Sir William Herschel: His Life and Works Part 10

In the introduction to his paper on the _Nature and Construction of the Sun and Fixed Stars_ (1795), HERSCHEL recounts what was known of the nature of the sun at that time. NEWTON had shown that it was the centre of the system; GALILEO and his successors had determined its rotation, the place of its equator, its real diameter, magnitude, density, distance, and the force of gravity on its surface. He says:

"I should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun." "The spots have been supposed to be solid bodies, the smoke of volcanoes, the sc.u.m floating on an ocean of fluid matter, clouds, opaque ma.s.ses, and to be many other things." "The sun itself has been called a globe of fire, though, perhaps, metaphorically." "It is time now to profit by the observations we are in possession of.

I have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena."

HERSCHEL then refers to the theories advanced by his friend, Prof.

WILSON, of Glasgow, in 1774. WILSON maintained that the spots were depressions below the sun's atmosphere, vast hollows as it were, at the bases of which the true surface of the sun could be seen.

The essence of his theory was the existence of two different kinds of matter in the sun: one solid and non-luminous--the nucleus--the other gaseous and incandescent--the atmosphere. Vacant places in the atmosphere, however caused, would show the black surface of the solid ma.s.s below. These were the spots. No explanation could be given of the _faculae_, bright streaks, which appear on the sun's surface from time to time; but his theory accounted for the existence of the black _nuclei_ of the spots, and for the existence of the _penumbrae_ about these. The penumbra of a spot was formed by the thinner parts of the atmosphere about the vacancy which surrounded the nucleus.

This theory of WILSON'S was adopted by HERSCHEL as a basis for his own, and he brought numerous observations to confirm it, in the modified shape which he gave to it.

According to HERSCHEL, the sun consisted of three essentially different parts. First, there was a solid nucleus, non-luminous, cool, and even capable of being inhabited. Second, above this was an atmosphere proper; and, lastly, outside of this was a layer in which floated the clouds, or bodies which gave to the solar surface its intense brilliancy:

"According to my theory, a dark spot in the sun is a place in its atmosphere which happens to be free from luminous decompositions"

above it.

The two atmospheric layers, which will be of varying thickness about a spot, will account for all the shades of darkness seen in the penumbra.

Ascending currents from the solar surface will elevate certain regions, and may increase the solar activity near by, and will thus give rise to faculae, which HERSCHEL shows to be elevated above the general surface.

It will not be necessary to give a further account of this theory. The data in the possession of the modern theorist is a thousand-fold that to be derived from HERSCHEL'S observations, and, while the subject of the internal construction of the sun is to-day unsettled, we know that many important, even fundamental, portions of his theory are untenable.

A remark of his should be recorded, however, as it has played a great part in such theories:

"That the emission of light must waste the sun, is not a difficulty that can be opposed to our hypothesis. Many of the operations of Nature are carried on in her great laboratory which we cannot comprehend. Perhaps the many telescopic comets may restore to the sun what is lost by the emission of light."

Arguments in favor of the habitability of both sun and moon are contained in this paper; but they rest more on a metaphysical than a scientific basis, and are to-day justly forgotten.

_Researches on the Motion of the Sun and of the Solar System in s.p.a.ce._

In 1782 HERSCHEL writes, in regard to some of his discoveries of double stars:

"These may serve another very important end. I will just mention it, though it is foreign to my present purpose. Several stars of the first magnitude have been observed or suspected to have a proper motion; hence we may surmise that our sun, with all its planets and comets, may also have a motion towards some particular point of the heavens. . . . If this surmise should have any foundation, it will show itself in a series of some years in a kind of systematical parallax, or change, due to the motion of the whole solar system."

In 1783 he published his paper _On the Proper Motion of the Solar System_, which contained the proofs of his surmises of a year before.

That certain of the stars had in fact a _proper_ motion had been well established by the astronomers of the eighteenth century. After all allowances had been made for the effects of precession and other displacements of a star's position which were produced by motions of the earth, it was found that there were still small outstanding differences which must be due to the motion of the star itself--its proper motion.

The quant.i.ty of this motion was not well known for any star when HERSCHEL'S researches began. Before they were concluded, however, MASKELYNE had deduced the proper motions of thirty-six stars--the fundamental stars, so called--which included in their number _Sirius_, _Procyon_, _Arcturus_, and generally the brightest stars.

It is _a priori_ evident that stars, in general, must have proper motions, when once we admit the universality of gravitation. That any fixed star should be entirely at rest would require that the attractions on all sides of it should be exactly balanced. Any change in the position of this star would break up this balance, and thus, in general, it follows that stars must be in motion, since all of them cannot occupy such a critical position as has to be a.s.sumed. If but one fixed star is in motion, this affects all the rest, and we cannot doubt but that every star, our sun included, is in motion by an amount which varies from small to great. If the sun alone had a motion, and the other stars were at rest, the consequence of this would be that all the fixed stars would appear to be retreating _en ma.s.se_ from that point in the sky towards which we were moving. Those nearest us would move more rapidly, those more distant less so. And in the same way, the stars from which the solar system was receding would seem to be approaching each other.

