fading away in brightness until it dissolves in the blackness of
the sky; as we look at the multitudes of bright stars which sparkle
out from the depths of the great glowing gas; as we ponder on the
marvelous outlines of a portion of the nebula, we are tempted to
ask what the true magnitude of this object must really be. Here,
again, we have to confess that science is unable to satisfy this very
legitimate curiosity. The only means of learning the true length
and breadth of a celestial object depends upon our first having
discovered the distance from us at which the object is situated.
Unhappily we are, as I have said, entirely ignorant of what this
distance may be in the case of the Great Nebula in Orion. Our
ordinary methods of conducting such an inquiry are hardly applicable
to such an object, and its position so near the Equator introduces
fresh difficulties into the problem. We shall, however, certainly not
err on the side of exaggeration if we assert that the Great Nebula
must be many millions of times larger than that group of bodies which
we call the Solar System.
COLORED, DOUBLE, MULTIPLE, BINARY, VARIABLE AND TEMPORARY STARS.
—J. E. GORE
On a clear night a careful observer will notice a marked difference
in the colors of the brighter stars. The brilliant white or
bluish-white light of Sirius, Rigel, and Vega contrasts strongly
with the yellowish color of Capella, the deeper yellow, or orange,
of Arcturus, and the ruddy light of Aldebaran and Betelgeuse. These
colors are, however, limited to various shades of yellow and red.
No star of a _decided_ blue or green color is known, at least among
those visible to the naked eye in the Northern Hemisphere. The third
magnitude star Beta Libræ is described by Webb as of a “beautiful
pale green hue,” but probably such a tint in the light of this star
will to most people prove quite imperceptible. Dr. Gould, observing
it in the Southern Hemisphere—under, of course, more favorable
conditions—says: “There is a decidedly greenish tinge to the light
of Beta Libræ, although its color can not properly be called
conspicuous.”
Among the ruddy stars visible to the naked eye, Mu Cephei, Herschel’s
“garnet star,” is generally admitted to be the reddest, but it is
not sufficiently bright to enable its color to be well distinguished
without the aid of an opera-glass. With such an instrument, however,
its reddish hue is striking and beautiful, and very remarkable when
compared with other stars in its vicinity. Like so many of the red
stars, Mu Cephei is variable in its light, but seems to have no
regular period, and often remains for many weeks without perceptible
change. It may be seen near the zenith in the early evening hours
toward the end of October, and when in this position its ruddy color
is very conspicuous.
Among the brightest stars, Betelgeuse is perhaps the reddest, and
the contrast between its ruddy tint and the white color of Rigel in
the same constellation (Orion) is very noticeable. Like Mu Cephei,
Betelgeuse is irregularly variable in its light, but not to such
an extent, and, like the “garnet star,” it frequently remains for
protracted periods nearly constant in brightness. There are other
cases of reddish color among the naked-eye stars. Among these may
be mentioned Antares (Alpha Scorpii), Alphard (Alpha Hydræ), noted
as red by the Persian astronomer Al-Sûfi, in the Tenth Century, and
called by the Chinese “The Red Bird”; Eta and Mu Geminorum; Mu and
Nu Ursæ Majoris; Delta and Lambda Draconis; Beta Ophiuchi; Gamma
Aquilæ, and others in the Southern Hemisphere.
But it is among the stars below the limit of naked-eye vision that
we meet with the finest examples of the red stars. Some of these are
truly wonderful objects. The small star, No. 592 of Birmingham’s
Catalogue of Red Stars (No. 713 of Espin’s edition), which lies
a little south of the 5½ magnitude star 79 Cygni, was described
as “splendid red” by Birmingham, “very deep red” by Copeland and
Dreyer, and “orange vermilion” by Franks. The star 248 Birmingham,
which lies about 5° south of Gamma Hydræ, is another fine specimen.
Birmingham described it as “fine red” and “ruby”; Copeland as “brown
red”; Dreyer as “copper red”; and Espin as “magnificent blood red.”
This star is variable in light, as the estimates of magnitude range
from 6.7 to below 9. About 3° to the northeast of this remarkable
object is another highly-colored star, known as R Crateris. It is
easily found, as it lies in the same telescopic field of view with
Alpha Crateris, a 4½ magnitude star. Sir John Herschel described it
as “scarlet, almost blood-color; a most intense and curious color.”
Birmingham called it “crimson”; and Webb “very intense ruby.”
