YET MORE ABOUT THE SUN.
Having seen something of storms taking place on the sun’s photosphere,
we must next give our attention to storms taking place at his edge. But
it should be remembered that the said edge, far from being a mere rim
to a flat surface, is a kind of horizon-line--is, in fact, just that
part of the photosphere which is passing out of or coming under our
sight. The surface there is, in kind, the same as the surface of the
broad disk facing us. In watching outbursts at the edge of the sun, we
have a side view instead of a bird’s-eye view.
In the year 1871, Professor Charles A. Young, of Princeton, was looking
at a large hydrogen cloud on the edge of the sun. When I speak of a
“cloud,” it must not be supposed that anything like a damp, foggy,
earthly cloud is meant. This solar cloud was a huge mass of red-hot
gas, about one hundred thousand miles long, rising to a height of fifty
thousand miles from the sun’s surface, and appearing to rest on glowing
pillars of fire.
The professor, while watching, was called away for half an hour. He
came back, expecting to find things much as he had left them. Instead
of this, a startling change had taken place. The whole mass of glowing
fire seemed to have been actually “blown to shreds” by some tremendous
outburst from below. In place of the motionless cloud were masses of
scattered fire, each from about four thousand to fourteen thousand
miles long, and a thousand miles wide.
As the professor gazed, these “bits” of broken cloud rose rapidly
upwards, away from the surface of the sun. When I say “rapidly,” I mean
that the real movement, which the professor could calculate, was rapid.
The seeming movements were of course slow, and over a small space. The
actual motions were not tardy, for in ten minutes these huge, fiery
cloud-pieces rushed upwards to a height of two hundred thousand miles
from the edge of the sun, moving at a rate of at least one hundred and
sixty-seven miles each second. Gradually they faded away.
But what caused this sudden change? Just before the professor was
interrupted, he had noticed a curious little brilliant lump--a sort of
suspicious thunder-cloud appearance--below the quiet, bright cloud. And
after this tremendous shattering, the little bright lump rose upwards
into a huge mass of rolling flame, reaching like a pyramid to a height
of fifty thousand miles. In the course of a few minutes these enormous
flames could be seen to move and bend, and to curl over their gigantic
tips. But they did not last long. At half-past twelve the professor
had been called away; by half-past two the rolling flames completely
vanished.
Now, whatever may be the full explanation of this sight, there is no
doubt that on that day was observed from earth a tremendous outburst,
compared with which our mightiest volcanoes are like the sputtering of
a farthing dip beside a roaring furnace. The awful force and greatness
of such a solar eruption are more than we can possibly picture to
ourselves. At our distance we may catch a faint glimpse of what is
going on, and calculate speed of movement. But vividly to realize the
actual terrific grandeur of what took place is past our power.
Possibly this was much the same kind of outburst as that seen by the
two English astronomers; only theirs was a bird’s-eye view, as it
were, looking down on the top of the sight, while the professor had a
side-view, certainly much the best for observation.
It does not follow from what he saw that the eruption must have taken
place exactly at the “edge” of the sun. Probably it happened near
the edge. All he could say, was that the flames rose fifty thousand
miles, and the pieces of cloud were carried two hundred thousand miles,
away from the edge. The eruption may have begun on the other side of
the sun, at any distance from the horizon-edge where it first became
visible to earthly eyes.
Also, while the professor found that the shattered cloudlets moved at
a rate of about one hundred and sixty-seven miles each second, it is
calculated that the first fearful outburst must have caused movement,
near the surface of the sun, at a rate of at least three hundred miles
each second. Probably the hydrogen cloud was borne upwards along with
a vast mass of fragments flung out from the sun. We are here upon
doubtful ground; but this tremendous power of eruption in the sun,
and of driving matter out of and away from his surface, should not be
forgotten.
Though such a sun-storm as that just described is not often to be seen,
yet there are at all times certain strange red prominences, or glowing
flames, rising up here and there from the sun’s “limb.” Doubtless they
rise also from other parts of the photosphere, though they are only
visible to us when near enough to the edge to stand out beyond it.
Seen during an eclipse, these prominences have clear, sharp outlines,
and are usually bright rose-red in color. They are described as
sometimes wide and low, sometimes tall and slender; sometimes jagged,
sometimes regular; sometimes keeping long the same shape, sometimes
changing quickly in a few minutes. They are said to be like flames,
like mountains, like the teeth of a saw, like icebergs, like floating
cloudlets.
As to their height, from fifty to eighty thousand miles is nothing
unusual. We must not speak of Mont Blanc or Mount Everest here. Jupiter
placed bodily on the surface of the sun, beside such a fire-mountain,
would not far overtop it. The earth, Venus, Mars, and Mercury, would
lie like little toy-balls at its foot. And these are common-sized
sun-flames. One was measured which reached to the enormous height
of two hundred thousand miles. The spectroscope shows these solar
prominences or jets to be made--at least in part--of burning hydrogen
gas.
Beyond the sierra or chromatosphere--that border of rippling, crimson
flame-billows round the edge of the sun, with red flame-mountains
rising out of it here and there--beyond these, stretches the corona.
