Anyone who takes a cursory glance at the heavens on a clear night can readily perceive that there exists considerable diversity of colour among the stars. The contrast between some is pronounced and well marked, whilst others exhibit refined gradations of hue.

The most numerous class of stars are those which are described as white or colourless. They comprise about one-half of the stars visible to the naked eye. Among the most conspicuous examples of this type are Sirius—whose diamond blaze is sometimes mingled with an occasional flash of blue and red—Altair, Spica, Castor, Regulus, Rigel, all the stars of Ursa Major with the exception of one, and Vega—a glittering gem of pale sapphire, almost colourless. The light emitted by stars of this class gives a continuous spectrum, the predominating element being hydrogen, having a very elevated temperature and under relatively high pressure. The vapours of iron, sodium, magnesium, and other metals, are indicated as existing in small quantities.

The second class of stars is that to which our Sun belongs. They are of a yellow colour, and embrace two-thirds of the remaining stars. The most prominent examples of this type are Arcturus, Capella, Aldebaran, Procyon, and Pollux. Hydrogen does not predominate so much in these as in the Sirian stars, and their spectra resemble closely the solar spectrum, indicating that they are composed of elements similar to those which exist in the Sun.

The star which bears the nearest resemblance to our Sun, both as regards the colour of its light and physical structure, is Capella, the most conspicuous star in the constellation Auriga, and one of the leading brilliants in the Northern Hemisphere. Its spectrum presents all the characteristics observed in the solar spectrum, and there exists an almost identical similarity in their physical constitution, though Capella is a much more magnificent orb than the Sun.

The third class of stars includes those which are of a ruddy hue, such as Betelgeux in the right shoulder of Orion, Antares in Scorpio, and Alpha Herculis. Their spectra present a banded or columnar appearance, and there is greater absorption, especially of the blue rays of light. It is believed that the temperature of stars of this colour is not so elevated as that of those belonging to the other two orders, and that this is a sufficient reason to account for the different appearance of their spectra.

The aid of a good telescope is, however, necessary to enable us to perceive the varied colours and tints of the sparkling gems with which Nature has adorned her star-built edifice of the universe. Most of the precious stones on Earth have their counterparts in the heavens, presenting in a jewelled form contrasts of colour, pleasing harmonies, and endless variety of shade. The diamond, sapphire, emerald, amethyst, topaz, and ruby sparkle among crowds of stars of more sombre hue. Agate, chalcedony, onyx, opal, beryl, lapis-lazuli, and aquamarine are represented by the radiant sheen emanating from distant suns, displaying an inexhaustible variety of colour, blended in tints of untold harmony.

It is among double stars that the richest and most varied colours predominate. There are pairs of white, yellow, orange, and red stars; yellow and blue, yellow and pale emerald, yellow and rose red, yellow and fawn, green and gold, azure and crimson, golden and azure, orange and emerald, orange and lilac, orange and purple, orange and green, white and blue, white and lilac, lilac and dark purple, &c., &c. There are companion stars revolving round their primaries, coloured olive, lilac, russet, fawn, dun, buff, grey, and other shades indistinguishable by any name.

Our knowledge of binary star systems brings us to what may be regarded as the threshold of the fabric of the heavens. For it is known that other systems exist into the construction of which numerous stars enter. These form intricate and complex stellar arrangements, in which the component stars are physically united and retained in their orbits by their mutual attraction.



CHAPTER VII

THE STARRY HEAVENS

TRIPLE, QUADRUPLE, AND MULTIPLE STARS.—These, when observed with the naked eye, appear as single stars, but, when examined with a high magnifying power, each lucid point can be resolved into several component stars. They vary in number from three to half a dozen or more, and form systems of a more complex character than what are observed in the case of binary stars. In the usual construction of a triple system, the secondary star of a binary is resolvable into two, each star being in mutual revolution, whilst they both gravitate round their primary. By another arrangement, a close pair control the movements of a distant attendant.

One of the most interesting of triple stars is the tricoloured Gamma Andromedae. The brilliant components of this system have their counterparts in the topaz, the emerald, and the sapphire—the larger star is of the third magnitude and of a golden yellow colour; the secondary of the fifth magnitude and of an emerald green. These stars are ten seconds apart, and, though they have been under observation since 1777, no orbital movement has as yet been detected, but their common proper motion indicates their close relationship and physical connection. In 1842, Otto Struve discovered that the companion star is itself double, and round it there gravitates a sapphire sun, which is believed to accomplish a revolution of its orbit in about 500 years. If round those suns there should be circling planetary systems of worlds inhabited by intelligent beings, the varied effects produced by the light emanating from those different coloured orbs would be of a very beautiful and pleasing nature.

A system suggestive of the endless variety of stellar arrangement that exists throughout the sidereal regions is apparent in the case of the triple star Zeta Cancri. Two of the stars, of magnitudes six and seven, form a binary in rapid revolution, the components of which complete a circuit of their orbits in fifty-eight years, whilst the more distant third star, of almost similar magnitude, accomplishes a wide orbital ellipse round the other two in 500 or 600 years. These stars have been closely observed by astronomers during the past forty years, with the result that their motions have appeared most perplexing, and complicated beyond precedent. 'If this be really a ternary system,' wrote Sir John Herschel, 'connected by the mutual attraction of its parts, its perturbations will present one of the most intricate problems in physical astronomy.' The second star revolves round its primary, whilst the third pursues a retrograde course, but its path, instead of being even, presents the appearance of a series of circular loopings, in traversing which the star alternately quickens and slackens its pace, or at times appears to be stationary.

Astronomers have arrived at the conclusion that these perturbations are produced by the presence of a fourth member, which, though invisible, is probably the most massive of the system—perhaps a magnificent world teeming with animated beings, and attended by three suns which gravitate round it, dispensing light and heat to meet the requirements of the various forms of life which exist on its surface. In this system we have an arrangement the reverse of what exists in the solar system, where all the planets revolve round a predominant sun; but here there is a strange verification of the old Ptolemaic belief with regard to the path of a sun, though in this instance there are three suns circling round a dark globe which they illumine and vivify.

Triple stars occur with comparative frequency throughout the heavens. In Monoceros there is a fine triple star, discovered by Herschel, which he describes as 'one of the most beautiful sights in the heavens.' The stars Xi and Beta Scorpii form triple systems in which the components are differently arranged. In Xi the primary and secondary consist of two revolving stars which control the movements of a distant attendant; in Beta the primary and secondary stars are in mutual revolution, whilst round the former there circles a very close minute companion. There are doubtless many binary stars which, if examined with adequate telescopic power, would resolve themselves into triple and multiple systems, but the profound distances of those objects render the detection of their components a most difficult task.

Quadruple stars are usually arranged in pairs, i.e. the primary and secondary of a binary system are each resolvable into two, forming two pairs, each pair being in mutual revolution, while they both gravitate round their common centre of gravity. Epsilon Lyrae, which has been described as a double double, is an example of a quadruple system, and Nu Scorpii is of a similar construction, but more beautiful because its components are in closer proximity to each other. Close upon twenty of those double double systems have been discovered in different parts of the heavens.

One of the most interesting of quadruple systems is Theta Orionis, which is situated in the Great Nebula, by which it is surrounded. This star, when observed with a telescope of low power, can be at once resolved into four separate lucent points, so arranged as to form a quadrilateral figure or trapezium. They are of the fifth, sixth, seventh, and eighth magnitudes, and described as pale white, garnet, faint lilac, and red. Though they have been under careful observation for upwards of two centuries, no perceptible motion has been perceived as occurring among them, nor has there been any change in their relative positions—they appear to be perfectly motionless; but we must not infer from this that no physical bond of union exists between them, for they are situated at an amazing distance from the Earth. Ascending higher in the scale of celestial architecture, we have multiple stars forming systems still more elaborate and complex, into the structure of which numerous stars enter, and they, as they increase in number, gradually merge into star-clusters.

