forty years ago, uncertain whether its period was forty-three or
sixty-six years, but now that two complete revolutions have been
performed since its discovery by Sir William Herschel in 1781, the
question has been finally decided in favor of the shorter period.
The brilliant star Sirius is also an interesting binary star. The
companion, which is relatively very faint—about tenth magnitude—was
discovered by Alvan Clark in 1862. The existence of some such
disturbing body was previously suspected by astronomers, owing to
observed irregularities in the proper motion of Sirius. Several
orbits, giving periods of about fifty years, have been computed.
The great brilliancy of Sirius, the brightest star in the heavens,
naturally suggests a sun of great size. Recent investigations do not
favor this idea. Its spectrum is, however, of the first type, and
the star is therefore not comparable with the sun in brilliancy. The
above result would indicate that stars of the first, or Sirian type,
are intrinsically brighter than our sun.
Sirius is about eleven magnitudes brighter than its faint companion.
This makes the light of Sirius about 25,000 times the light of
the small star. The two bodies must, therefore, be differently
constituted, and, indeed, the companion must be nearly a dark body.
If Sirius has any planets revolving round it—like those of our solar
system—they must forever remain invisible in our largest telescopes.
This remark, of course, applies to all the fixed stars, single and
double. They may possibly have attendant families of planets, like
our sun, but if so, the fact can never be ascertained by direct
observation.
The star Zeta Cancri is a well-known triple star, the close pair
revolving in a period of about sixty years. Nearly two revolutions
have now been completed since its discovery by Sir William Herschel
in 1781. All three stars probably form a connected system, but the
motion of the third star round the binary pair is very slow and
irregular.
[Illustration: Fig. 18.—System of the Double Sun Alpha Centauri]
Another interesting binary star is Xi Ursæ Majoris. As already
stated, this was the first pair for which an orbit was computed. More
than a complete revolution has now been performed since its discovery
by Sir William Herschel in 1780. The period has, therefore, been well
determined, and seems to be about sixty years.
The bright southern star, Alpha Centauri, the nearest of all the
fixed stars to the earth, so far as is known at present, is also
a remarkable binary star. It seems to have been first noticed as a
double star by Richaud in 1690.
Assuming my value of the sun’s stellar magnitude (about 27), I find
that the sun, if placed at the distance of Alpha Centauri, would
appear of about the same brightness as the star does to us. As,
according to Professor Pickering, the spectrum of Alpha Centauri is
of the second or solar type, it would seem that in mass, brightness,
and physical condition the star closely resembles our sun.
We next come to another very interesting binary star, known to
astronomers as 70 Ophiuchi. It is a very fine double star, the
magnitudes of the components being about four and six, and the
colors yellow and orange. More than a complete revolution has now
been described by the components since its discovery by Sir William
Herschel in 1779. Placed at the distance indicated by Krüger’s
parallax, I find that our sun would be reduced to a star of about
magnitude 3½, which shows that the sun and star are of about equal
brightness. The spectrum is of the solar type, according to Vogel.
A very famous binary star is that known to astronomers as Gamma
Virginis. Its history is a very interesting one. It lies close to the
celestial equator, about one degree to the south and about fifteen
degrees to the northwest of the bright star Spica (Alpha of the same
constellation), with which it forms the stem of a Y-shaped figure
formed by the brightest stars of the constellation Virgo, or the
Virgin, Gamma being at the junction of the two upper branches. The
brightness of Gamma Virginis is a little greater than an average
star of the third magnitude. Variation of light has, however, been
suspected in one or both components. The Persian astronomer, Al-Sûfi,
in his description of the heavens, written in the Tenth Century,
rates it of the third magnitude, and describes it as “the third of
the stars of _al-auvâ_, which is a mansion of the moon,” the first
and second stars of this “mansion” being Beta and Eta Virginis, the
fourth star Delta, and the fifth Epsilon, these five stars forming
the two upper branches of the Y-shaped figure above referred to.
Gamma was called _Zawiyah-al-auvâ_, “the corner of the barkers!”
perhaps from its position in the figure, which formed the thirteenth
Lunar Mansion of the old astrologers. It was also called _Porrima_
and _Postvarta_ in the old calendars. The fact that Gamma Virginis
really consists of two stars very close together seems to have been
discovered by the famous astronomer, Bradley, in 1718. The rapid
decrease in the apparent distance from 1780-1834 indicated that the
apparent orbit is very elongated, and that possibly the two stars
might “close up” altogether, and appear as a single star even in
telescopes of considerable power. This actually occurred in the year
1836, or, at least, the stars were then so close together that the
most powerful telescopes of that day failed to show Gamma Virginis as
anything but a single star. Of course, it would not have been beyond
the reach of the giant telescopes of our day. From the year 1836 the
pair began to open out again.
