THE SUN
But now let us turn to the Solar System, and consider the members of our own little colony.
Within the Solar System there are a large number of problems that interest us. What is the size, mass, and distance of each of the planets? What satellites, like our Moon, do they possess? What are their temperatures? And those other, sporadic members of our system, comets and meteors, what are they? What are their movements? How do they originate? And the Sun itself, what is its composition, what is the source of its heat, how did it originate? Is it running down?
These last questions introduce us to a branch of astronomy which is concerned with the physical constitution of the stars, a study which, not so very many years ago, may well have appeared inconceivable. But the spectroscope enables us to answer even these questions, and the answer opens up questions of yet greater interest. We find that the stars can be arranged in an order of development—that there are stars at all stages of their life-history. The main lines of the evolution of the stellar universe can be worked out. In the sun and stars we have furnaces with temperatures enormously high; it is in such conditions that substances are resolved into their simplest forms, and it is thus we are enabled to obtain a knowledge of the most primitive forms of matter. It is in this direction that the spectroscope (which we shall refer to immediately) has helped us so much. It is to this wonderful instrument that we owe our knowledge of the composition of the sun and stars, as we shall see.
"That the spectroscope will detect the millionth of a milligram of matter, and on that account has discovered new elements, commands our admiration; but when we find in addition that it will detect the nature of forms of matter trillions of miles away, and moreover, that it will measure the velocities with which these forms of matter are moving with an absurdly small per cent. of possible error, we can easily acquiesce in the statement that it is the greatest instrument ever devised by the brain and hand of man."
Such are some of the questions with which modern astronomy deals. To answer them requires the employment of instruments of almost incredible refinement and exactitude and also the full resources of mathematical genius. Whether astronomy be judged from the point of view of the phenomena studied, the vast masses, the immense distances, the æons of time, or whether it be judged as a monument of human ingenuity, patience, and the rarest type of genius, it is certainly one of the grandest, as it is also one of the oldest, of the sciences.
The Solar System
In the Solar System we include all those bodies dependent on the sun which circulate round it at various distances, deriving their light and heat from the sun—the planets and their moons, certain comets and a multitude of meteors: in other words, all bodies whose movements in space are determined by the gravitational pull of the sun.
The Sun
Thanks to our wonderful modern instruments and the ingenious methods used by astronomers, we have to-day a remarkable knowledge of the sun.
Look at the figure of the sun in the frontispiece. The picture represents an eclipse of the sun; the dark body of the moon has screened the sun's shining disc and taken the glare out of our eyes; we see a silvery halo surrounding the great orb on every side. It is the sun's atmosphere, or "crown" (corona), stretching for millions of miles into space in the form of a soft silvery-looking light; probably much of its light is sunlight reflected from particles of dust, although the spectroscope shows an element in the corona that has not so far been detected anywhere else in the universe and which in consequence has been named Coronium.
We next notice in the illustration that at the base of the halo there are red flames peeping out from the edges of the hidden disc. When one remembers that the sun is 866,000 miles in diameter, one hardly needs to be told that these flames are really gigantic. We shall see what they are presently.
Regions of the Sun
The astronomer has divided the sun into definite concentric regions or layers. These layers envelop the nucleus or central body of the sun somewhat as the atmosphere envelops our earth. It is through these vapour layers that the bright white body of the sun is seen. Of the innermost region, the heart or nucleus of the sun, we know almost nothing. The central body or nucleus is surrounded by a brilliantly luminous envelope or layer of vaporous matter which is what we see when we look at the sun and which the astronomer calls the photosphere.
Above—that is, overlying—the photosphere there is a second layer of glowing gases, which is known as the reversing layer. This layer is cooler than the underlying photosphere; it forms a veil of smoke-like haze and is of from 500 to 1,000 miles in thickness.
A third layer or envelope immediately lying over the last one is the region known as the chromosphere. The chromosphere extends from 5,000 to 10,000 miles in thickness—a "sea" of red tumultuous surging fire. Chief among the glowing gases is the vapour of hydrogen. The intense white heat of the photosphere beneath shines through this layer, overpowering its brilliant redness. From the uppermost portion of the chromosphere great fiery tongues of glowing hydrogen and calcium vapour shoot out for many thousands of miles, driven outward by some prodigious expulsive force. It is these red "prominences" which are such a notable feature in the picture of the eclipse of the sun already referred to.
During the solar eclipse of 1919 one of these red flames rose in less than seven hours from a height of 130,000 miles to more than 500,000 miles above the sun's surface. This immense column of red-hot gas, four or five times the thickness of the earth, was soaring upward at the rate of 60,000 miles an hour.
These flaming jets or prominences shooting out from the chromosphere are not to be seen every day by the naked eye; the dazzling light of the sun obscures them, gigantic as they are. They can be observed, however, by the spectroscope any day, and they are visible to us for a very short time during an eclipse of the sun. Some extraordinary outbursts have been witnessed. Thus the late Professor Young described one on September 7, 1871, when he had been examining a prominence by the spectroscope:
It had remained unchanged since noon of the previous day—a long, low, quiet-looking cloud, not very dense, or brilliant, or in any way remarkable except for its size. At 12:30 p.m. the Professor left the spectroscope for a short time, and on returning half an hour later to his observations, he was astonished to find the gigantic Sun flame shattered to pieces. The solar atmosphere was filled with flying debris, and some of these portions reached a height of 100,000 miles above the solar surface. Moving with a velocity which, even at the distance of 93,000,000 miles, was almost perceptible to the eye, these fragments doubled their height in ten minutes. On January 30, 1885, another distinguished solar observer, the late Professor Tacchini of Rome, observed one of the greatest prominences ever seen by man. Its height was no less than 142,000 miles—eighteen times the diameter of the earth. Another mighty flame was so vast that supposing the eight large planets of the solar system ranged one on top of the other, the prominence would still tower above them.
FIG. 5.—DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN
Compare with frontispiece.
FIG. 6.—SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, May 29, 1919. TAKEN AT SOBRAL, BRAZIL.
The small Corona is also visible.
FIG. 8.—THE SUN
Photographed in the light of glowing hydrogen, at the Mount Wilson Observatory of the Carnegie Institution of Washington: vortex phenomena near the spots are especially prominent.
FIG. 7.—THE VISIBLE SURFACE OF THE SUN
A photograph taken at the Mount Wilson Observatory of the Carnegie Institution at Washington.
The fourth and uppermost layer or region is that of the corona, of immense extent and fading away into the surrounding sky—this we have already referred to. The diagram (Fig. 5) shows the dispositions of these various layers of the sun. It is through these several transparent layers that we see the white light body of the sun.