If we assume that around each of the components of a multiple star there circles a retinue of planetary worlds, we are confronted with a most perplexing problem as to how the dynamical stability of a system so different from, and so vastly more complicated than, that of our solar system is maintained—where, as it were, suns and planets intermingle—how numerous circling orbs can accomplish their revolutions without being swayed and deflected from their paths by the gravitational attraction of adjacent members of the same system. Perplexing though the arrangement of such a scheme may be to our conception, yet, each orb has been weighed, poised, and adjusted by Infinite Wisdom, to perform its intricate motions in synchronous harmony with other members of the system—all moving in unison like the parts of a complicated piece of mechanism, and maintained in stable equilibrium by their mutual attraction—

Mystical dance, which yonder starry sphere Of planets and of fixed in all her wheels Resembles nearest; mazes intricate, Eccentric, intervolved, yet regular Then most, when most irregular they seem; And in their motions harmony divine So smooths her charming tones that God's own ear Listens delighted.—v. 620-27.

All the natural phenomena with which we are familiar would, in the case of planets revolving round the component suns of a multiple system, be of a different kind or altogether absent. Instead of being illumined by one sun, those worlds would, at certain times, have several suns—some more distant than others—above their horizons, and upon very rare occasions, if ever, would there be an entire absence of all of those orbs from their skies. Consequently there would be no year such as we are familiar with; no regular sequence of seasons similar to what is experienced on Earth; no alternation of day and night, for there would be 'no night there,' though, in the absence of the primary orb, the light emitted by distant suns, whilst sufficient to banish night, and beyond comparison brighter than the Moon when at full, would, in the diminution of its intensity from that of noonday, be as grateful a change as that of from day to night which occurs on our globe.

Should those suns be differently coloured, each emitting its own peculiar shade of light as it appears above the horizon, the varied aspects of the perpetual day enjoyed by the inhabitants of those circling worlds present to the imagination harmonies of light and shade over which it is pleasant to linger.

TEMPORARY, PERIODICAL, AND VARIABLE STARS.—It may seem remarkable that among so many thousands of stars which spangle the firmament, there should occur no very perceptible change or variation in their aspect and brilliancy. From age to age they present the same appearance, shine with the same undiminished splendour, and rise and set with the same regularity. So that from time immemorial the stars have been regarded by mankind as the embodiment of all that is eternal and unchangeable. Yet, the serenity of the celestial regions does not always remain undisturbed—at occasional times a 'Nova,' or new star, blazes forth unexpectedly in the heavens, and perplexes astronomers; and, after shining with a varying degree of brilliancy for a few weeks or months, gradually diminishes in size and brightness and eventually becomes lost to sight.

A record has been kept of about twenty temporary stars that have been observed at various periods since the time that reliable data of those objects have been published. Pliny mentions the appearance of a new star in the time of Hipparchus (134 B.C.); it was seen in the constellation of the Scorpion, and it is said that it was the apparition of this star which induced the celebrated astronomer to construct what is known as the earliest star catalogue. A new star is said to have become visible when the Emperor Honorius ruled, and another during the reign of the Emperor Otho, about 945 A.D. In May 1012 a new star appeared in Aries, and in July 1203 another was observed in Scorpio, which resembled Saturn. The most remarkable star of this kind was one observed by Tycho Brahe, which appeared in the constellation Cassiopeia. He first perceived it on November 11, 1572. In lustre it equalled Jupiter, and when at its brightest rivalled Venus; it was visible at noonday, and at night its light could be perceived through strata of cloud which rendered all other stars invisible. The star maintained its brilliancy for three weeks, when it became of a yellowish colour and perceptibly decreased in size; it afterwards assumed a ruddy hue resembling Aldebaran, and, diminishing gradually in magnitude and brightness, ceased to be visible in March 1574. It twinkled more than the other stars, and during the time it could be perceived its position remained unchanged. In 1604 a conspicuous new star burst forth in Ophiuchus. It surpassed in brilliancy stars of the first magnitude, and outshone the planet Jupiter, which was in its proximity. Kepler observed this star, and described it as 'sparkling like a diamond with prismatic tints.' It soon began to decline after its appearance; in March 1605 it had shrunk to the dimensions of a third-magnitude star, and in a year later it became entirely lost to view. Other stars of the same class, though of a less conspicuous character, have been observed at occasional times. Anthelme, a Carthusian monk, discovered one near Beta Cygni in 1670; another appeared in Ophiuchus in 1848; one in Scorpio in 1860; one in Corona Borealis in 1866; in Cygnus in 1876; in Andromeda in 1885; and in Auriga in 1892.