If the stars, instead of being quite at rest, as just supposed, had motions proper to themselves, then we should have a double complexity.

They would still appear to an observer in the solar system to have motions, and part of these motions would be truly proper to the stars, and part would be due to the advance of the sun itself in s.p.a.ce.

Observations can show us only the _resultant_ of these two motions.

It is for reasoning to separate this resultant into its two components.

At first the question is to determine whether the results of observation indicate any solar motion at all. If there is none, the proper motions of stars will be directed along all possible lines. If the sun does truly move, then there will be a general agreement in the resultant motions of the stars near the ends of the line along which it moves, while those at the sides, so to speak, will show comparatively less systematic effect. It is as if one were riding in the rear of a railway train and watching the rails over which it has just pa.s.sed. As we recede from any point, the rails at that point seem to come nearer and nearer together.

If we were pa.s.sing through a forest, we should see the trunks of the trees from which we were going apparently come nearer and nearer together, while those on the sides of us would remain at their constant distance, and those in front would grow further and further apart.

These phenomena, which occur in a case where we are sensible of our own motion, serve to show how we may deduce a motion, otherwise unknown, from the appearances which are presented by the stars in s.p.a.ce.

In this way, acting upon suggestions which had been thrown out previously to his own time by LAMBERT, MAYER, and BRADLEY, HERSCHEL demonstrated that the sun, together with all its system, was moving through s.p.a.ce in an unknown and majestic orbit of its own. The centre round which this motion is directed cannot yet be a.s.signed. We can only know the point in the heavens towards which our course is directed--"the apex of solar motion."

By a study of the proper motions a.s.signed by MASKELYNE to the brighter stars, HERSCHEL was able to define the position of the solar apex with an astonishing degree of accuracy. His calculations have been several times repeated with the advantage of modern a.n.a.lytical methods, and of the hundred-fold material now at our disposition, but nothing essential has been added to his results of 1805, which were based upon such scanty data; and his paper of 1782 contains the announcement of the discovery itself.

His second paper on the _Direction_ and _Velocity_ of the solar system (1805) is the best example that can possibly be given of his marvellous skill in reaching the heart of a matter, and it may be the one in which his philosophical powers appear in their highest exercise. For sustained reflection and high philosophic thought it is to be ranked with the researches of NEWTON in the _Principia_.

_Researches on the Construction of the Heavens._

HERSCHEL'S papers on the Construction of the Heavens, as he named it, extended over his whole scientific life. By this he specially means the method according to which the stars, the cl.u.s.ters, the nebulae, are spread through the regions of s.p.a.ce, the causes that have led to this distribution, and the laws to which it is subjected.

No single astronomical fact is unimportant in the light which it may throw on the scheme of the whole, and each fact is to be considered in this light. As an instance: his discovery of the variable star _[alpha] Herculis_, which has a period of sixty days, was valuable in itself as adding one more to the number of those strange suns whose light is now brighter, now fainter, in a regular and periodic order.

But the chief value of the discovery was that now we had an instance of a periodic star which went through all its phases in sixty days, and connected, as it were, the stars of short periods (three to seven days) with those of very long ones (three hundred to five hundred days), which two groups had, until then, been the only ones known. In the same way all his researches on the parallaxes of stars were not alone for the discovery of the distance of any one or two single stars, but to gain a unit of celestial measure, by means of which the depths of s.p.a.ce might be sounded.

Astronomy in HERSCHEL'S day considered the bodies of the solar system as separated from each other by distances, and as filling a cubical s.p.a.ce.

The ideas of near and far, of up and down, were preserved, in regard to them, by common astronomical terms. But the vast number of stars seemed to be thought of, as they appear in fact to exist, lying on the surface of a hollow sphere. The immediate followers of BRADLEY used these fixed stars as points of reference by which the motions within the solar system could be determined, or, like LACAILLE and LALANDE, gathered those immense catalogues of their positions which are so indispensable to the science. MICh.e.l.l and HERSCHEL alone, in England, occupied their thoughts with the nature and construction of the heavens--the one in his study, the other through observation.[34] They were concerned with all three of the dimensions of s.p.a.ce.

In his memoir of 1784, HERSCHEL says:

"Hitherto the sidereal heavens have, not inadequately for the purpose designed, been represented by the concave surface of a sphere, in the centre of which the eye of an observer might be supposed to be placed.

"It is true the various magnitudes of the fixed stars even then plainly suggested to us, and would have better suited, the idea of an expanded firmament of three dimensions; but the observations upon which I am now going to enter still farther ill.u.s.trate and enforce the necessity of considering the heavens in this point of view. In future, therefore, we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed, as well as consisting of very different materials. The surface of a globe or map, therefore, will but ill delineate the interior parts of the heavens."

