VIRGO AND HER NEIGHBORS
... "that region
Where still by night is seen
The Virgin goddess near to bright Booetes."--POSTE'S ARATUS.
[Illustration: MAP NO. 8.]
Following the order of right ascension, we come next to the little
constellations Crater and Corvus, which may be described as standing on
the curves of Hydra (map No. 8). Beginning with Crater, let us look
first at alpha, a yellow fourth-magnitude star, near which is a
celebrated red variable R. With a low power we can see both alpha and R
in the same field of view, like a very wide double. There is a third
star of ninth magnitude, and bluish in color, near R on the side toward
alpha. R is variable both in color and light. When reddest, it has been
described as "scarlet," "crimson," and "blood-colored"; when palest, it
is a deep orange-red. Its light variation has a period the precise
length of which is not yet known. The cycle of change is included
between the eighth and ninth magnitudes.
While our three-inch telescope suffices to show R, it is better to use
the five-inch, because of the faintness of the star. When the color is
well seen, the contrast with alpha is very pleasing.
There is hardly anything else in Crater to interest us, and we pass over
the border into Corvus, and go at once to its chief attraction, the star
delta. The components of this beautiful double are of magnitudes three
and eight; distance 24", p. 211 deg.; colors yellow and purple.
The night being dark and clear, we take the five-inch and turn it on the
nebula 3128, which the map shows just under the border of Corvus in the
edge of Hydra. Herschel believed he had resolved this into stars. It is
a faint object and small, not exceeding one eighth of the moon's
diameter.
Farther east in Hydra, as indicated near the left-hand edge of map No.
8, is a somewhat remarkable variable, R Hydrae. This star occasionally
reaches magnitude three and a half, while at minimum it is not much
above the tenth magnitude. Its period is about four hundred and
twenty-five days.
[Illustration: MAP NO. 9.]
While we have been examining these comparatively barren regions, glad to
find one or two colored doubles to relieve the monotony of the search, a
glittering white star has frequently drawn our eyes eastward and upward.
It is Spica, the great gem of Virgo, and, yielding to its attraction, we
now enter the richer constellation over which it presides (map No. 9).
Except for its beauty, which every one must admire, Spica, or alpha
Virginis, has no special claim upon our attention. Some evidence has
been obtained that, like beta Aurigae and Capella, it revolves with an
invisible companion of great mass in an orbit only six million miles in
diameter. Spica's spectrum resembles that of Sirius. The faint star
which our larger apertures show about 6' northeast of Spica is of the
tenth magnitude.
Sweeping westward, we come upon Sigma 1669, a pretty little double with
nearly equal components of about the sixth magnitude, distance 5.6", p.
124 deg.. But our interest is not fully aroused until we reach gamma, a star
with a history. The components of this celebrated binary are both
nearly of the third magnitude, distance about 5.8", p. 150 deg.. They
revolve around their common center in something less than two hundred
years. According to some authorities, the period is one hundred and
seventy years, but it is not yet certainly ascertained. It was noticed
about the beginning of the seventeenth century that gamma Virginis was
double. In 1836 the stars were so close together that no telescope then
in existence was able to separate them, although it is said that the
disk into which they had merged was elongated at Pulkowa. In a few years
they became easily separable once more. If the
one-hundred-and-seventy-year period is correct, they should continue to
get farther apart until about 1921. According to Asaph Hall, their
greatest apparent distance is 6.3", and their least apparent distance
0.5"; consequently, they will never again close up beyond the separating
power of existing telescopes.
There is a great charm in watching this pair of stars even with a
three-inch telescope--not so much on account of what is seen, although
they are very beautiful, as on account of what we know they are doing.
It is no slight thing to behold two distant stars obeying the law that
makes a stone fall to the ground and compels the earth to swing round
the sun.
In theta we discover a fine triple, magnitudes four and a half, nine,
and ten; distances 7", p. 345 deg., and 65", p. 295 deg.. The ninth-magnitude
star has been described as "violet," but such designations of color are
often misleading when the star is very faint. On the other hand it
should not be assumed that a certain color does not exist because the
observer can not perceive it, for experience shows that there is a wide
difference among observers in the power of the eye to distinguish color.
