It is often said that the Sun exhibits a granulated structure. If we wish to realise in the most precise manner what is meant by the word “granulation” as applied to the structure of the Sun, we must abandon the method of projection and examine the Sun directly with a powerful eyepiece, taking advantage of a moment when the atmosphere is perfectly calm, and before the eyepiece has had time to get hot. It may then be seen that the Sun’s surface is covered with a multitude of little grains, nearly all of about the same size, but of different shape, though for the most part more or less oval. The small interstices which separate these grains form a network which is dark without being positively black. Secchi considered it difficult to name any known object which exactly answers in appearance to this structure, but he thought that we can find something resembling it in examining with a microscope milk which has been a little dried up, and the globules of which have lost their regular form. Exceptionally good atmospheric conditions are under all circumstances indispensable for the study of these details.
In point of fact, there is a mysterious uncertainty about the normal condition of the Sun’s surface, in a visual sense, which a few years ago engendered a very vehement controversy, and led to the use of such expressions as “willow leaves,” “rice grains,” “sea beach,” and “straw thatching,” to indicate what was seen. All these words are too precise to be quite suitable to be taken literally, but perhaps, on the whole, “rice grains” is not altogether a bad expression to recall what certainly seems to be the granular surface of the Sun as we see it.
By making use of moderate magnifying powers, what we see will often convey the impression of a multitude of white points on a black network. This is very apparent during the first few moments that the telescope is brought to bear on the Sun, but its clearness quickly passes away because the eye gets fatigued, and the lenses becoming warm the air in the telescope tube gets disturbed because also warmed. Sometimes the appearance is a little different from that just described, and along with the white and brilliant points little black holes are intermixed. Oftentimes the grains appear as if suspended in a black network and heaped together in knots more or less shaded and more or less broad. Sometimes the grains exhibit a very elongated form, especially in the neighbourhood of the spots. It is these elongated forms to which Nasmyth applied the term “willow leaf,” whilst Huggins thought “rice grains” a very suitable expression.
This granular or leaf-like structure—call it what we will—cannot be made out except with considerable optical assistance, for the grains being intrinsically very small, diffraction in enlarging them and causing them to encroach on one another necessarily produces a general confusion of image. The real dimensions of these grains cannot therefore readily be determined by direct measurement, but by comparing them with the wires used in micrometer eye-pieces it has been thought that their diameters may usually be regarded as equal to ¼ or ⅓ of a second—say from 120 to 150 miles. The granules seem to be possessed of sensible movement, but presumably it is not always or even generally a movement of translation from place to place; only an undulatory movement like that of still water when a stone is cast into it. Nevertheless, probably in certain cases the granules actually are affected by a motion of translation, for in the vicinity of spots they may sometimes be seen flowing over the edges of the penumbræ. In order to explain the existence of the granules the strangest theories have been broached. Sir William Herschel having observed the granulations, applied to them the term “corrugations” or “furrows”—words somewhat inexact, perhaps, but by which, as his descriptions clearly show, he meant to designate the features which I am now treating of. He even noticed the dark network which separates the grains, and he applied to it the word “indentations.”
These granulations are without doubt prominences, probably of hydrogen gas, which rise above the general surface, for this structure is much more sharp and distinct at the centre of the sun’s disc than at the limbs; that is to say, near the limbs of the Sun they partially overlap one another, as indeed Herschel remarked. The idea of flames would satisfy these appearances: and as the spectroscope suggests to us that the Sun is habitually covered over with a multitude of little jets of flame, the observations which have been made compel the opinion that the grains are the summits of those prominences which exist all over the Sun’s surface.
The surface is sometimes so thickly covered over with these granulations—the network is so conspicuous—that we can readily imagine that we see everywhere pores and the beginnings of spots, but this aspect is not permanent, and seems to depend to some extent on atmospheric causes combined also with actual changes in the Sun’s surface itself. There seems however no doubt that the joints, so to speak, of the dark network already referred to do sometimes burst asunder and develope into spots.
The circumstances which accompany the formation of a spot cannot readily be specified with certainty. It is impossible to say that there exists any law as to this matter. Whilst some spots develope very slowly by the expansion of certain pores, others spring into existence quite suddenly. Yet it cannot be said that the formation of a spot is ever completely instantaneous however rapid it may be. The phenomenon is often announced some days in advance: we may perceive in the photosphere a great agitation which often manifests itself by some very brilliant faculæ (to be described presently) giving birth to one or more pores. Very often we next notice some groups of little black spots, as if the luminous stratum was becoming thinner in such a way as to disappear little by little and leave a large black nucleus uncovered. At the commencement of the business there is usually no clearly defined penumbra. This developes itself gradually and acquires a regular outline, just as the spot itself often takes a somewhat circular form. This tranquil and peaceable formation of a spot only happens at a time when calm seems to reign in the solar atmosphere: in general the development is more tumultuous and the stages more complicated.
As a rule a spot passes through three stages of existence:—(1) the Period of birth; (2) a Period of calm; and (3) the Period of dissolution. When a spot is on the point of closing up, the flow of the luminous matter which it, as it were, attracts, is not directed uniformly towards the centre; it seems that the photospheric masses, no longer meeting with resistance, are precipitated promiscuously anywhere so as to fill up the hole. It is impossible to describe in detail the phases which irregular spots go through, but two things may always be remarked: that their structure is characterized by the existence of luminous filaments, and that these filaments converge towards one or several centres.
Secchi thus sums up certain conclusions which he arrived at relating to spots generally:—(1) It is not on the surface of any solid body that the solar spots are manifested; they are produced in a fluid mass, the fluidity of which is represented by a gas, so that the constitution of this medium may be likened to that of flames or clouds; (2) the known details respecting the constitution of the penumbra and the phenomena exhibited prove that the penumbra is not a mass of obscure matter which floats across luminous matter, but that it is on the contrary a case of luminous matter invading and floating about over darker materials and so producing a half tint.
All the available evidence which we possess may be said to show that the spots are not merely superficial appearances, but that they have their origin deep in the interior of the Sun, and are produced by the operation of causes still unknown to us which affect and disturb the Sun’s mass to an extent which is sometimes very considerable. The spots then are only the results of a great agitation in the materials of which the Sun is composed, and this agitation extends far down below the limits of the visible dark nucleus whatever that may consist of.