The corona, as seen from earth, is a bright, far-reaching glory of
light, shining round the sun in a total eclipse. The moon then comes
between the sun and the earth, her dark, round body creeping over the
face of the sun till the bright photosphere is completely covered. But
the sierra and the tall, red flames stand out from behind the black
moon, and the beautiful, soft corona-light stretches far beyond.
[Illustration: SOLAR CORONA AND PROMINENCE.]
It was long doubted whether the corona really belonged to the sun or to
the moon. There seems now no doubt that it is a part of the sun.
Various descriptions of the corona have been given at different
times, as observed during different eclipses. It has been seen as a
steady, beamy, white cloud behind the moon, showing no flickering.
It has been seen marked with bright lines of light, and seeming to
move rapidly round and round. It has been seen silvery white, sending
off long streams of brightness. It has been seen in the form of
white light, with bluish rays running over it. It has been seen with
entangled jets of light, like “a hank of thread in disorder.” It has
been seen silvery-white again, with a faint tinge of greenish-violet
about the outer edge. It has been seen from a high mountain-top as a
mass of soft, bright light, “through which shot out, as if from the
circumference of the moon, straight, massive, silvery rays, seeming
distinct and separate from each other, to a distance of two or three
diameters of the lunar disk, the whole spectacle showing as upon a
background of diffused, rose-colored light.”
Majestic, indeed, are the proportions of some of those mighty flames
which leap from the surface of the sun, yet these flames flicker, as
do our terrestrial flames, when we allow them time comparable to their
gigantic dimensions. Drawings of the same prominence often show great
changes in a few hours, or even less. The magnitude of the changes
could not be less than many thousands of miles, and the actual velocity
with which such masses move is often not less than one hundred miles
a second. Still more violent are the solar convulsions, which some
observers have been so fortunate as to behold, when from the sun’s
surface, as from a mighty furnace, vast incandescent masses are
projected upwards. All indications point to the surface of the sun as
the seat of the most frightful storms and tempests, in which the winds
sweep along incandescent vapors.
The corona consists of two parts--the inner and brighter corona, the
outer and fainter corona. The shape of the whole seems to change much
at different times. The outer edge is usually blurred and indistinct,
fading gently away.
Many explanations have been suggested. At one time the corona was
supposed to be a solar atmosphere, reflecting light like our own
atmosphere. Some have thought the light might be caused by countless
myriads of meteorite systems, revolving in the close neighborhood of
the sun. Some suppose it may be owing, in part at least, to solar
eruptions, and the pouring outward of burning gas and matter. But our
knowledge of the true nature of the corona is yet in its infancy.
A few closing words as to the size and weight of the sun. In diameter,
eight hundred and fifty-eight thousand miles, and in bulk equal to
one million two hundred and eighty thousand earths, his weight is in
proportion less. Our earth is about four times as dense as the sun. If
her size were increased to the sun’s size, her density being the same
as now, she would be very much heavier than the sun, and would attract
much more strongly. Still, though the sun is of lighter materials than
the earth, his immense size gives him weight equal to seven hundred and
fifty times as much as all the planets put together.
The attraction on the surface of the sun is also very great--so great
that we can hardly picture it to ourselves. If life exists there at
all--supposing it possible that any kind of life can be in such a
fiery atmosphere--it must be life very different from any known in
this world. A man who on earth weighs one hundred and sixty pounds,
and walks lightly erect, would, on the sun, lie helplessly bound to
the ground, crushed by his own overpowering weight. It is said that a
cannon-ball, reposing on the sun, if lifted one inch and allowed to
fall, would dash against the ground with a speed three times greater
than that of our fastest express-trains. For weight on earth is merely
caused by the amount of force with which the earth draws downward a
body towards herself--a force greater or less according to the density
of that body. So weight on the sun would be immensely increased by his
immensely greater power of attraction.
[Illustration: COMPARATIVE SIZE OF THE EARTH AND SUN.]
It is an interesting question how far the sun’s attractive influence
reaches effectually through space. The nearer a body is to the sun,
the greater the attraction which he exercises over it. At the distance
of the planet Mercury, a speed of twenty-nine miles each second is
needful to overcome or balance it sufficiently for the planet to remain
in his orbit. At the distance of the planet Neptune, about three miles
each second is enough. If a planet were journeying at four times the
distance of Neptune, the speed would need to be not over two miles each
second, lest the planet should break loose and wander away. But even
two miles a second is no mean speed--more than seven thousand miles an
hour. If we come to speak of that which we on earth call rapid motion,
we shall gain a clearer idea as to the extent of the sun’s power.
Suppose a planet were traveling through space at the rate of one of our
fastest express-trains--sixty miles an hour. It has been calculated
that, unless the sun’s attraction were interfered with and overpowered
by some nearer sun, the said planet, though placed at a distance ten
or twelve times as great as that of the far-off star Alpha Centauri,
would still be forced by the sun’s attraction to journey round him in a
closed orbit. At such a speed it would not be free to wander off into
the depths of space.