Part 4
The constellation of Hercules is a very rich field for the possessor of even a small telescope. Here are to be found beautifully colored double stars in profusion, and, in addition, two remarkable clusters of stars. The brighter of the two is known as the Great Hercules Cluster. Its position is shown on the chart, and, under favorable conditions--that is, on a clear, dark night, when there is no moonlight--it may be seen without the aid of a telescope as a small, faint patch of light. One would never suspect from such a view what a wonderful object this cluster becomes when seen with the aid of a powerful telescope. Photographs taken with the great telescopes show this faint wisp of light as a magnificent assemblage of thousands of stars, each a sun many times more brilliant than our own sun. The crowded appearance of the stars in the cluster is due partly to the fact that it is very distant from the earth, though neighboring stars in the cluster are indeed much nearer to one another than are the stars in the vicinity of our solar system. It has been found that this cluster is so far away that its light takes over thirty-six thousand years to reach the earth. At the distance of this cluster, a sun equal in brightness to our own sun would be so faint that the most powerful telescope in the world would not show it. So we know that the stars that are visible in the Hercules cluster are far more brilliant than our sun. A fair-sized telescope will show about four thousand stars in this cluster, but the greatest telescopes show over one hundred thousand in it, and there are without doubt many more too faint to be seen at all. The Hercules cluster is called a globular star-cluster, because the stars in it are arranged nearly in the form of a sphere. There are in the heavens about ninety such clusters whose distances have been found, and they are among the most distant of all objects. Most of them are very faint, and a few are over two hundred thousand light-years distant from the earth. The Hercules cluster is one of the nearest and is the most noted of all of these globular clusters. It is considered to be one of the finest objects in the heavens. The other cluster in Hercules is also very fine, but not to be compared with this one.
Just to the south of Hercules are two constellations, Ophiuchus, The Serpent-Bearer, and Serpens, The Serpent, which are so intermingled that it is difficult to distinguish them. There are in these two constellations, as in Hercules, no stars of unusual brilliancy, but a large number of fairly bright stars. The brightest star in Ophiuchus is known as Alpha Ophiuchi and it marks the head of the Serpent-Bearer. The two stars, Alpha Ophiuchi and Alpha Herculis, are close together, being separated by a distance about equal to that between the Pointers of the Big Dipper. Alpha Ophiuchi is the brighter of the two, and it is farther east.
Ophiuchus, according to one legend, was once a physician on earth, and was so successful as a healer that he could raise the dead. Pluto, the god of the lower world, became alarmed for fear his kingdom would become depopulated, and persuaded Jupiter to remove Ophiuchus to a heavenly abode, where he would be less troublesome. The serpent is supposed to be a symbol of his healing powers. The head of Serpens is marked by a group of faint stars just south of Corona Borealis and southwest of Hercules. From here a line of fairly bright stars marks the course of Serpens southward to the hand of Ophiuchus. Two stars close together and nearly equal in brightness mark the hand with which the hero grasps the body of the serpent. The other hand is marked by an equally bright single star some distance to the eastward where the two constellations again meet. Ophiuchus is thus represented as holding the serpent with both hands. It is not an easy matter to make out the outlines of these straggling groups, but there are in them several pairs of stars nearly equal in brightness and about as evenly spaced as the two stars in the one hand of Ophiuchus, and these, as well as the diagram, will be of aid in tracing the two groups.
Just south of Serpens and Ophiuchus lies one of the most beautiful and easily recognized constellations in the heavens. This is the constellation of Scorpio, The Scorpion, which will be found not far above the southern horizon at this time. The small constellation of Libra, The Scales, which lies just to the northwest of Scorpio, was at one time a part of this constellation and represented the creature's claws, but some centuries ago its name was changed to Libra. Both Scorpio and Libra are numbered among the twelve zodiacal constellations--that is, they lie along the ecliptic, or apparent yearly path of the sun among the stars. Scorpio is the most brilliant and interesting of all the zodiacal groups. The heart of the Scorpion is marked by the magnificent first-magnitude star Antares, which is of a deep reddish color. The name signifies Rival of Ares (Mars). It is so called because it is the one star in the heavens that most closely resembles Mars, and it might be mistaken for the ruddy planet if one were not familiar with the constellations. At times, when Mars is at a considerable distance from the earth, it is almost equal in brightness and general appearance to this glowing red star in the heart of the Scorpion. In its trips around the sun, Mars passes occasionally very close to Antares, and the two then present a very striking appearance.
With a telescope of medium size, one will find an exquisite little green companion-star close to Antares. The little companion is so close to Antares that it is difficult to find it in the glare of light from its more brilliant neighbor. Antares is one of the giant stars of the universe. In fact it is, so far as we know, the greatest of all the giants. Its diameter is more than five hundred times that of our own sun and nearly twice that of the giant star Betelgeuze in Orion. If placed at the center of the solar system its surface would lie far beyond the orbit of Mars.