Another interesting binary star is Eta Cassiopeiæ. Periods ranging
from 149 to 222½ years have been found by different computers. The
most recent computation makes it about 196 years.
The bright star Gamma Leonis, situated in the well-known “Sickle in
Leo,” is also a binary star, but only a small portion of the orbit
has been described since its discovery by Sir William Herschel in
1782. Dr. Doberck finds a period of 407 years. It is remarkable for
its very high “relative brightness.” This pair forms a fine object
for a small telescope.
The star known as 12 Lyncis is a triple star, the components being
5, 6, and 7½ magnitude. The close pair forms a binary system, for
which an orbit has been computed by the present writer, who finds a
period of about 486 years. Sir John Herschel predicted in 1823 that
the angular motion of the pair would “bring the three stars into a
straight line in 57 years.” This prediction was fulfilled in 1887,
when measures by Tarrant showed that the stars were then exactly in a
straight line.
The bright star Castor is a famous double star, and has been known
since the year 1718, when it was observed by Bradley and Pond. It was
also observed by Maskelyne in 1759, and frequently by Sir William
Herschel from 1799 to 1803. Numerous orbits have been computed, with
periods ranging from 199 years by Mädler and 1,001 years by Doberck.
I find that the mass of the system of Castor is only 1/19th of the
sun’s mass, a result which would imply that the components are masses
of glowing gas! Dr. Bélopolsky has found, with the spectroscope,
that the brighter component is a close binary star with a dark
companion, like Algol. The period of revolution is about three days,
and the relative orbital velocity about 20¾ miles a second. Dr.
Bélopolsky’s observations show that the system is receding from the
earth at the rate of about 4½ miles per second.
With reference to the colors of the components of binary stars, the
following relation between color and relative brightness has been
established:
(1) When the magnitudes of the components are equal, or approaching
equality, the colors are generally the same, or similar.
(2) When the magnitudes of the components differ considerably, there
is also a considerable difference in color.
A new class of binary stars has been discovered within the last
few years by means of the spectroscope. These have been called
“spectroscopic binaries,” and the brighter component of Castor,
referred to above, is an example of the class. They are supposed to
consist of two component stars, so close together that the highest
powers of the largest telescopes fail to show them as anything but
single stars. Indeed, the velocities indicated by the spectroscope
show that they must be so close that the components must forever
remain invisible by the most powerful telescopes which could ever be
constructed by man. In some of these remarkable objects, the doubling
of the spectral lines indicates that the components are both bright
bodies, but in others, as in Algol, the lines are merely shifted from
their normal position, not doubled, thus denoting that one of the
components is a dark body. In either case, the motion in the line
of sight can be measured by the spectroscope, and we can, therefore,
calculate the actual dimensions of the system in miles, and thence
its mass in terms of the sun’s mass, although the star’s distance
from the earth remains unknown. Judging, however, from the brightness
of the star, and the character of its spectrum, we can make an
estimate of its probable distance from the earth.
The bright star Spica has also been found by the spectroscope to be a
close binary star. Vogel finds a period of four days with a distance
between the components of about 6¼ millions of miles, and assuming
that the components have equal mass and are moving in a circular
orbit, he finds the mass of the system about 2.6 times the mass of
our sun. In addition to its orbital motion, Vogel finds that Spica is
approaching the sun at the rate of over nine miles per second.
To ordinary observers, the light of the stars seems to be constant.
Even to those who are familiar with the constellations, the stars
appear to maintain their relative brilliancy unchanged. To a great
extent this is, of course, true; the great majority of the stars
remaining of the same brightness from day to day, and from year to
year. There are, however, numerous exceptions to this rule. Many
of the stars, when carefully watched, are found to fluctuate in
their light, being sometimes brighter and sometimes fainter. These
are known as “variable stars”—one of the most interesting class of
objects in the heavens. Some of these have been known for a great
number of years, and their variations having been carefully watched,
the laws governing their light changes have been well determined.
We will first consider the variable stars with long periods of
variation, as these generally show the largest fluctuations of light.
Among these, the first star in which variation of light seems to have
been noticed is the extraordinary object, Omicron Ceti, popularly
known as Mira, or the “wonderful” star. It appears to have been first
noticed by David Fabricius in the year 1596. He observed that the
star now called Omicron, in the constellation Cetus, was of the third
magnitude on April 13 of that year, and that in the following year it
had disappeared. Bayer saw it again in 1603, when forming his maps
of the constellations, and assigned to it the Greek letter Omicron,
but does not seem to have noticed the fact that it was the same star
which had been observed by Fabricius seven years previously. No
further attention seems to have been paid to it until 1638 and 1639,
when it was observed at Francker by Professor Phocylides Holwarda
to be of the third magnitude in December, 1638, invisible in the
following summer, and again visible in October, 1639. From 1648 to
1662 it was carefully observed by Hevelius, and in subsequent years
by several observers. Its variations are now regularly followed
from year to year, and it forms one of the most interesting objects
of its kind in the heavens. Its light varies from about the second
magnitude to the ninth, but its brightness at maximum is variable to
a considerable extent.
