Pleasures of the telescope Part 3

234; 54, magnitudes four and a half and seven, distance 6.2", p. 102; and 49, magnitudes six and nine, distance 2.4", p. 158.

Leo contains a remarkable variable star, R, deep red in color, and varying in a s.p.a.ce of a hundred and forty-four days from the fifth to the tenth magnitude. It has also several nebulae, of which only one needs special mention, No. 1861. This is spindle-shaped, and large telescopes show that it consists of three nebulae. The observer with ordinary instruments finds little to see and little to interest him in these small, faint nebulae.

We may just glance at two double stars in the small constellation of s.e.xtans, situated under Leo. These are: 9, magnitudes seven and eight, distance 53", p. 292; and 35, magnitudes six and seven, distance 6.9", p. 240.

[Ill.u.s.tration: MAP NO. 6.]

Coma Berenices (map No. 6) includes several interesting objects. Let us begin with the star 2, a double, of magnitudes six and seven and a half, distance 3.6", p. 240. The color of the smaller star is lilac.

This hue, although not extremely uncommon among double stars elsewhere, recurs again and again, with singular persistence, in this little constellation. For instance, in the very next star that we look at, 12, we find a double whose smaller component is _lilac_. The magnitudes in 12 are five and eight, distance 66", p. 168. So also the wide double 17, magnitudes five and a half and six, distance 145", exhibits a tinge of _lilac_ in the smaller component; the triple 35, magnitudes five, eight, and nine, distances 1", p. 77, and 28.7", p. 124, has four colors yellow, _lilac_, and blue, and the double 24, magnitudes five and six, distance 20", p. 270, combines an orange with a _lilac_ star, a very striking and beautiful contrast. We should make a mistake if we regarded this wonderful distribution of color among the double stars as accidental. It is manifestly expressive of their physical condition, although we can not yet decipher its exact meaning.

The binary 42 Comae Berenicis is too close for ordinary telescopes, but it is highly interesting as an intermediate between those pairs which the telescope is able to separate and those--like beta Aurigae--which no magnifying power can divide, but which reveal the fact that they are double by the periodical splitting of their spectral lines. The orbit in 42 Comae Berenicis is a very small one, so that even when the components are at their greatest distance apart they can not be separated by a five-or six-inch gla.s.s. Burnham, using the Lick telescope, in 1890 made the distance 0.7"; Hall, using the Was.h.i.+ngton telescope, in 1891 made it a trifle more than 0.5". He had measured it in 1886 as only 0.27". The period of revolution is believed to be about twenty-five years.

In Coma Berenices there is an outlying field of the marvelous nebulous region of Virgo, which we may explore on some future evening. But the nebulae in Coma are very faint, and, for an amateur, hardly worth the trouble required to pick them up. The two cl.u.s.ters included in the map, 2752 and 3453, are bright enough to repay inspection with our largest aperture.

[Ill.u.s.tration: MAP NO. 7.]

Although Hydra is the largest constellation in the heavens, extending about seven hours, or 105, in right ascension, it contains comparatively few objects of interest, and most of these are in the head or western end of the constellation, which we examined during our first night at the telescope. In the central portion of Hydra, represented on map No. 7, we find its leading star alpha, sometimes called Alphard, or Cor Hydrae, a bright second-magnitude star that has been suspected of variability. It has a decided orange tint, and is accompanied, at a distance of 281", p. 153, by a greenish tenth-magnitude star. Bu. 339 is a fine double, magnitudes eight and nine and a half, distance 1.3", p. 216. The planetary nebula 2102 is about 1' in diameter, pale blue in color, and worth looking at, because it is brighter than most objects of its cla.s.s. Tempel and Secchi have given wonderful descriptions of it, both finding mult.i.tudes of stars intermingled with nebulous matter.

For a last glimpse at celestial splendors for the night, let us turn to the rich cl.u.s.ter 1630, in Argo, just above the place where the stream of the Milky Way--here bright in mid-channel and shallowing toward the sh.o.r.es--separates into two or three currents before disappearing behind the horizon. It is by no means as brilliant as some of the star cl.u.s.ters we have seen, but it gains in beauty and impressiveness from the presence of one bright star that seems to captain a host of inferior luminaries.

CHAPTER IV

VIRGO AND HER NEIGHBORS

... "that region Where still by night is seen The Virgin G.o.ddess near to bright Bootes."--POSTE'S ARATUS.

[Ill.u.s.tration: 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 pa.s.s 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; 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.

[Ill.u.s.tration: 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 ma.s.s 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. 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. 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, and 65", p. 295. 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 a.s.sumed 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. Colors yellow and blue. Sigma 1846 is a fifth-magnitude star with a tenth-magnitude companion, distance only 4", p. 108. 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 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 a.s.semblage 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.

[Ill.u.s.tration: 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 gla.s.s. 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 princ.i.p.al stars are easily seen, their magnitudes being six and seven and a half; distance 15", p. 290. 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. Burnham discovered that the ninth-magnitude star consists of two of the tenth less than 2" apart, p. 24.

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.

[Ill.u.s.tration: MAP NO. 11.]

We may now conveniently turn northward from Virgo in order to explore Bootes, 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 a.s.sumed distance, which show that it _outs.h.i.+nes 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...o...b..t, 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!

a.s.suming 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.