Both Ophiuchus and Scorpio are crossed by the Milky Way, that broad belt of numberless faint stars that encircles the heavens. Some of the most wonderful and beautiful regions of the Milky Way are to be found in these two constellations.
At various times in the past, there have suddenly flashed forth brilliant stars in the Milky Way which are known as "temporary stars," or "novæ." These outbursts signify that some celestial catastrophe has taken place, the nature or cause of which is not clearly understood. Some of the most brilliant of these outbursts have occurred in these two constellations. The life of a nova is very short, a matter of a few months, and it rapidly sinks into oblivion, so nothing is to be seen of some of the most brilliant of all these stars that have appeared in this region in the past. A few are still faintly visible in large telescopes.
IX
AUGUST
It was one of the twelve labors of Hercules, the hero of Grecian mythology, to vanquish the dragon that guarded the golden apples in the garden of the Hesperides. Among the constellations for July we found the large group of stars that represents the hero himself, and this month we find just to the north of Hercules the head of Draco, The Dragon. The foot of the hero rests upon the dragon's head, which is outlined by a group of four fairly bright stars forming a quadrilateral or four-sided figure. The brightest star in this group passes in its daily circuit of the pole almost through the zenith of London. That is, as it crosses the meridian of London, it is almost exactly overhead. From the head of Draco, the creature's body can be traced in a long line of stars curving first eastward, then northward, toward the pole-star to a point above Hercules, where it bends sharply westward. The body of the monster lies chiefly between its head and the bowl of the Little Dipper. The tail extends in a long line of faint stars midway between the two Dippers, or the constellations of Ursa Major and Ursa Minor, the tip of the tail lying on the line connecting the Pointers of the Big Dipper with the pole-star Polaris.
Draco, as well as Ursa Major and Ursa Minor, is a circumpolar constellation in our latitude; that is, it makes its circuit of the pole without at any time dipping below the horizon in latitudes north of 40°. It is, therefore, visible at all hours of the night in mid-latitudes of the northern hemisphere, but is seen to the best advantage during the early evening hours in the summer months. There are no remarkable stars in this constellation with the exception of Alpha, which lies halfway between the bowl of the Little Dipper and Mizar, the star at the bend in the handle of the Big Dipper.
About four thousand seven hundred years ago, this star was the pole-star--lying even nearer to what was then the north pole of the heavens than Polaris does to the present position of the pole. The sun and moon exert a pull on the bulging equatorial regions of the earth, which tends to draw the plane of the earth's equator down into the plane of the ecliptic. This causes the "Precession of the Equinoxes" and at the same time a slow revolution of the earth's axis of rotation about the pole of the ecliptic. The north pole of the heavens as a result describes a circle about the pole of the ecliptic of radius 23-1/2° in a period of 25,800 years.
Each bright star that lies near the circumference of this circle becomes in turn the pole-star sometime within this period. The star Alpha, in Draco, had its turn at being pole-star some forty-seven centuries ago. Polaris is now a little over a degree from the north pole of the heavens. During the next two centuries it will continue to approach the pole until it comes within a quarter of a degree of it, when its distance from the pole will begin to increase again. About twelve thousand years hence the magnificent Vega, whose acquaintance we will now make, will be the most brilliant and beautiful of all pole-stars.
Vega (Arabic for "Falling Eagle") is the resplendent, bluish-white, first-magnitude star that lies in the constellation of Lyra, The Lyre or Harp, a small, but important, constellation just east of Hercules and a little to the southeast of the head of Draco. Vega is almost exactly equal in brightness to Arcturus, the orange-colored star in Boötes, now lying west of the meridian in the early evening hours. It is also a near neighbor of the solar system, its light taking something like forty years to travel to the earth. Vega is carried nearly through the zenith of Washington and all places in the same latitude by the apparent daily rotation of the heavens. It is a star that we have no difficulty in recognizing, owing to the presence of two nearby stars that form, with it, a small equal-sided triangle with sides only two degrees in extent. If our own sun were at the distance of Vega, it would not appear as bright as one of these faint stars, so much more brilliant is this magnificent sun than our own. The two faint stars that follow so closely after Vega and form the little triangle with it are also of particular interest. Epsilon Lyræ, which is the northern one of these two stars, may be used as a test of keen eyesight. It is the finest example in the heavens of a quadruple star--that is, "a double-double star." A keen eye can just separate this star into two without a telescope, and with the aid of a telescope, each of the two splits up into two stars, making four stars in place of the one visible to the average eye. Zeta, the other of the two stars that form the little triangle with Vega, is also a fine double star. The star that lies almost in a straight line with Epsilon and Zeta and a short distance to the south of them is a very interesting variable star known as Beta Lyræ. Its brightness changes very considerably in a period of twelve days and twenty-two hours. This change of brightness is due to the presence of a companion star. The two stars are in mutual revolution, and their motion is viewed at such an angle from the earth that, in each revolution, one star is eclipsed by the other, producing a variation in the amount of light that reaches our eyes. By comparing this star from day to day with the star just a short distance to the southeast of it, which does not vary in brightness, we can observe for ourselves this change in the light of Beta Lyræ. There are a number of stars in the heavens that vary in brightness in the same manner as Beta Lyræ, and they are called eclipsing-variable stars.