I have known persons who could not perceive the difference of hue in
some of the most beautifully contrasted colored doubles to be found in
the sky. I am acquainted with an astronomer of long experience in the
use of telescopes, whose eye is so deficient in color sense that he
denies that there are any decided colors among the stars. Such persons
miss one of the finest pleasures of the telescope. In examining theta
Virginis we shall do best to use our largest aperture, viz., the
five-inch. Yet Webb records that all three of the stars in this triple
have been seen with a telescope of only three inches aperture. The
amateur must remember in such cases how much depends upon practice as
well as upon the condition of the atmosphere. There are lamentably few
nights in a year when even the best telescope is ideally perfect in
performance, but every night's observation increases the capacity of the
eye, begetting a kind of critical judgment which renders it to some
extent independent of atmospheric vagaries. It will also be found that
the idiosyncrasies of the observer are reflected in his instrument,
which seems to have its fits of excellence, its inspirations so to
speak, while at other times it behaves as if all its wonderful powers
had departed.
Another double that perhaps we had better not try with less than four
inches aperture is 84 Virginis. The magnitudes are six and nine;
distance, 3.5", p. 233 deg.. Colors yellow and blue. Sigma 1846 is a
fifth-magnitude star with a tenth-magnitude companion, distance only 4",
p. 108 deg.. Use the five-inch.
And now we approach something that is truly marvelous, the "Field of the
Nebulae." This strange region, lying mostly in the constellation Virgo,
is roughly outlined by the stars beta, eta, gamma, delta, and epsilon,
which form two sides of a square some 15 deg. across. It extends, however,
for some distance into Coma Berenices, while outlying nebulae belonging
to it are also to be found in the eastern part of Leo. Unfortunately
for those who expect only brilliant revelations when they look through a
telescope, this throng of nebulae consists of small and inconspicuous
wisps as ill defined as bits of thistle-down floating high in the air.
There are more than three hundred of them all told, but even the
brightest are faint objects when seen with the largest of our
telescopes. Why do they congregate thus? That is the question which
lends an interest to the assemblage that no individual member of it
could alone command. It is a mystery, but beyond question it is
explicable. The explanation, however, is yet to be discovered.
The places of only three of the nebulae are indicated on the map. No.
2806 has been described as resembling in shape a shuttle. Its length is
nearly one third of the moon's diameter. It is brightest near the
center, and has several faint companions. No. 2961 is round, 4' in
diameter, and is accompanied by another round nebula in the same field
of view toward the south. No. 3105 is double, and powerful telescopes
show two more ghostly companions. There is an opportunity for good and
useful work in a careful study of the little nebulae that swim into view
all over this part of Virgo. Celestial photography has triumphs in store
for itself here.
Scattered over and around the region where the nebulae are thickest we
find eight or nine variable stars, three of the most remarkable of
which, R, S, and U, may be found on the map. R is very irregular,
sometimes attaining magnitude six and a half, while at other times its
maximum brightness does not exceed that of an eighth-magnitude star. At
minimum it sinks to the tenth or eleventh magnitude. Its period is one
hundred and forty-five days. U varies from magnitude seven or eight down
to magnitude twelve or under and then regains its light, in a period of
about two hundred and seven days. S is interesting for its brilliant red
color. When brightest, it exceeds the sixth magnitude, but at some of
its maxima the magnitude is hardly greater than the eighth. At minimum
it goes below the twelfth magnitude. Period, three hundred and
seventy-six days.
[Illustration: MAP NO. 10.]
Next east of Virgo is Libra, which contains a few notable objects (map
No. 10). The star alpha has a fifth-magnitude companion, distant about
230", which can be easily seen with an opera glass. At the point marked
A on the map is a curious multiple star, sometimes referred to by its
number in Piazzi's catalogues as follows: 212 P. xiv. The two principal
stars are easily seen, their magnitudes being six and seven and a half;
distance 15", p. 290 deg.. Burnham found four other faint companions, for
which it would be useless for us to look. The remarkable thing is that
these faint stars, the nearest of which is distant about 50" from the
largest member of the group and the farthest about 129", do not share,
according to their discoverer, in the rapid proper motion of the two
main stars.
In iota we find a double a little difficult for our three-inch. The
components are of magnitudes four and a half and nine, distance 57", p.
110 deg.. Burnham discovered that the ninth-magnitude star consists of two
of the tenth less than 2" apart, p. 24 deg..