Perhaps the long period variable star next in order of interest—at
least to observers in the Northern Hemisphere—is that known as
Chi Cygni. It was discovered by Kirch in 1686. The star varies at
maximum from 4 to 6½ magnitude, and at the minimum it sinks to below
the thirteenth magnitude. At some maxima, therefore, it is easily
visible to the naked eye, and at others it is just below the limit
of ordinary vision. At the maximum of 1847, it was visible to the
naked eye for a period of 97 days. The average period is about 406
days; but according to Schönfeld—a well-known authority on the
variables—observations indicate a small lengthening of the period.
Chi Cygni is said to be “strikingly variable in color.” Espin’s
observations in different years show it “sometimes quite red, at
others only pale orange-red.” In the spectroscope, its light shows
a splendid spectrum of the third type (or banded spectrum, very
characteristic of these long period variables), in which bright lines
were observed by Espin in May, 1889.
R Leonis is another remarkable variable star, which is sometimes
visible to the naked eye at maximum. It lies closely south of the
star known as 19 Leonis. It was discovered by Koch in 1782. At the
maximum, its brightness varies from 5.2 to 7 magnitude, and at
minimum it fades to about the tenth magnitude. The mean period is
about 313 days. The star is red in all phases of its light, and forms
a fine telescopic object. Close to it are two small stars, which
form, with the variable, an isosceles triangle.
There is a very remarkable variable star in the Southern Hemisphere
known as Eta Argûs. It lies in the midst of the great nebula in Argo,
and the history of its fluctuations in light is very interesting.
Observed by Halley in 1677 as a star of the fourth magnitude, it was
seen of the second magnitude by Lacaille in 1751. After this, it must
have again faded, for Burchell found it of only the fourth magnitude
from 1811 to 1815. From 1822 to 1826 it was again of the second
magnitude, as observed by Fallows and Brisbane; but on February 1,
1827, it was estimated of the first magnitude by Burchell. It then
faded again, for on February 29, 1828, Burchell found it of the
second magnitude. From 1829 to 1833 Johnson and Taylor rated it of
the second magnitude; and it was still of this magnitude, or a little
brighter, when Sir John Herschel commenced his observations at the
Cape of Good Hope in 1834. It does not seem to have varied much in
brightness from that time until December, 1837, when Herschel was
astonished to find its light “nearly tripled.” He says: “It very
decidedly surpassed Procyon, which was about the same altitude,
and was far superior to Aldebaran. It exceeded Alpha Orionis, and
the only star (Sirius and Canopus excepted) which could at all be
compared with it was Rigel.”
From this time its light continued to increase. On the 28th December
it was far superior to Rigel, and could only be compared with Alpha
Centauri, which it equaled, having the advantage of altitude, but
fell somewhat short of it as the altitudes approached equality. The
maximum of brightness seems to have been obtained about the 2d of
January, 1838, on which night, both stars being high and the sky
clear and pure, it was judged to be very nearly matched, indeed,
with Alpha Centauri. In 1843 it again increased in brightness, and
in April of that year it was observed by Maclear to be brighter than
Canopus, and nearly equal to Sirius! It then faded slightly, but
seems to have remained nearly as bright as Canopus until February,
1850, since which time its brilliancy gradually decreased. It was
still of the first magnitude in 1856, according to Abbott, but was
rated a little below the second magnitude by Powell in 1858. Tebbutt
found it of the third magnitude in 1860; Abbott a little below the
fourth in 1861. Ellery rated it fifth magnitude in 1863, and Tebbutt
sixth magnitude in 1867. In 1874 it was estimated 6.8 magnitude at
Cordoba, and only 7.4 in November, 1878. Tebbutt’s observations from
1877-86 show that it did not rise above the seventh magnitude in
those years, and in March, 1886, it was rated 7.6 magnitude by Finlay
at the Cape of Good Hope. This seems to have been the minimum of
light, for in May, 1888, Tebbutt found that it “had increased fully
half a magnitude” since April, 1887. The star is very reddish in
color.
