Part 8

To the west of Boötes, below the handle of the Big Dipper, is a region where there are few conspicuous stars. Here will be found Canes Venatici (The Hunting Dogs with which Boötes is supposed to be pursuing the Great Bear around the north pole), and, further south, Coma Berenices (Bernice's Hair).

The brighter of the two Hunting Dogs, which is also the brightest star in the entire region covered by these two constellations, appears as a beautiful blue-and-yellow double star in the telescope. It was named Cor Caroli (Heart of Charles) by the astronomer Halley in honor of Charles II of England, at the suggestion of the court physician, who imagined it shone more brightly than usual the night before the return of Charles to London. Of more interest to astronomers is the magnificent spiral nebula in this constellation, known as the "Whirlpool Nebula," appearing as a faint, luminous patch in the sky, of which many photographs have been taken with the great telescopes. This entire region, from Canes Venatici to Virgo, abounds in faint spiral nebulæ that for some reason not yet understood by astronomers are crowded together in this part of the heavens where stars are comparatively few. It is believed that there are between five hundred thousand and a million of these spiral nebulæ in the entire heavens, and the problem of their nature and origin and distance is one that the astronomers are very anxious to solve. Many wonderful facts are now being learned concerning these faint nebulous wisps of light which, with a few exceptions, are observable only with great telescopes. They reveal their spiral structure more clearly to the photographic plate than to the human eye, and some magnificent photographs of them have been taken with powerful telescopes.

Coma Berenices, south of Canes Venatici and southwest of Boötes, is a constellation that consists of a great number of stars closely crowded together, and just barely visible to the unaided eye. As a result, it has the appearance of filmy threads of light, which doubtless suggested its name to the imaginative ancients, who loved to fill the heavens with fanciful creations associated with their myths and legends. These stars form a moving cluster of stars estimated to be at a distance of about 270 light-years from the solar systems.

This region, so lacking in interesting objects for the naked-eye observer, is a mine of riches to the fortunate possessors of telescopes; and the great telescopes of the world are frequently pointed in this direction, exploring the mysteries of space that abound here.

Just to the east of Boötes is the exquisite little circlet of stars known as Corona Borealis, the Northern Crown. It consists of six stars arranged in a nearly perfect semicircle, and one will have no difficulty in recognizing it. Its brightest star, Alpha, known also by the name of Alphacca, is a star of the second magnitude.

Boötes is one of the largest and finest of the northern constellations. It can be easily distinguished by its peculiar kite-shaped grouping of stars or by the conspicuous pentagon (five-sided figure) of stars which it contains. The most southerly star in this pentagon is known as Epsilon Boötes and is one of the finest double stars in the heavens. The two stars of which it consists are respectively orange and greenish-blue in color.

By far the finest object in Boötes, however, is the magnificent Arcturus, which is the brightest star in the northern hemisphere of the heavens. This star will be conspicuous in the evening hours throughout the summer months, as will also the less brilliant Spica in Virgo.

Some recent measurements show that Arcturus is one of our nearer neighbors among the stars. Its distance is now estimated to be about twenty-one light-years. That is, a ray of light from this star takes twenty-one years to reach the earth, traveling at the rate of one hundred and eighty-six thousand miles per second. It would seem as if we should hardly speak of Arcturus, twenty-one light-years away, as a near neighbor, yet there are millions of stars that are far more distant from the earth, and very few that are nearer to us than Arcturus.

The brightness of Arcturus is estimated to be about forty times that of the sun. That is, if the two bodies were side by side, Arcturus would give forth forty times as much light and heat as the sun.

Arcturus is also one of the most rapidly moving stars in the heavens. In the past sixteen centuries it has traveled so far as to have changed its position among the other stars by as much as the apparent width of the moon. Most of the stars, in spite of their motions through the heavens in various directions, appear today in the same relative positions in which they were several thousand years ago. It is for this reason that the constellations of the Egyptians and of the Greeks and Romans are the same constellations that we see in the heavens today. Were all the stars as rapidly moving as Arcturus, the distinctive forms of the constellations would be preserved for only a very few centuries.

Virgo, which lies south and southwest of Boötes, is a large, straggling constellation, consisting of a Y-shaped configuration of rather inconspicuous stars. It lies in the path of our sun, moon and planets, and so is one of the zodiacal constellations. The cross in the diagram indicates the present position of the autumnal equinox, the point where the sun crosses the equator going south, and the position the sun now occupies at the beginning of fall.

Spica, the brightest star in Virgo, is a bluish-white, first-magnitude star, standing very much alone in the sky. In fact, the Arabs referred to this star as "The Solitary One." Its distance from the earth is not known, but must be very great as it cannot be found by the usual methods. The spectroscope shows that it consists of two suns very close together, revolving about a common center in a period of only four days.

Within the branches of the "Y" in Virgo, and just to the north of it, is the wonderful nebulous region of this constellation, but it takes a powerful telescope to show the faint spiral nebulæ that exist here in great profusion. All of these spirals are receding from the plane of the Milky Way with enormous velocities. The spiral nebulæ are, in fact, the most rapidly moving objects in the heavens.

VIII

JULY

Due east of the little circlet of stars known as Corona Borealis, and almost directly overhead in our latitude (40° N.) about nine o'clock in the evening in the early part of July, is the large constellation of Hercules, named for the famous hero of Grecian mythology. There are no stars of great brilliancy in this group, but it contains a large number of fairly bright stars arranged in the form outlined in the chart. The hero is standing with his head, marked by the star Alpha Herculis, toward the south, and his foot resting on the head of Draco, The Dragon, a far-northern constellation with which we become acquainted in August.

Alpha Herculis, the best known star in this constellation, is of unusual interest. Not only is it a most beautiful double star, the brighter of the two stars of which it is composed being orange, and the fainter greenish-blue, but it is also a star that changes in brightness irregularly. Both the orange and the blue star share in this change of brightness. There are a number of stars in the heavens that vary in brightness, some in very regular periods, and others, like Alpha Herculis, irregularly. These latter stars are nearly always deep orange or reddish in color. One may note this variation in the brightness of Alpha Herculis by comparing it from time to time with some nearby star that does not vary in brightness.

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.

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.