No astronomer who happens to be engaged in this part of the sky ever
fails, unless his attention is absorbed by something of special
interest, to glance at beta Librae, which is famous as the only naked-eye
star having a decided green color. The hue is pale, but manifest.[3]
[3] Is the slight green tint perceptible in Sirius variable? I am
sometimes disposed to think it is.
The star is a remarkable variable, belonging to what is called the Algol
type. Its period, according to Chandler, is 2 days 7 hours, 51
minutes, 22.8 seconds. The time occupied by the actual changes is about
twelve hours. At maximum the star is of magnitude five and at minimum of
magnitude 6.2.
[Illustration: MAP NO. 11.]
We may now conveniently turn northward from Virgo in order to explore
Booetes, one of the most interesting of the constellations (map No. 11).
Its leading star alpha, Arcturus, is the brightest in the northern
hemisphere. Its precedence over its rivals Vega and Capella, long in
dispute, has been settled by the Harvard photometry. You notice that the
color of Arcturus, when it has not risen far above the horizon, is a
yellowish red, but when the star is near mid-heaven the color fades to
light yellow. The hue is possibly variable, for it is recorded that in
1852 Arcturus appeared to have nearly lost its color. If it should
eventually turn white, the fact would have an important bearing upon the
question whether Sirius was, as alleged, once a red or flame-colored
star.
But let us sit here in the starlight, for the night is balmy, and talk
about Arcturus, which is perhaps actually the greatest sun within the
range of terrestrial vision. Its parallax is so minute that the
consideration of the tremendous size of this star is a thing that the
imagination can not placidly approach. Calculations, based on its
assumed distance, which show that it _outshines the sun several thousand
times_, may be no exaggeration of the truth! It is easy to make such a
calculation. One of Dr. Elkin's parallaxes for Arcturus is 0.018". That
is to say, the displacement of Arcturus due to the change in the
observer's point of view when he looks at the star first from one side
and then from the other side of the earth's orbit, 186,000,000 miles
across, amounts to only eighteen one-thousandths of a second of arc. We
can appreciate how small that is when we reflect that it is about equal
to the apparent distance between the heads of two pins placed an inch
apart and viewed from a distance of a hundred and eighty miles!
Assuming this estimate of the parallax of Arcturus, let us see how it
will enable us to calculate the probable size or light-giving power of
the star as compared with the sun. The first thing to do is to multiply
the earth's distance from the sun, which may be taken at 93,000,000
miles, by 206,265, the number of seconds of arc in a radian, the base of
circular measure, and then divide the product by the parallax of the
star. Performing the multiplication and division, we get the following:
19,182,645,000,000 / .018 = 1,065,702,500,000,000.
The quotient represents miles! Call it, in round numbers, a thousand
millions of millions of miles. This is about 11,400,000 times the
distance from the earth to the sun.
Now for the second part of the calculation: The amount of light received
on the earth from some of the brighter stars has been experimentally
compared with the amount received from the sun. The results differ
rather widely, but in the case of Arcturus the ratio of the star's light
to sunlight may be taken as about one twenty-five-thousand-millionth--i.
e., 25,000,000,000 stars, each equal to Arcturus, would together shed
upon the earth as much light as the sun does. But we know that light
varies inversely as the square of the distance; for instance, if the sun
were twice as far away as it is, its light would be diminished for us to
a quarter of its present amount. Suppose, then, that we could remove the
earth to a point midway between the sun and Arcturus, we should then be
5,700,000 times as far from the sun as we now are. In order to estimate
how much light the sun would send us from that distance we must square
the number 5,700,000 and then take the result inversely, or as a
fraction. We thus get 1 / 32,490,000,000,000, representing the ratio of
the sun's light at half the distance of Arcturus to that at its real
distance. But while receding from the sun we should be approaching
Arcturus. We should get, in fact, twice as near to that star as we were
before, and therefore its light would be increased for us fourfold. Now,
if the amount of sunlight had not changed, it would exceed the light of
Arcturus only a quarter as much as it did before, or in the ratio of
25,000,000,000 / 4 = 6,250,000,000 to 1. But, as we have seen, the
sunlight would diminish through increase of distance to one
32,490,000,000,000th part of its original amount. Hence its altered
ratio to the light of Arcturus would become 6,250,000,000 to
32,490,000,000,000, or 1 to 5,198.
