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.[1]
[1] _The Romance of Astronomy_, by H. Macpherson.
[Illustration: FIG. 5.--DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN