Part 6

Among the constellations of the southern heavens near the meridian in February we see in addition to Argo Navis the constellations of Dorado, The Goldfish; Hydrus, The Serpent, and Tucana, The Toucan. Though insignificant in appearance Dorado contains what was described by Sir John Herschel as one of the most extraordinary objects in the heavens, a worthy rival of The Great Orion Nebula and in some respects very similar to it, The Great Looped Nebula, "the center of a great spiral." In Dorado also is located The Greater Magellanic Cloud which looks like a detached portion of the Milky Way though it is far removed from it. To the naked eye it resembles a small white cloud about 4° in extent. In the telescope it bears a close resemblance to a typical portion of the Milky Way. A similar formation known as The Lesser Magellanic Cloud is located in Hydrus. It has been estimated that the distance of The Lesser Cloud is 80,000 light-years and that it is receding from us.

In Tucana is located one of the finest globular star clusters in the heavens, known as 47 Tucanæ. This cluster and Omega Centauri, a globular star cluster in Centaurus, are the two nearest of all the globular clusters. They are distant from the earth about 22,000 light-years and it is known that the combined light of the thousands of stars of which each cluster is composed is about one million times that of our own sun.

In the western sky in the southern hemisphere in February may be seen the brilliant, white, first-magnitude star Achernar in the river Eridanus, the long, winding constellation that, we recall, starts near the brilliant Rigel in Orion and disappears from the view of northern observers below the southern horizon, extending its course far into the southern hemisphere. Achernar means "The End of the River" and this is nearly its position in the constellation.

Though Argo Navis is the largest and most important constellation of the southern hemisphere, Crux, The Southern Cross, far-famed in story and legend as well as for its historical associations, is beyond a doubt the most popular.

The best time to view the Southern Cross is in June or July when it is near the meridian. It is not seen to advantage in the months of January or February. It then lies on its side and close to the horizon and therefore is dimmed by atmospheric haze so that it almost invariably is a disappointing object to the tourist from the north who usually views it for the first time in one of these months. The Cross is viewed to advantage in the latitude of Rio or Valparaiso and it is best seen from the Straits where it rides high overhead. It is not seen to advantage from the latitudes of Cuba or Jamaica. It is small, only 6° in extent from north to south and less in width and it lies in the most brilliant portion of the Milky Way which is here a narrow stream only three or four degrees wide. Directly below the Cross is the noted Coal Sack, apparently a yawning chasm in the midst of its brilliant surroundings though probably in reality a dark nebula. Viewed with the telescopes a number of stars are to be seen projected on this dark background.

The Southern Cross is to the inhabitants of the southern hemisphere what the Big Dipper is to those who dwell in the northern hemisphere--an infallible timepiece. The upright of the Cross points toward the south pole of the heavens which lies in a region where there is a singular dearth of bright stars, the nearest star to the south pole being a faint fifth-magnitude star called Sigma Octantis. When seen in the southeast or southwest the Cross lies on its side, but when passing the meridian it stands nearly upright. Humboldt, referring to this fact, says:

"How often have we heard our guides exclaim in the savannahs of Venezuela and in the desert extending from Lima to Truxillo, 'Midnight is past, the Cross begins to bend.'"

By the explorers of the sixteenth century the Cross was taken as a sign of heaven's approval of their attempt to establish the Christian religion in the wilds of the New World. This thought is beautifully expressed in Mrs. Hemans' lines in "The Cross of the South."

"But to thee, as thy lode-stars resplendently burn In their clear depths of blue, with devotion I turn Bright Cross of the South! and beholding thee shine, Scarce regret the loved land of the olive and vine. Thou recallest the ages when first o'er the main My fathers unfolded the ensign of Spain, And planted their faith in the regions that see Its imperishing symbol ever blazoned in thee."

Alpha Crucis, the brightest star in Crux, is at the foot of the Cross. It consists in reality of two second-magnitude stars forming a beautiful double while a third fifth-magnitude star one and one-half minutes of arc distant makes with this pair a combination similar to our Mizar and Alcor of the Big Dipper though the separation is not great enough to be visible to the naked eye. The second-magnitude star at the head of the Cross is a deep orange in color and the two stars that mark the ends of the cross-arm are white third-magnitude stars.

One of the finest constellations of the southern hemisphere is Centaurus, The Centaur, which surrounds Crux on the north and is more than 60° in length. Its center lies about 50° south of Spica in Virgo and below the tail of Hydra. Alpha Centauri, its brightest star and the nearest star to the solar system, four and one-third light-years away, is a golden-yellow double star that forms with the star Beta Centauri on the west a configuration similar to that of Castor and Pollux in Gemini, only one that is far more striking because of the superior brilliancy of the stars. Alpha Centauri lies in the Milky Way and transits the meridian at the same time with Arcturus though it cannot be seen north of the 29th parallel. Alpha Centauri, like Canopus, was an object of worship in Egypt and a number of temples in northern Egypt were oriented to its emergence from the sun's rays in the morning at the autumnal equinox, between 3000 and 2575 B.C.

North of Centaurus is the constellation Lupus, The Wolf, which is also crossed by the Milky Way. According to one myth Lupus is held in the right hand of the Centaur as an offering upon the altar which is represented by the constellation of Ara next to Centaurus on the east. Ara also is crossed by the Milky Way. Neither Lupus nor Ara contain any objects that are worthy of special attention.

Triangulum Australe, The Southern Triangle, a little to the southeast of Alpha Centauri, is far more conspicuous than the Triangulum of the northern hemisphere.

The accompanying charts give two views of these principal southern constellations that lie within 40° of the south pole of the heavens and that are below the horizon in 40° north latitude. The first of these charts shows the constellations that are nearest the meridian in the early evening hours in February. Canopus in Argo Navis and the Greater Magellanic Cloud then lie close to the meridian. Argo Navis with its subdivisions Puppis, Vela and Carina are found east of the meridian lying directly in the path of the Milky Way, which stretches diagonally across the sky from the northwest to the southeast. Far over in the southeast appears Crux, the Southern Cross, also directly in the path of the Milky Way. In the western heavens may be seen the Lesser Magellanic Cloud in Hydrus, brilliant Achernar in Eridanus and the inconspicuous star-group of Tucana.

In the early evening hours of July we find as shown on the second chart, Alpha and Beta Centauri in Centaurus close to the meridian, Lupus due north of Centaurus, Ara and Triangulum Australe in the southeast and Crux in fine position for observation just west of the meridian. Carina of Argo Navis lies to the southwest of Crux. The Milky Way now arches magnificently across the heavens from Carina through Crux, Centaurus and Lupus and Ara to the zodiacal groups of Scorpio and Sagittarius in the northeast.

In the northern part of the heavens, as seen from the southern hemisphere, appear the familiar zodiacal constellations that we of the northern hemisphere find south of the zenith, as well as the constellations of Orion, Lepus and Canis Major, Hydra, Corvus and Crater, Ophiuchus and Serpens and Aquila, all finely in view in their appropriate seasons.

It is only our familiar circumpolar constellations of the north--The Two Bears, Draco, Cassiopeia, and Cepheus, Andromeda and Perseus and Auriga that are invisible in mid-latitudes of the southern hemisphere just as the constellations shown in the diagrams, and a few additional groups such as Pavo, Grus, Phoenix, Apus, Mensa and Volans which we have not shown, lie hidden from view beneath the southern horizon in mid-latitudes of the northern hemisphere.

The northern visitor to the southern hemisphere familiar with the constellations of his own land is filled with a queer sensation of being in topsy-turvydom as he sees familiar Orion standing on his head and all of the zodiacal constellations passing in their daily motions to the north instead of to the south of his zenith while by day the sun passes across the northern part of the heavens and culminates north instead of south of his zenith. He misses the familiar Dippers of his own land and searches in vain for a pole-star in the unfamiliar circumpolar regions of the south.

XV

THE MILKY WAY OR GALAXY

"Broad and ample road whose dust is gold, And pavement stars, as stars to thee appear Seen in the galaxy, that milky way Which nightly as a circling zone thou seest Powder'd with stars."

--MILTON, _Paradise Lost_.

On clear, winter evenings one may see a portion of the zone of the Milky Way, which encircles the heavens, arching magnificently across the heavens as it passes from Cassiopeia and Cepheus in the northwest, through Perseus and Auriga and the eastern part of Taurus, across the feet of Gemini, between Canis Minor and Orion and through the eastern part of Canis Major to the southern horizon.

At this point it passes beyond our range of vision into the star-groups of Puppis, Vela and Carina, subdivisions of the huge southern constellation of Argo Navis. It reaches its greatest distance south of the celestial equator and also attains its greatest brilliancy in Crux, the far-famed Southern Cross. From here it turns northward once more, passing into Centaurus, Musca, Circinus Ara and Lupus constellations of the southern hemisphere and comes within our range of vision again in Sagittarius and Scorpio. Here the Milky Way divides into two branches, though some astronomers now believe that this apparent division into two branches is due to the presence of an enormous cloud of non-luminous matter lying along the course of the Milky Way at this point, similar in its nature to the dark "holes" and "caves" and streaks that appear in all portions of the Milky Way and most noticeably athwart its course in Argo and Centaurus.

One of these branches of the Milky Way passes from Sagittarius through Aquila to Cygnus and the other through Scorpio, Ophiuchus and Serpens to Cygnus, the two extending diagonally across the heavens in the late summer and early fall evenings from the northeast to the southwest. From Cygnus, the Milky Way passes into Cepheus and Cassiopeia and thus completes its circuit of the heavens.

It is not seen to advantage in spring or early summer evenings because it then rests nearly on the horizon. Its plane is inclined about 63° to the celestial equator and its poles lie in the constellations of Coma Berenicis and Cetus. These are the two points that lie farthest from the Milky Way.

The Milky Way has been called the groundwork of the universe. By far the greater number of all the stars are crowded towards its plane in the form of an enormous flattened disk or lens.

Our solar system, it has been estimated, lies close to the plane of the Milky Way and at a distance of some 50,000 or 60,000 light-years from its center. The diameter of the Milky Way as deduced from Dr. Harlow Shapley's work on globular star clusters is about 300,000 light-years in extent, or ten times greater than the limit set some years ago.

The apparent crowding together of the stars into dense clouds in the Milky Way is partly an effect due to our position in the Milky Way. When we look at the heavens in a direction at right angles to this plane we find comparatively few stars lying along our line of vision because the stars are actually fewer in number in this direction. If we look _along_ the plane of the Milky Way, however, we see to a greater distance through an enormous depth of stars. Though individual stars may not be much closer together in the Milky Way than they are outside of it, there are on the whole more of them and the effect of greater density is produced.

Father Hagen of the Vatican Observatory, who has for years made a study of the dark clouds of obscuring matter and dark nebulæ that abound in space, has found evidence of the existence of many vast clouds of dark obscuring matter over the entire heavens above and below the plane of the Milky Way as well as surrounding the Milky Way in its own plane. The existence of such clouds of non-luminous matter would account partly for the comparative fewness of stars in space outside of the plane of the Milky Way since many stars would be concealed from our eyes by these obscuring clouds. There is, however, in addition, an actual crowding of all the visible stars toward this plane.

The peoples of all ages have honored the Milky Way in story and legend. It has been universally referred to as The Sky River and The Pathway of Souls. To the little Hiawatha, we remember, the "wrinkled old Nakomis"

"Showed the broad white road in heaven Pathway of the ghosts, the shadows, Running straight across the heavens, Crowded with the ghosts, the shadows. To the Kingdom of Ponemah To the land of the hereafter."

In _The Galaxy_, Longfellow thus describes the Milky Way:

"Torrent of light and river of the air Along whose bed the glimmering stars are seen Like gold and silver sands in some ravine Where mountain streams have left their channels bare!"

In Sweden, where the Milky Way arches high through the zenith in winter, it is called the Winter Street, and Miss Edith Thomas writes thus beautifully of it in her poem entitled, "The Winter Street":

"Silent with star dust, yonder it lies-- The Winter Street, so fair and so white; Winding along through the boundless skies, Down heavenly vale, up heavenly height.

Faintly it gleams, like a summer road When the light in the west is sinking low, Silent with star dust! By whose abode Does the Winter Street in its windings go?

And who are they, all unheard and unseen-- O who are they, whose blessèd feet Pass over that highway smooth and sheen? What pilgrims travel the Winter Street?

Are they not those whom here we miss In the ways and the days that are vacant below? As the dust of that Street their footfalls kiss Does it not brighter and brighter grow?"

Beautiful indeed are these poetic fancies but none of them picture even remotely the awe-inspiring grandeur of the Milky Way as it actually exists.

Millions upon millions of far distant suns equal to or surpassing our own sun in brilliancy are gathered within this vast encircling zone of the heavens, their combined light giving to the naked eye the impression of a milky band of light. Nine-tenths of all the stars, it has been estimated, lie close to the plane of the Galaxy, as well as all the vast expanses of luminous gaseous nebulæ and clouds of dark obscuring matter all seemingly intermingled in chaotic confusion; yet law and order govern the motions of all. Here also are the great moving star clusters such as the Pleiades and the Hyades and all of the brilliant "Orion" stars.

The structure of the Milky Way is not clearly understood but many astronomers believe there is evidence that it takes the form of a vast spiral nebula along whose arms the stars pass to and fro.

Beyond the Milky Way at enormous distances of many thousands of light-years, but apparently influenced by it, lie the globular star-clusters and the spiral nebulæ. The spirals appear to avoid the plane of the Milky Way for they are receding in the direction of its poles at high velocities; the globular clusters on the other hand are drawing in toward the Milky Way on either side, and in time will cross it.

Whether these objects external to the Milky Way form with it one enormous universe or whether the spiral nebulæ are in turn galaxies or "island universes," as the astronomer calls them, similar in form and structure to our own galaxy and at inconceivably great distances of millions of light-years from it, is still one of the riddles of the universe which the astronomers are attempting to solve.

XVI

THE SURFACE OF THE SUN

The visible surface of the sun is called the _photosphere_. Even the smallest telescopes will show its peculiar "rice-grain" structure, consisting of intensely brilliant flecks or nodules about 500 miles in diameter, which can be resolved by the more powerful telescopes into smaller particles about 100 miles in diameter, against a darker background. It has been estimated that these bright nodules or rice-grains occupy only one-fifth of the total surface of the sun, yet radiate three-fourths of the total light.

It is generally believed that the "rice grains" are the summits of highly heated columns of gas, arising from the sun's interior, and that the darker portions between are cooler descending currents.

It is well known that the photosphere or visible surface of the sun appears to be much brighter in the center of the disk than near its circumference. This is due entirely to its gaseous nature and to the fact that it is surrounded by an atmosphere of dense enveloping cooler gases. Rays from the center of the disk travel a shorter distance through this atmosphere than the rays from the rim and therefore are absorbed less by surrounding gases. We look further down into the sun's interior near the center of the disk than in the direction of its circumference and so the light appears more intense there.

The photosphere is the region where sun-spots appear and they are found in zones extending from 8° to 35° on either side of the solar equator, never appearing exactly at the equator or near the poles.

The disturbances that produce sun-spots and many allied phenomena occur cyclically in periods of eleven years on the average. The first outburst of the disturbance is manifested by the appearance of sun-spots in high solar latitudes. These break out and disappear and break out again with increased vigor, working gradually downward toward the solar equator, the maximum spottedness for a given period occurring in solar latitude about 16°. The disturbance finally dies out within 8° or 10° of the equator, but even before one cycle of disturbance has entirely passed away a new cycle has broken forth in high latitudes. So during the period of minimum spottedness there are four distinct belts, two in low latitudes, due to the dying disturbance, and two in high latitudes, due to the new disturbance. At sun-spot maximums there are two well-marked zones of great intensity, approximately 16° north and south of the sun's equator.

Sun-spots are solar cyclones, occurring usually in groups, though large single spots appear less frequently. Each spot is quite sharply divided into an umbra and a penumbra. The umbra is the darker central portion, the funnel of the whirling cyclone, and the penumbra is composed of the outspreading gases, and is less dark than the umbra. The peculiar "thatch-straw" structure of the penumbra is due, it is believed, to the fact that the columns of gases that usually rise vertically from the sun's interior and from the "rice grains" of the photosphere are drawn into a horizontal position by the whirling motion that exists in the penumbra regions of a sun-spot and therefore we get a longitudinal rather than a cross sectional view of them.

The umbra of a sun-spot is anywhere from a few hundred miles to fifty thousand miles in diameter, frequently exceeding the earth in size, while the penumbra occasionally reaches a diameter of two hundred thousand miles. Sun-spots of exceptional size can be seen even without the aid of a telescope.

The darkness of sun-spots is only by comparison with their more brilliant background. Owing to the rapid expansion and cooling of gases the temperature in sun-spot regions is far below the normal solar temperature of 6,000° Centigrade, lying between 3,000° and 4,000° Centigrade. At this temperature it is possible for the more refractory chemical compounds to form, the oxides and the hydrides, and the spectra of sun-spots reveal the presence of titanium oxide and magnesium and calcium hydride. At the higher solar temperatures that exist elsewhere in the photosphere and in its overlying gaseous envelopes all chemical elements occur in a free state, intermingling as incandescent vapors without the formation of any chemical compounds.

Strong magnetic fields exist in sun-spot regions and magnetic storms in our own atmosphere frequently accompany the appearance of exceptionally large sun-spots.

Directly above the photosphere of the sun lies the "reversing layer," which is about five hundred miles in depth and is composed of the incandescent vapors of all the chemical elements that exist on the sun, which are also the same familiar elements that exist on the earth, with the exception of coronium, the unknown element in the solar corona, there is no element in the sun that has not been found on our own planet.

The "reversing layer" receives its name from the fact that it reverses the solar spectrum. It produces by its absorption of the rays of light from gases below the dark absorption lines found in the spectrum that serve to identify all the elements existing in the sun. During the time immediately preceding and following a total eclipse of the sun this reversing layer produces what is known as the flash spectrum. When the photosphere, which gives the bright continuous background of the solar spectrum, is concealed by the moon, the normally dark lines of the reversing layer--dark only by contrast with the bright background--become momentarily intensely bright lines against a dark background. The flash spectrum only lasts a second or so, as the reversing layer itself is soon covered by the moon.

Just above the reversing layer lies the _chromosphere_, which is between five thousand and ten thousand miles in depth. Many of the gaseous vapors of the reversing layer are found in the chromosphere, thrown there continually by the vast upheavals of gases that are constantly disturbing the surface of the sun. The greater the solar activity the more is the chromosphere charged with the vapors of the lower strata of the sun's atmosphere. The gases that are most characteristic of the chromosphere, however, are the incandescent gases of hydrogen and calcium, which give it the pink or reddish tinge so noticeable during total solar eclipse. Helium is also found in great abundance in the solar chromosphere.