This means that if the earth were situated midway between the sun and
Arcturus, it would receive 5,198 times as much light from that star as
it would from the sun! It is quite probable, moreover, that the heat of
Arcturus exceeds the solar heat in the same ratio, for the spectroscope
shows that although Arcturus is surrounded with a cloak of metallic
vapors proportionately far more extensive than the sun's, yet, smothered
as the great star seems in some respects to be, it rivals Sirius itself
in the intensity of its radiant energy.
If we suppose the radiation of Arcturus to be the same per unit of
surface as the sun's, it follows that Arcturus exceeds the sun about
375,000 times in volume, and that its diameter is no less than
62,350,000 miles! Imagine the earth and the other planets constituting
the solar system removed to Arcturus and set revolving around it in
orbits of the same forms and sizes as those in which they circle about
the sun. Poor Mercury! For that little planet it would indeed be a jump
from the frying pan into the fire, because, as it rushed to perihelion,
Mercury would plunge more than 2,500,000 miles beneath the surface of
the giant star. Venus and the earth would melt like snowflakes at the
mouth of a furnace. Even far-away Neptune, the remotest member of the
system, would swelter in torrid heat.
But stop! Look at the sky. Observe how small and motionless the disks of
the stars have become. Back to the telescopes at once, for this is a
token that the atmosphere is steady, and that "good seeing" may be
expected. It is fortunate, for we have some delicate work before us. The
very first double star we try in Booetes, Sigma 1772, requires the use of
the four-inch, and the five-inch shows it more satisfactorily. The
magnitudes are sixth and ninth, distance 5", p. 140 deg.. On the other side
of Arcturus we find zeta, a star that we should have had no great
difficulty in separating thirty years ago, but which has now closed up
beyond the reach even of our five-inch. The magnitudes are both fourth,
and the distance less than a quarter of a second; position angle
changing. It is apparently a binary, and if so will some time widen
again, but its period is unknown. The star 279, also known as Sigma
1910, near the southeastern edge of the constellation, is a pretty
double, each component being of the seventh magnitude, distance 4", p.
212 deg.. Just above zeta we come upon pi, an easy double for the
three-inch, magnitudes four and six, distance 6" p. 99 deg.. Next is xi, a
yellow and purple pair, whose magnitudes are respectively five and
seven, distance less than 3", p. 200 deg.. This is undoubtedly a binary with
a period of revolution of about a hundred and thirty years. Its distance
decreased about 1" between 1881 and 1891. It was still decreasing in
1899, when it had become 2.5". The orbital swing is also very apparent
in the change of the position angle.
The telescopic gem of Booetes, and one of "the flowers of the sky," is
epsilon, also known as Mirac. When well seen, as we shall see it
to-night, epsilon Booetis is superb. The magnitudes of its two component
stars are two and a half (according to Hall, three) and six. The
distance is about 2.8", p. 326 deg.. The contrast of colors--bright orange
yellow, set against brilliant emerald green--is magnificent. There are
very few doubles that can be compared with it in this respect. The
three-inch will separate it, but the five-inch enables us best to enjoy
its beauty. It appears to be a binary, but the motion is very slow, and
nothing certain is yet known of its period.
In delta we have a very wide and easy double; magnitudes three and a
half and eight and a half, distance 110", p. 75 deg.. The smaller star has a
lilac hue. We can not hope with any of our instruments to see all of the
three stars contained in , but two of them are easily seen; magnitudes
four and seven, distance 108", p. 172 deg.. The smaller star is again
double; magnitudes seven and eight, distance 0.77", p. 88 deg.. It is
clearly a binary, with a long period. A six-inch telescope that could
separate this star at present would be indeed a treasure. Sigma 1926 is
another object rather beyond our powers, on account of the contrast of
magnitudes. These are six and eight and a half; distance 1.3", p. 256 deg..
Other doubles are: 44 (Sigma 1909), magnitudes five and six, distance
4.8", p. 240 deg.; 39 (Sigma 1890), magnitudes both nearly six, distance
3.6", p. 45 deg.. Smaller star light red; iota, magnitudes four and a half
and seven and a half, distance 38", p. 33 deg.; kappa, magnitudes five and a
half and eight, distance 12.7", p. 238 deg.. Some observers see a greenish
tinge in the light of the larger star, the smaller one being blue.