Observing it with a 3-inch refractor in India in 1875, I noted it as
“full scarlet.” It varies in light from above the eighth magnitude to
below the ninth, and has near it a star of the ninth magnitude of a
paler blue tint.
Another very red star is No. 4 of Birmingham’s Catalogue, which will
be found about 5° north, preceding the great nebula in Andromeda. It
is of about the eighth magnitude, and may be well seen with a 3-inch
refractor. Krüger describes it as “_intensiv roth_”; Birmingham as
“fine red” and “crimson”; Franks as “fine color, almost vermilion”;
and Espin as “intense red color, most wonderful.”
Another fine object is R Leporis, which forms roughly an equilateral
triangle with Kappa and Mu Leporis. This is also variable from 6½
to 8½ magnitude. It was discovered by Hind in 1845, and described
by him as “of the most intense crimson, resembling a blood-drop on
the background of the sky; as regards depth of color, no other star
visible in these latitudes could be compared with it.” Schönfeld
called it “_intensiv blutroth_,” but Dunér, observing its spectrum in
1880, gives its color as a less intense red than that of other stars.
Possibly it may vary in color as well as in light.
The variable star U Cygni, which lies between Omicron and Omega
Cygni, is also very red. Webb described it as showing “one of the
loveliest hues in the sky.” It varies from about the seventh to 11½
magnitude, with a period of about 461 days.
Another deeply colored star is the well-known variable R Leonis.
Hind says: “It is one of the most fiery-looking variables on our
list—fiery in every stage from maximum to minimum, and is really
a fine telescopic object in a dark sky about the time of greatest
brilliancy, when its color forms a striking contrast with the steady
white light of the sixth magnitude a little to the north.” This
latter star is 19 Leonis.
In the Southern Hemisphere there are some fine examples of red stars.
Epsilon Crucis, one of the stars in the Southern Cross, is very red.
Mu Muscæ is described by Dr. Gould as of “an intense orange red.”
Delta^2 Gruis is a very reddish star of about the fourth magnitude.
Pi^1 Gruis was observed by Gould as “deep crimson,” and forming a
striking contrast with its white neighbor Pi^2 Gruis, which he notes
as “conspicuously white.” The variable L_{2} Puppis is described as
“red in all its stages, and remarkably so when faint.” Miss Clerke,
observing—at the Cape of Good Hope—R Doradûs, another southern
variable, says: “This extraordinary object strikes the eye with the
glare of a stormy sunset,” and with reference to the variable R
Sculptoris, described by Gould as “an intense scarlet,” she says:
“The star glows like a live coal in the field,” a description I have
found myself very applicable to other small red stars.
An eighth magnitude star about 5° north of Beta Pictoris is noted by
Sir John Herschel, in his _Cape Observations_, as “vivid sanguine
red, like a blood-drop. A superb specimen of its class.” With
reference to a star of about 8½ magnitude in the field with Beta
Crucis, Herschel says: “The fullest and deepest maroon red; the most
intense blood-red of any star I have seen. It is like a drop of blood
when contrasted with the whiteness of Beta Crucis.”
Of stars of other colors, the asserted green tint of Beta Libræ has
already been referred to. Among the brighter stars of the Southern
Hemisphere, Theta Eridani, Epsilon Pavonis, Upsilon Puppis, and
Gamma Tucanæ are said to be decidedly blue. The wonderful cluster
surrounding the star Kappa Crucis contains several bluish, greenish
and red stars, and is described by Sir John Herschel as resembling “a
superb piece of fancy jewelry.”
Among the double stars we find many examples of colored suns. Of
these may be mentioned Epsilon Boötis, of which the colors are
“most beautiful yellow” and “superb blue,” according to Secchi;
Beta Cephei, “yellow and violet”; Beta Cygni, “golden yellow and
smalt blue”; Gamma Delphini, of which I noted the colors in 1874
as “reddish yellow and grayish lilac”; Alpha Herculis, “orange and
emerald or bluish green,” and described by Admiral Smyth as “a lovely
object, one of the finest in the heavens”; Zeta Lyræ, “pale yellow
and lilac” (Franks); and Beta Piscis Australis, of which I observed
the colors in India as white and reddish lilac.
Some distant telescopic companions to red stars have been described
as blue. This may be in some case due, partly at least, to the effect
of contrast. In others the blue color seems to be real. This has been
shown spectroscopically to be the case with the bluish companions of
Beta Cygni.
The physical cause of the difference of color is still more or less
a matter of mystery. Although we can not consider it proved that the
red stars are cooling and “dying out” suns, as has been suggested,
we may, I think, conclude that their temperature, although doubtless
very high, must be lower than that of the white stars. We know that
a bar of iron when heated to redness is not so hot as when raised to
“white heat,” and although the analogy between hot iron and stellar
photospheres may not be a perfect one, it seems probable that the
higher the temperature of a star, the whiter its color will be.
Most of the white stars, as Sirius, Vega, and those only yellow or
slightly colored, show spectra of Secchi’s first and second types,
while the great majority of the red stars exhibit banded spectra of
the third and fourth types.
To this rule there are, however, like other rules, some notable
exceptions. For instance, Aldebaran, Alpha Hydræ, Xi Cygni, and 31
Orionis, although distinctly reddish stars, show well-marked spectra
of the second or solar type. On the other hand, Rho Ursæ Majoris and
Omega Virginis, which, according to Dunér, are only slightly yellow,
have well-marked spectra of the third type.
An apparent change of color seems in some cases to be well
established. The supposed red color of Sirius in ancient times is
well known. A certain established change is found in the case of the
famous variable star Algol, which is distinctly described as red by
Al-Sûfi in the Tenth Century. It is now pure white, or nearly so, and
this is probably the best attested instance on record of change of
color in a bright star.
Schmidt’s Nova Cygni of 1876 was noted as “golden yellow” on the
night of its discovery. When it had faded to the eighth magnitude,
Dr. Copeland called it “decided red,” but when examined at Lord
Crawford’s observatory in September, 1877, its color was recorded as
“faint blue”! The new star in the Andromeda nebula was considered to
be yellowish or reddish by most observers when near its maximum, but
about a month later its color was noted as “bluish.”
Among the red and variable stars, there are many suspected cases
of color variation. Espin and other observers have noted that the
wonderful variable Mira Ceti is much less red at maximum than at
minimum. My own observations confirm this. When at its maximum
brightness, Mira does not seem to me a very highly-colored star,
while at one of its minima I noted it as “fiery red.” Possibly,
however, the great difference between its maximum and minimum
brilliancy may have an influence on estimations of its color. The
remarkable variable 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.” The star Birmingham 118
was described by Schjellerup in 1863 as “decided red,” but it was
found yellow by Secchi in 1868; “bluish” by Birmingham, 1873-76; “no
longer red” by Schjellerup in March, 1876; and “white” by Franks
in 1885. Espin omits it from his revised edition of Birmingham’s
Catalogue.
Birmingham 169 was found red by Struve, blue or bluish-white by
Birmingham in 1874, and white at Greenwich in the same year. Espin
also saw it white in March, 1888. The star Birmingham 30, which lies
close to Phi Persei (54 Andromedæ), was described by Schweizer
as a “red star with a little disk” in January, 1843; Birmingham
noted it as “light red” in December, 1875; Copeland “deep red” in
January, 1876; and Dreyer “reddish” in September, 1878; but Espin,
in November and December, 1887, found it “certainly not red, and
nothing peculiar in the star’s appearance.” It might be expected
that these curious changes of color, if real, would be accompanied
by corresponding changes in the star’s spectrum. Such may be the
case, and observations in this direction would probably lead to some
interesting results.
There seems to be some law governing the distribution of the colored
stars. The white stars appear to be most numerous, as a rule, in
those constellations where bright stars are most abundant, for
instance, in Orion, Cassiopeia, and Lyra; yellow and orange stars in
large and ill-defined constellations, such as Cetus, Pisces, Hydra,
Virgo, etc. The very reddish stars are most numerous in or near the
Milky Way, and one portion of the Galaxy—between Aquila, Lyra, and
Cygnus—was termed by Birmingham “the red region in Cygnus.”
Many of the stars when examined with a good telescope are seen to be
double, some triple, and a few quadruple, and even multiple. These
when viewed with the naked eye, or even a powerful binocular, seem
to be single, and show no sign of consisting of two components.
These telescopic double stars should be carefully distinguished
from those which appear very close together with the naked eye,
and which in opera-glasses or telescopes of small power might be
mistaken for wide double stars by the inexperienced observer. These
latter stars, such as Mizar—the middle star in the tail of the Great
Bear—and its small companion, Alcor, have been called “naked-eye
doubles,” but they are not, properly speaking, double stars at all.