HERSCHEL'S method of study was founded on a mode of observation which he called _star-gauging_. It consisted in pointing a powerful telescope toward various parts of the heavens, and ascertaining by actual count how thick the stars were in each region. His twenty-foot reflector was provided with such an eye-piece that, in looking into it, he saw a portion of the heavens about 15' in diameter. A circle of this size on the celestial sphere has about one quarter the apparent surface of the sun, or of the full moon. On pointing the telescope in any direction, a greater or less number of stars were visible. These were counted, and the direction in which the telescope pointed was noted. Gauges of this kind were made in all parts of the sky, and the results were tabulated in the order of right ascension.

The following is an extract from the gauges, and gives the average number of stars in each field at the points noted in right ascension and north polar distance:

---------------------------------------------------------- | N. P. D. || | N. P. D.

R. A. | 78 to 80. || R. A. | 92 to 94.

| No. of Stars. || | No. of Stars.

------------|-----------------||-----------|-------------- H. M. | || H. M. | 11 6 | 3.1 || 15 10 | 9.4 12 31 | 3.4 || 15 22 | 10.6 12 44 | 4.6 || 15 47 | 10.6 12 49 | 3.9 || 16 8 | 12.1 13 5 | 3.8 || 16 25 | 13.6 14 30 | 3.6 || 16 37 | 18.6 ----------------------------------------------------------

In this small table, it is plain that a different law of cl.u.s.tering or of distribution obtains in the two regions. Such differences are still more marked, if we compare the extreme cases found by HERSCHEL, as R. A. = 19h 41m, N. P. D. = 74 33', number of stars per field = 588; and R. A. = 16h 10m, N. P. D. = 113 4', number of stars = 1.1.

The number of stars in certain portions is very great. For example, in the Milky Way, near _Orion_, six fields of view promiscuously taken gave 110, 60, 70, 90, 70, and 74 stars each, or a mean of 79 stars per field.

The most vacant s.p.a.ce in this neighborhood gave 60 stars. So that as HERSCHEL'S sweeps were two degrees wide in declination, in one hour (15) there would pa.s.s through the field of his telescope 40,000 or more stars. In some of the sweeps this number was as great as 116,000 stars in a quarter of an hour.

When HERSCHEL first applied his telescope to the Milky Way, he believed that it completely resolved the whole whitish appearance into small stars. This conclusion he subsequently modified. He says:

"It is very probable that the great stratum called the Milky Way is that in which the sun is placed, though perhaps not in the very centre of its thickness.

"We gather this from the appearance of the Galaxy, which seems to encompa.s.s the whole heavens, as it certainly must do if the sun is within it. For, suppose a number of stars arranged between two parallel planes, indefinitely extended every way, but at a given considerable distance from each other; and calling this a sidereal stratum, an eye placed somewhere within it will see all the stars in the direction of the planes of the stratum projected into a great circle, which will appear lucid on account of the acc.u.mulation of the stars, while the rest of the heavens, at the sides, will only seem to be scattered over with constellations, more or less crowded according to the distance of the planes, or number of stars contained in the thickness or sides of the stratum.

"If the eye were placed somewhere without the stratum, at no very great distance, the appearance of the stars within it would a.s.sume the form of one of the smaller circles of the sphere, which would be more or less contracted according to the distance of the eye; and, if this distance were exceedingly increased, the whole stratum might at last be drawn together into a lucid spot of any shape, according to the length, breadth, and height of the stratum.

"Suppose that a smaller stratum should branch out from the former in a certain direction, and that it also is contained between two parallel planes, so that the eye is contained within the great stratum somewhere before the separation, and not far from the place where the strata are still united. Then this second stratum will not be projected into a bright circle like the former, but it will be seen as a lucid branch proceeding from the first, and returning into it again at a distance less than a semicircle. If the bounding surfaces are not parallel planes, but irregularly curved surfaces, a.n.a.logous appearances must result."

The Milky Way, as we see it, presents the aspect which has been just accounted for, in its general appearance of a girdle around the heavens and in its bifurcation at a certain point, and HERSCHEL'S explanation of this appearance, as just given, has never been seriously questioned. One doubtful point remains: are the stars scattered all through s.p.a.ce? or are they near its bounding planes, or cl.u.s.tered in any way within this s.p.a.ce so as to produce the same result to the eye as if uniformly distributed?

HERSCHEL a.s.sumed that they were nearly equably arranged all through the s.p.a.ce in question. He only examined one other arrangement, _viz._, that of a ring of stars surrounding the sun, and he p.r.o.nounced against such an arrangement, for the reason that there is absolutely nothing in the size or brilliancy of the sun to cause us to suppose it to be the centre of such a gigantic system. No reason, except its importance to us personally, can be alleged for such a supposition. Every star will have its own appearance of a Galaxy or Milky Way, which will vary according to the situation of the star.