There are one or two interesting things to be seen in that part of Canes
Venatici which is represented on map No. 11. The first of these is the
star cluster 3936. This will reward a good look with the five-inch. With
large telescopes as many as one thousand stars have been discerned
packed within its globular outlines.
The star 25 (Sigma 1768) is a close binary with a period estimated at
one hundred and twenty-five years. The magnitudes are six and seven or
eight, distance about 1", p. 137 deg.. We may try for this with the
five-inch, and if we do not succeed in separating the stars we may hope
to do so some time, for the distance between them is increasing.
Although the nebula 3572 is a very wonderful object, we shall leave it
for another evening.
Eastward from Booetes shines the circlet of Corona Borealis, whose form
is so strikingly marked out by the stars that the most careless eye
perceives it at once. Although a very small constellation, it abounds
with interesting objects. We begin our attack with the five-inch on
Sigma 1932, but not too confident that we shall come off victors, for
this binary has been slowly closing for many years. The magnitudes are
six and a half and seven, distance 0.84", p. 150 deg.. Not far distant is
another binary, at present beyond our powers, eta. Here the magnitudes
are both six, distance 0.65", p. 3 deg.. Hall assigns a period of forty
years to this star.
The assemblage of close binaries in this neighborhood is very curious.
Only a few degrees away we find one that is still more remarkable, the
star gamma. What has previously been said about 42 Comae Berenicis
applies in a measure to this star also. It, too, has a comparatively
small orbit, and its components are never seen widely separated. In 1826
their distance was 0.7"; in 1880 they could not be split; in 1891 the
distance had increased to 0.36", and in 1894 it had become 0.53", p.
123 deg.. But in 1899 Lewis made the distance only 0.43". The period has
been estimated at one hundred years.
While the group of double stars in the southern part of Corona Borealis
consists, as we have seen, of remarkably close binaries, another group
in the northern part of the same constellation comprises stars that are
easily separated. Let us first try zeta. The powers of the three-inch
are amply sufficient in this case. The magnitudes are four and five,
distance 6.3", p. 300 deg.. Colors, white or bluish-white and blue or green.
Next take sigma, whose magnitudes are five and six, distance 4", p.
206 deg.. With the five-inch we may look for a second companion of the tenth
magnitude, distance 54", p. 88 deg.. It is thought highly probable that
sigma is a binary, but its period has simply been guessed at.
Finally, we come to nu, which consists of two very widely separated
stars, nu^1 and nu^2, each of which has a faint companion. With the
five-inch we may be able to see the companion of nu^2, the more
southerly of the pair. The magnitude of the companion is variously given
as tenth and twelfth, distance 137", p. 18 deg..
With the aid of the map we find the position of the new star of 1866,
which is famous as the first so-called temporary star to which
spectroscopic analysis was applied. When first noticed, on May 12, 1866,
this star was of the second magnitude, fully equaling in brilliancy
alpha, the brightest star of the constellation; but in about two weeks
it fell to the ninth magnitude. Huggins and Miller eagerly studied the
star with the spectroscope, and their results were received with deepest
interest. They concluded that the light of the new star had two
different sources, each giving a spectrum peculiar to itself. One of the
spectra had dark lines and the other bright lines. It will be
remembered that a similar peculiarity was exhibited by the new star in
Auriga in 1893. But the star in Corona did not disappear. It diminished
to magnitude nine and a half or ten, and stopped there; and it is still
visible. In fact, subsequent examination proved that it had been
catalogued at Bonn as a star of magnitude nine and a half in 1855.
Consequently this "blaze star" of 1866 will bear watching in its
decrepitude. Nobody knows but that it may blaze again. Perhaps it is a
sun-like body; perhaps it bears little resemblance to a sun as we
understand such a thing. But whatever it may be, it has proved itself
capable of doing very extraordinary things.
We have no reason to suspect the sun of any latent eccentricities, like
those that have been displayed by "temporary" stars; yet, acting on the
principle which led the old emperor-astrologer Rudolph II to torment his
mind with self-made horoscopes of evil import, let us unscientifically
imagine that the sun _could_ suddenly burst out with several hundred
times its ordinary amount of heat and light, thereby putting us into a
proper condition for spectroscopic examination by curious astronomers in
distant worlds.
But no, after all, it is far pleasanter to keep within the strict
boundaries of science, and not imagine anything of the kind.