Telescopic double stars are far closer, and even the widest of them
could not possibly be seen double without optical aid, even by
those who are gifted with the keenest vision. Of these so-called
“naked-eye doubles,” we may mention Alpha Capricorni, which on a
very clear night may be seen with the naked eye to consist of two
stars. On a very fine night two stars may be seen in Iota Orionis,
the most southern star in Orion’s Sword. The star Zeta Ceti has near
it a fifth magnitude star, Chi, which may be easily seen with the
unaided vision. The star Epsilon Lyræ (near Vega) is a severe test
for naked-eye vision. Bessel, the famous German astronomer, is said
to have seen it when thirteen years of age. Omicron Cygni (north of
Alpha and Delta Cygni) forms another naked-eye double, and other
objects of this class may be noticed by a sharp-eyed observer.
The star Mizar, already referred to, is itself a wide telescopic
double, and it seems to have been the first double star discovered
with the telescope (by Riccioli in 1650). It consists of two
components, of which one is considerably brighter than the other. It
will give an idea of the closeness of even a “wide” telescopic double
when we say that the apparent distance between Mizar and Alcor is
nearly forty times the distance which separates the close components
of the bright star. From this it will be seen that even a powerful
binocular field-glass would fail to show Mizar as anything but a
single star. The components may, however, be well seen with a 3-inch
telescope, or even with a good 2-inch. The colors of the two stars
are pale green and white. Between Mizar and Alcor is a star of the
eighth magnitude, and others fainter. Mizar was the first double star
photographed by Bond.
The Pole Star has a small companion at a little greater distance
than that which separates the components of Mizar, but owing to the
faintness of this small star, the object is not so easy as Mizar.
The star Beta Cygni is composed of a large and small star, of which
the colors are described as “golden yellow and smalt blue.” This is
a very wide double, and may be seen with quite a small telescope.
Another fine double star is that known to astronomers as Gamma
Andromedæ. The magnitudes of the components are about the same as
those of Mizar, but a little closer. Their colors are beautiful
(“gold and blue”). This is one of the prettiest double stars in the
heavens. It is really a triple star, the fainter of the pair being
a very close double star; but this is beyond the reach of all but
the largest telescopes. The star Gamma Delphini is another beautiful
object, the components being a little more unequal in magnitude, but
the distance between them about the same as in Gamma Andromedæ. I
have noted the colors with a 3-inch telescope as “reddish yellow and
grayish lilac.” Gamma Arietis, the faintest of the three well-known
stars in the head of Aries, is another fine double star, a little
closer than Gamma Delphini. This is an interesting object, from the
fact that it was one of the first double stars discovered with the
telescope—by Hooke, in 1664, when following the comet of that year.
Another beautiful double star is Eta Cassiopeiæ, the components
being about equal in brightness to those of Gamma Delphini, but the
distance less than one-half. The colors are, according to Webb,
yellow and purple; but other observers have found the smaller star
garnet or red. This is a very interesting object, the components
revolving round each other, and forming what is called a binary star.
Another fine double star is Castor, which is composed of two nearly
equal stars separated by a distance about half that between the
components of Gamma Andromedæ. This is also a binary, or revolving
double star, but the period is long. Gamma Virginis is another fine
double star, with components at about the same distance as those of
Castor, and the colors very similar. It is also a remarkable binary
star.
Among double stars of which the components are closer than those
mentioned above, but which are within the reach of a good 3-inch
telescope—a common size with amateur observers—the following may be
noticed: Alpha Herculis, colors, orange or emerald green; the light
of this star is slightly variable. Gamma Leonis, another binary star
with a long period; colors, pale yellow and purple. Epsilon Boötis,
a lovely double star, the colors of which Secchi described as “most
beautiful yellow, superb blue.”
For observers in the Southern Hemisphere, the following fine double
stars may be seen with a 3-inch telescope: Alpha Centauri; this
famous star, the nearest of all the fixed stars to the earth, is also
a remarkable binary; its period, as recently computed by Dr. See,
is eighty-one years. Theta Eridani is a splendid pair, but closer
than Alpha Centauri. It is, however, an easy object with a 3-inch
telescope, and with a telescope of this size I noted the colors in
India as light yellow and dusky yellow. The star known as ƒ Eridani
is a very similar double to Theta, but the components are fainter. I
noted the colors in India as yellowish-white and very light green.