We will now consider the variables of short period, which are
particularly interesting objects, owing to the comparative rapidity
of their light changes. The periods vary in length from about 17¼
days down to a few hours. Perhaps the most interesting of these short
period variables, at least to the amateur observer, is the star Beta
Lyræ, which is easily visible to the naked eye in all phases of its
light. It can be readily identified, as it is the nearest bright star
to the south of the brilliant Vega, and one of two stars of nearly
the same magnitude, the second being Gamma Lyræ. The variability of
Beta Lyræ was discovered by Goodricke in the year 1784. The period is
about 12 days, 21 hours, 46 minutes, 58 seconds. Recent observations
with the spectroscope indicate that the star is a very close double
or “spectroscopic binary,” although it does not seem certain that
an actual eclipse of one component by the other takes place, as in
the case of Algol. Bright lines were detected in the star’s spectrum
by Secchi so far back as 1866. In 1883 M. Von Gothard noticed that
the appearance of these bright lines varied in appearance, and from
an examination of photographs taken at Harvard Observatory in 1891,
Mrs. Fleming found displacements of bright and dark lines in a double
spectrum, the period of which agreed fairly well with that of the
star’s light changes.
Another interesting star of short period is Delta Cephei, which is
one of three stars forming an isosceles triangle a little to the
west of Cassiopeia’s Chair, the variable being at the vertex of the
triangle, and the nearest of the three to Cassiopeia. Its variability
was also discovered by Goodricke in 1784. It varies from 3.7 to 4.9
magnitude, with a period of 5 days, 8 hours, 47 minutes, 40 seconds.
The amount of the variation is, therefore, the same as in the case
of Algol, the star’s light at maximum being about three times its
light at minimum. The observations also show that Delta Cephei is
approaching the earth at the rate of about 8¾ miles a second. The
color of the star is yellow, and it has a distant bluish companion
of about the fifth magnitude, which may possibly have some physical
connection with the brighter star, as both stars have a common proper
motion through space.
Another remarkable star of short period is Eta Aquilæ, the
variability of which was discovered by Pigott in 1784. It varies from
magnitude 3.5 to 4.7, with a period of 7 days, 4 hours, 14 minutes,
but Schönfeld found marked deviations from a uniform period. Its
color is yellow, and its spectrum, like that of Delta Cephei, of the
second or solar type.
A remarkable variable star of short period was discovered in 1888
by Mr. Paul in the southern constellation Antlia. It varies from
magnitude 6.7 to 7.3, with the wonderfully short period of 7 hours,
46 minutes, 48 seconds, all the light changes being gone through
no less than three times in twenty-four hours! It was for some
years believed that the variation was of the Algol type, but recent
measures made at the Harvard College Observatory show that it belongs
to the same class as Delta Cephei and Eta Aquilæ.
A telescopic variable with a wonderfully short period was discovered
by Chandler in 1894. It lies a little to the west of the star Gamma
Pegasi, and has been designated U Pegasi. It varies from magnitude
8.9 to 9.7, and was first supposed to be of the Algol type with a
period of about two days, but further observations showed that the
period was much shorter, and only 5 hours, 31 minutes, 9 seconds.
The remarkable rapidity of its light changes, which are gone through
four times in less than twenty-four hours, make this remarkable star
a most interesting object. Possibly there may be other stars in the
heavens with a similar rapidity of variation which have hitherto
escaped detection.
Unlike the variable stars of long period which seemed scattered
indifferently over the surface of the heavens, the great majority of
the short period variables are found in a zone which nearly coincides
with the course of the Milky Way. The most notable exceptions to this
rule are W Virginis with the comparatively long period of 17¼ days,
and U Pegasi, above described, which has the shortest known period
of all the variable stars. Another peculiarity is that most of them
are situated in what may be called the following hemisphere, that is
between 12 hours and 24 hours of right ascension. The most remarkable
exception to this rule is Zeta Geminorum.
Algol, or Beta Persei, is a famous variable star, and the typical
star of the class to which it belongs. Its name, Algol, is derived
from a Persian word, meaning the “demon,” which suggests that
the ancient astronomers may have detected some peculiarity in
its behavior. The real discovery of its variation was, however,
made by Montanari in 1667, and his observations were confirmed by
Maraldi in 1692. Its fluctuations of light were also noticed by
Kirch and Palitzsch, but the true character of its variations was
first determined by the English astronomer, Goodricke, in 1782. Its
fluctuations of light are very curious and interesting. Shining with
a constant, or nearly constant, brightness for a period of about
59 hours as a star of a little less than the second magnitude, it
suddenly begins to diminish in brightness, and in about 4½ hours
it is reduced to a star of about magnitude 3½. In other words, its
light is reduced to about one-third of its normal brightness. If we
suppose three candles placed side by side at such a distance that
their combined light is merged into one, and equal to the usual
brightness of Algol, then, if two of these candles are extinguished,
the remaining candle will represent the light of Algol at its minimum
brilliancy. The star remains at its minimum, or faintest, for only
about 15 minutes. It then begins to increase, and in about 5 hours
recovers its normal brightness, all the light changes being gone
through in a period of about 10 hours out of nearly 69 hours, which
elapse between successive minima. These curious changes take place
with great regularity, and the exact hour at which a minimum of light
may be expected can be predicted with as much certainty as an eclipse
of the sun.