new stars were noticed until they had nearly
or quite attained their greatest brilliancy renders it probable that
they all blazed up very suddenly.
360. _Cause of the Variability of Stars._--The changes in the brightness
of variable and temporary stars are probably due to operations similar
to those which produce the spots and prominences in our sun. We have
seen (188) that the frequency of solar spots shows a period of eleven
years, during one portion of which there are few or no spots to be seen,
while during another portion they are numerous. If an observer so far
away as to see our sun like a star could from time to time measure its
light exactly, he would find it to be a variable star with a period of
eleven years, the light being least when we see most spots, and greatest
when few are visible. The variation would be slight, but it would
nevertheless exist. Now, we have reason to believe that the physical
constitution of the sun and the stars is of the same general nature. It
is therefore probable, that, if we could get a nearer view of the stars,
we should see spots on their disks as we do on the sun. It is also
likely that the varying physical constitution of the stars might give
rise to great differences in the number and size of the spots; so that
the light of some of these suns might vary to a far greater degree than
that of our own sun does. If the variations had a regular period, as in
the case of our sun, the appearances to a distant observer would be
precisely what we see in the case of a periodic variable star.
The spectrum of the new star of 1866 was found to be a continuous one,
crossed by bright lines, which were apparently due to glowing hydrogen.
The continuous spectrum was also crossed by dark lines, indicating that
the light had passed through an atmosphere of comparatively cool gas.
Mr. Huggins infers from this that there was a sudden and extraordinary
outburst of hydrogen gas from the star, which by its own light, as well
as by heating up the whole surface of the star, caused the extraordinary
increase of brilliancy. Now, the spectroscope shows that the red flames
of the solar chromosphere (197) are largely composed of hydrogen; and it
is not unlikely that the blazing-forth of this star arose from an action
similar to that which produces these flames, only on an immensely larger
scale.
Distance of the Stars.
361. _Parallax of the Stars._--Such is the distance of the stars, that
only in a comparatively few instances has any displacement of these
bodies been detected when viewed from opposite parts of the earth's
orbit, that is, from points a hundred and eighty-five million miles
apart; and in no case can this displacement be detected except by the
most careful and delicate measurement. Half of the above displacement,
or the displacement of the star as seen from the earth instead of the
sun, is called the _parallax_ of the star. In no case has a parallax of
one second as yet been detected.
362. _The Distance of the Stars._--The distance of a star whose parallax
is one second would be 206,265 times the distance of the earth from the
sun, or about nineteen million million miles. It is quite certain that
no star is nearer than this to the earth. Light has a velocity which
would carry it seven times and a half around the earth in a second; but
it would take it more than three years to reach us from that distance.
Were all the stars blotted out of existence to-night, it would be at
least three years before we should miss a single one.
_Alpha Centauri_, the brightest star in the constellation of the
_Centaur_, is, so far as we know, the nearest of the fixed stars. It is
estimated that it would take its light about three years and a half to
reach us. It has also been estimated that it would take light over
sixteen years to reach us from _Sirius_, about eighteen years to reach
us from _Vega_, about twenty-five years from _Arcturus_, and over forty
years from the _Pole-Star_. In many instances it is believed that it
would take the light of stars hundreds of years to make the journey to
our earth, and in some instances even thousands of years.
Proper Motion of the Stars.
363. _Why the Stars appear Fixed._--The stars seem to retain their
relative positions in the heavens from year to year, and from age to
age; and hence they have come universally to be denominated as _fixed_.
It is, however, now well known that the stars, instead of being really
stationary, are moving at the rate of many miles a second; but their
distance is so enormous, that, in the majority of cases, it would be
thousands of years before this rate of motion would produce a sufficient
displacement to be noticeable to the unaided eye.
[Illustration: Fig. 420.]
364. _Secular Displacement of the Stars._--Though the proper motion of
the stars is apparently slight, it will, in the course of many ages,
produce a marked change in the configuration of the stars. Thus, in Fig.
420, the left-hand portion shows the present configuration of the stars
of the Great Dipper. The small arrows attached to the stars show the
direction and comparative magnitudes of their motion. The right-hand
portion of the figure shows these stars as they will appear thirty-six
thousand years from the present time.
[Illustration: Fig. 421.]
Fig. 421 shows in a similar way the present configuration and proper
motion of the stars of _Cassiopeia_, and also these stars as they will
appear thirty-six thousand years hence.
[Illustration: Fig. 422.]
Fig. 422 shows the same for the constellation _Orion_.
365. _The Secular Motion of the Sun._--The stars in all parts of the
heavens are found to move in all directions and with all sorts of
velocities. When, however, the motions of the stars are averaged, there
is found to be an apparent proper motion common to all the stars. The
stars in the neighborhood of _Hercules_ appear to be approaching us, and
those in the opposite part of the heavens appear to be receding from us.