Of triple, quadruple, and multiple stars, there are several which may
be well seen with a small telescope. Of these may be mentioned Iota
Orionis, the lowest star in the Sword of Orion, which consists of a
bright star accompanied by two small companions. In Theta Orionis,
the middle star of the Sword, four stars may be seen forming a
quadrilateral figure, known to observers as the “trapezium.” There
are two fainter stars in this curious object, which lie in the midst
of the Orion nebula, but a somewhat larger telescope is required
to see them. Within the trapezium are two very faint stars, which
are only visible in the largest telescopes. In Sigma Orionis—a star
closely south of Zeta, the lowest star in Orion’s Belt—six stars may
be seen with a 3-inch telescope.
Double and multiple stars may be either optical or real. Optical
double stars are those in which the component stars are merely
apparently close together, owing to their being seen in nearly the
same direction in space. Two stars may _seem_ to be close together,
while, in reality, one of them may be placed at an immense distance
behind the other. Just as two lighthouses at sea may, on a dark
night, appear close together when viewed from a certain point,
whereas they may be really miles apart. In the case of double stars
it is, of course, always difficult to determine whether the apparent
closeness of the stars is real or merely optical. But when, from a
long series of observations of their relative position, we find that
one is apparently moving round the other, we know that the stars must
be comparatively close, and linked together by some physical bond
of union. These most interesting objects are known to astronomers
as binary, or revolving double stars. The probable existence of
such objects was predicted from abstract reasoning by Mitchell in
the Eighteenth Century; but the discovery of their actual existence
was made by Sir William Herschel, while engaged on an attempt to
determine the distance of some of the double stars from the earth.
Unlike the planetary orbits, which are nearly circular, at least
those of the larger planets of the Solar System, it is found that the
orbits of these double stars differ, in many cases, widely from the
circular form, in some cases, indeed, approaching in shape more the
orbit of a comet than a planet.
The binary stars are among the most interesting objects in the
heavens. The number now known probably amounts to nearly one
thousand. In most of them, however, the motion is very slow, and
in only about seventy cases has the change of position, since their
discovery, been sufficient to enable an orbit to be computed.
Savary, in 1830, was the first astronomer who attempted to compute
the orbit of a binary star, namely, the star Xi Ursæ Majoris. This
remarkable pair was discovered by Sir William Herschel in 1780, and
as the period of revolution is about sixty-one years, a considerable
portion of the ellipse had been described in 1830, when it was
attacked by Savary.
The binary star with the shortest period known at present seems to be
the fourth magnitude star Kappa Pegasi. It was discovered as a wide
double star by Sir William Herschel in 1786, the companion star being
of the ninth magnitude. In August, 1880, Mr. Burnham, the famous
American double star observer, examining the star with the 18½-inch
refractor of the Dearborn Observatory, found the brighter star to be
a very close double, with a distance between the components of only
a quarter of a second of arc. A few years’ observations showed that
this pair were in rapid motion round each other (about eleven years).
Another binary star, with a period of about the same length, is Delta
Equulei, which was discovered to be a close double by Otto Struve
in 1851. Next in order of shortness of period comes the southern
binary star Zeta Sagittarii, for which an orbit was first computed
in the year 1886 by the present writer. The orbit of this star will,
I think, require still further revision, but the period of about
eighteen years is probably not far from the truth.
Another remarkably rapid binary star is 85 Pegasi. Next in order of
rapidity of motion we have the southern binary star 9 Argûs.
The star 42 Comæ Berenices has a period of about 25¾ years, according
to Otto Struve. The orbit is remarkable from the fact that its plane
passes through, or nearly through, the earth, and is, therefore,
projected into a straight line, the companion star oscillating
backward and forward on each side of its primary.
The star Beta Delphini—the most southern of the four stars in the
“Dolphin’s Rhomb”—is also a fast-moving binary, discovered by
Burnham in 1873. Burnham thinks the period will prove to be about
twenty-eight years. The spectrum of the light of Beta Delphini
is similar to that of our sun, so that the two bodies should be
comparable in intrinsic brilliancy.
Another remarkable binary star with a comparatively short period
is Zeta Herculis. This pair have now performed three complete
revolutions since their discovery in 1782 by Sir William Herschel.
Several orbits have been computed, but Dr. See’s period of
thirty-five years is probably the best. The companion is, however,
rather faint, being only 6½ magnitude, while the primary star is of
the third.