On the line connecting Beta Lyræ with the star southeast of it and one-third of the distance from Beta to this star, lies the noted Ring Nebula in Lyra, which is a beautiful object even in a small telescope. It consists of a ring of luminous gas surrounding a central star. The star shines with a brilliant, bluish-white light and is visible only in powerful telescopes though it is easily photographed since it gives forth rays to which the photographic plate is particularly sensitive. In small telescopes the central part of this nebula appears dark but with a powerful telescope a faint light may be seen even in the central portion of the nebula. This is one of the most interesting and beautiful telescopic objects in the heavens.
It is in the general direction of the constellation of Lyra that our solar system is speeding at the rate of more than a million miles a day. This point toward which we are moving at such tremendous speed lies a little to the southwest of Vega, on the border between the constellations of Lyra and Hercules, and is spoken of as The Apex of the Sun's Way.
In the southern sky we have this month the constellation of Sagittarius, The Archer, which is just to the east of Scorpio and a considerable distance south of Lyra. It can be recognized by its peculiar form, which is that of a short-handled milk dipper, with the bowl turned toward the south and a trail of bright stars running from the end of the handle toward the southwest. This is one of the zodiacal groups which contain no first-magnitude stars, but a number of the second and third magnitude. It is crossed by the Milky Way, which is very wonderful in its structure at this point. Some astronomers believe that here--among the star-clouds and mists of nebulous light which are intermingled with dark lanes and holes, in reality dark nebulæ--lies the center of the vast system of stars and nebulæ in which our entire solar system is but the merest speck. Some of the grandest views through the telescope are also to be obtained in this beautiful constellation of Sagittarius, which is so far south that it is seen to better advantage in the tropics than in the mid-latitudes of the northern hemispheres.
X
SEPTEMBER
One of the most beautiful constellations of the northern hemisphere is Cygnus, The Swan, which is in the zenith in mid-latitudes about nine o'clock in the evening the middle of September. It lies directly in the path of the Milky Way which stretches diagonally across the heavens from the northeast to the southwest at this time. In Cygnus, the Milky Way divides into two branches, one passing through Ophiuchus and Serpens to Scorpio, and the other through Sagitta and Aquila to Sagittarius, to meet again in the southern constellation of Ara, just south of Scorpio and Sagittarius. On clear, dark evenings, when there is no moonlight, this long, dark rift in the Milky Way can be seen very clearly. In Cygnus, as in Ophiuchus, Scorpio, and Sagittarius we find wonderful star-clouds, consisting of numberless stars so distant from us and, therefore, so faint that they do not appear as distinct points of light except in the greatest telescopes. It is the combined light from these numberless stars that cannot be seen separately that produces the impression of stars massed in clouds of nebulous light and gives to this girdle of the heavens its name of the Milky Way. In Cygnus, as in a number of other constellations of both hemispheres, the Milky Way is crossed by dark rifts and bars and is very complicated in its structure. It is in Cygnus, also, that one may see with the aid of powerful telescopes the vast, irregular, luminous nebulæ, that are like great clouds of fiery mist. These nebulæ are of enormous extent, for they cover space that could be occupied by hundreds of stars.
Cygnus is a constellation that is filled with the wonders and mysteries of space and that abounds in beautiful objects of varied kinds. It is a region one never tires of exploring with the telescope. The principal stars in Cygnus form the well-known Northern Cross, with the beautiful, white, first-magnitude star Deneb, or Arided, as it is sometimes called, at the top of the cross, and Albireo, the orange-and-blue double star at the foot. Albireo, among all the pairs of contrasting hues, has the distinction of being considered the finest double star in the heavens for small telescopes. This star marks the head of The Swan, as well as the foot of the Northern Cross, and Deneb marks the tail of The Swan. A short distance to the southeast of Deneb, on the right wing of The Swan, is a famous little star, 61 Cygni, barely visible to the naked eye and forming a little triangle with two brighter stars to the east. This star has the distinction of being the first one to have its distance from the solar system determined. The famous mathematician and astronomer Bessel accomplished this difficult feat in the year 1838. Since that day, the distances of many stars have been found by various methods, but of all these stars only four or five are known to be nearer to us than 61 Cygni. Its distance is about eight light-years, so its light takes about eight years to travel the distance that separates it from the solar system. As a result, we see it not as it is tonight, but as it was at the time when the light now entering our eyes first started on its journey eight years ago. 61 Cygni is also a double star, and the combined light of the two stars gives forth only one-tenth as much light as our own sun. Most of the brilliant first-magnitude stars give forth many times as much light as the sun; but among the fainter stars, we find some that appear faint because they are very distant, and some that are faint because they are dwarf stars and have little light to give forth. To the class of nearby, feebly-shining dwarf stars 61 Cygni belongs. Deneb, on the other hand, is one of the giant stars, and is at an immeasurably great distance from the solar system.
Just south of Cygnus in the eastern branch of the Milky Way lie Sagitta, The Arrow, and Aquila, The Eagle. Not far to the northeast of Aquila is the odd little constellation of Delphinus, The Dolphin, popularly referred to as Job's Coffin. There will be no difficulty in finding this small star-group, owing to its peculiar diamond-shaped configuration. Its five principal stars are of the fourth magnitude. It is in the constellation of Delphinus that the most distant known object in the heavens is located. This is the globular star cluster known only by its catalogue number of N.G.C. 7006. It is estimated to be at a distance of 220,000 light-years from the earth.
Sagitta, The Arrow, lies midway between Albireo and the brilliant Altair in Aquila. The point of the arrow is indicated by the star that is farthest east; and the feather, by the two faint stars to the west. Like Delphinus, this constellation is very small and contains no objects of particular interest.
Altair (Flying Eagle) is the brilliant white star of the first magnitude in Aquila and is attended by two fainter stars, one on either side, at nearly equal distances from it. These two stars serve readily to distinguish this star, all three stars being nearly in a straight line. Altair is one of the nearer stars, its distance from the earth being about sixteen light-years. It gives forth about ten times as much light as the sun.
We cannot leave the constellation of Aquila without referring to the wonderful temporary star or nova, known as Nova Aquilæ No. 3 (because it was the third nova to appear in this constellation), which appeared in the position indicated on the chart upon the eighth of June, 1918. A few days previous to this date, there was in this position an extremely faint star, invisible to the naked eye and in small telescopes. This fact became known from later examinations of old photographs of this region that had been taken at the Harvard College Observatory, where the photographing of the heavens is carried on regularly for the purpose of having a record of celestial changes and happenings. Clouds prevented the obtaining of any photographs of this part of the heavens on the four nights preceding the eighth of June, but on this evening there shone in the place of the faint telescopic star, a wonderful temporary star, or nova, which was destined on the next evening to outshine all stars in the heavens, with the exception of the brightest of all, Sirius, which it closely rivaled in brilliancy at the height of its outburst. Within less than a week's time, this faint star in the Milky Way for some mysterious reason increased in brightness many thousandfold. Such outbursts have been recorded before, but on rare occasions, however. No star since the appearance of the nova known as Kepler's Star, in the year 1604, which at its greatest brilliancy rivaled Jupiter, shone with such splendor or attracted so much attention as Nova Aquilæ. In the year 1901, there appeared in the constellation of Perseus a star known as Nova Persei which at its brightest surpassed Vega, but its splendor was not as great as that of the wonderful nova of 1918.
It speaks well for the zeal and interest of amateur astronomers, as well as for their acquaintance with the stars, that Nova Persei was discovered by an amateur astronomer, Dr. Anderson, and that among the deluge of telephone calls and telegrams received at the Harvard College Observatory on the night of June 8th, announcing independent discoveries of the "new star," were many from non-professional astronomers.
Like all stars of this class, Nova Aquilæ No. 3 sank rapidly into oblivion. In a few weeks it was only a third-magnitude star; a few weeks more and it was invisible without a telescope. Many wonderful and interesting changes have been recorded in the appearance of this star, however, even after it became visible only in the telescope. Soon after its outburst it appeared to develop a nebulous envelope, as have other novas before it. It showed in addition many of the peculiarities of the nebulæ, though the central star remained visible as before the outburst.
Astronomers are still in doubt as to the cause of these outbursts, which certainly indicate celestial catastrophies of some form on a gigantic scale. All novas possess one characteristic in common--that of appearing exclusively in the Milky Way; and another characteristic is the development of a nebular envelope after the outburst of greatest brightness. In some cases temporary stars have been known to be variable in brightness for years before the great outburst. Such a star was Nova Aquila, for the examination of photographs of this region taken some years previous showed variations in its brightness for a period of thirty years at least.