Part 12

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.