In other words, all the stars appear to be moving away from Hercules,
and towards the opposite part of the heavens.
[Illustration: Fig. 423.]
This apparent motion common to all the stars is held by astronomers to
be due to the real motion of the sun through space. The point in the
heavens towards which our sun is moving at the present time is indicated
by the small circle in the constellation Hercules in Fig. 423. As the
sun moves, he carries the earth and all the planets along with him. Fig.
424 shows the direction of the sun's motion with reference to the
ecliptic and to the axis of the earth. Fig. 425 shows the earth's path
in space; and Fig. 426 shows the paths of the earth, the moon, Mercury,
Venus, and Mars in space.
[Illustration: Fig. 424.]
[Illustration: Fig. 425.]
[Illustration: Fig. 426.]
Whether the sun is actually moving in a straight line, or around some
distant centre, it is impossible to determine at the present time. It is
estimated that the sun is moving along his path at the rate of about a
hundred and fifty million miles a year. This is about five-sixths of the
diameter of the earth's orbit.
366. _Star-Drift._--In several instances, groups of stars have a common
proper motion entirely different from that of the stars around and among
them. Such groups probably form connected systems, in the motion of
which all the stars are carried along together without any great change
in their relative positions. The most remarkable case of this kind
occurs in the constellation _Taurus_. A large majority of the brighter
stars in the region between _Aldebaran_ and the _Pleiades_ have a common
proper motion of about ten seconds per century towards the east. Proctor
has shown that five out of the seven stars which form the Great Dipper
have a common proper motion, as shown in Fig. 427 (see also Fig. 420).
He proposes for this phenomenon the name of _Star-Drift_.
[Illustration: Fig. 427.]
367. _Motion of Stars along the Line of Sight._--A motion of a star in
the direction of the line of sight would produce no displacement of the
star that could be detected with the telescope; but it would cause a
change in the brightness of the star, which would become gradually
fainter if moving from us, and brighter if approaching us. Motion along
the line of sight has, however, been detected by the use of the
tele-spectroscope (152), owing to the fact that it causes a displacement
of the spectral lines. As has already been explained (169), a
displacement of a spectral line towards the red end of the spectrum
indicates a motion away from us, and a displacement towards the violet
end, a motion towards us.
* * * * *
By means of these displacements of the spectral lines, Huggins has
detected motion in the case of a large number of stars, and calculated
its rate:--
STARS RECEDING FROM US.
Sirius 20 miles per second.
Betelgeuse 22 miles per second.
Rigel 15 miles per second.
Castor 25 miles per second.
Regulus 15 miles per second.
STARS APPROACHING US.
Arcturus 55 miles per second.
Vega 50 miles per second.
Deneb 39 miles per second.
Pollux 49 miles per second.
Alpha Ursæ Majoris 46 miles per second.
These results are confirmed by the fact that the amount of motion
indicated is about what we should expect the stars to have, from their
observed proper motions, combined with their probable distances. Again:
the stars in the neighborhood of Hercules are mostly found to be
approaching the earth, and those which lie in the opposite direction to
be receding from it; which is exactly the effect which would result from
the sun's motion through space. The five stars in the Dipper, which have
a common proper motion, are also found to have a common motion in the
line of sight. But the displacement of the spectral lines is so slight,
and its measurement so difficult, that the velocities in the above table
are to be accepted as only an approximation to the true values.
Chemical and Physical Constitution of the Stars.
368. _The Constitution of the Stars Similar to that of the Sun._--The
stellar spectra bear a general resemblance to that of the sun, with
characteristic differences. These spectra all show Fraunhofer's lines,
which indicate that their luminous surfaces are surrounded by
atmospheres containing absorbent vapors, as in the case of the sun. The
positions of these lines indicate that the stellar atmospheres contain
elements which are also found in the sun's, and on the earth.
[Illustration: Fig. 428.]
369. _Four Types of Stellar Spectra._--The spectra of the stars have
been carefully observed by Secchi and Huggins. They have found that
stellar spectra may be reduced to four types, which are shown in Fig.
428. In the spectrum of _Sirius_, a representative of _Type I._, very
few lines are represented; but the lines are very thick.
Next we have the solar spectrum, which is a representative of _Type
II._, one in which more lines are represented. In _Type III._ fluted
spaces begin to appear, and in _Type IV._, which is that of the red
stars, nothing but fluted spaces is visible; and this spectrum shows
that something is at work in the atmosphere of those red stars different
from what there is in the simpler atmosphere of _Type I._