Various theories have been advanced in order to account for the sudden outbursts of those stars, the light from which has probably occupied not much less than one hundred years in its passage hither. It has been suggested that the collision of two suns, or of two great masses of matter, would create such phenomena; but, apart from the improbability of such a catastrophe occurring among the celestial orbs, the rapid subsidence in the luminosity of the observed objects would indicate that the outburst was produced by causes of a more rapidly transitory nature than what would result from the collision of two condensed masses of matter. A collision occurring between two swarms of meteors has been suggested as one way of accounting for the sudden appearance of those stars; but another, and more plausible, explanation is that they are produced by a great eruption of glowing gas from the interior of a sun, causing an enormous increase in its luminosity, which subsides after a time, and is succeeded by a normal condition of things. It has been observed that all those temporary stars, with the exception of two, have appeared in the region of the Milky Way. In this luminous zone the condensation of small gaseous stars and nebulae is more pronounced than in any other part of the heavens, and this would seem to indicate that there may be cosmical changes taking place among them which need not be associated with the occurrence of catastrophes resulting in the conflagration of worlds, and that Nature, in accomplishing her purposes, does not overstep the uniform working of her laws, upon which depend the stability and existence of the universe.

PERIODICAL AND VARIABLE STARS are distinguished from other similar objects by the fluctuations which occur in the quantity of light emitted by them. The difference in the luminosity of some stars is at times so marked that, in a few weeks or months, they decline from the first or second magnitudes to invisibility, and, after the expiration of a certain period, they again gradually regain their pristine condition. When these changes take place with regular recurrence, they are called 'periodical;' when they occur in a variable and uncertain manner, they are called 'irregular.' About 300 stars are known as variable, but the majority of them are telescopic objects. Their periodical changes of brilliancy present every degree of variety; in some stars they are scarcely perceptible and occur at long intervals; in others, changes of brightness occur in a few hours or days, by which the light emitted is intensified many hundreds of times.

Some stars accomplish their cycle of change in a few days, many in a few weeks or months, and there are others which do not complete their periods until the expiration of a number of years.

One of the most remarkable of variable stars is called Mira 'the wonderful,' in the constellation Cetus. When at its maximum brilliancy it shines for two or three weeks as a star of the second magnitude. It then begins to gradually decline, and at the end of three months becomes invisible. It remains invisible for five months, and then reappears, and during the ensuing three months it regains by degrees its former brilliancy. Mira completes a cycle of its changes in 334 days, and, during that time, oscillates between a star of the second and tenth magnitude. The variability of Mira Ceti was first observed by David Fabricius in the sixteenth century.

Another remarkable star is Eta Argus, which is surrounded by the great nebula in the constellation Argo Navis. It is invisible to the naked eye, but in the telescope it has a reddish appearance, and is slightly brighter than the stars in its vicinity. It was first observed by Halley in 1677, and it was then of the fourth magnitude. In 1751 it had risen to the second magnitude, and maintained its position as a star of this class until 1837, when, on December 16 of that year, its brilliancy suddenly increased, and it equalled in a short time Alpha Centauri. It reached its maximum in 1843, and then it was surpassed only by Sirius. It maintained its brilliancy for about ten years. In 1858, it declined to the second magnitude, in 1859 to the third, and, gradually diminishing, it became invisible to the naked eye in 1868. It is now of the seventh magnitude, and is again increasing, and may soon resume its position among the other stars. It is believed to have a period of seventy years, and in that time its light ebbs and flows between the seventh and first magnitudes.

The most interesting variable star in the heavens is Algol (the demon), in the constellation Perseus. Its light fluctuations can be observed without the aid of a telescope, and it completes a cycle of its changes in two or three days. For about two days and thirteen hours it is conspicuously visible as a star of the second magnitude; it then begins to decline, and in about four hours sinks to the dimensions of a fourth-magnitude star; it remains in this condition for twenty minutes, and then increases gradually until, at the expiration of four hours, it regains its former brilliancy, which it sustains for two days and thirteen hours, when it again goes through the same cycle of changes in a precisely similar manner to what has been described. Astrologers have ascribed many evil influences to the demon star, which adorned the head of Medusa; nor did it escape the observation of ancient astronomers that this malevolent orb is—as a modern writer amusingly remarks—slowly winking at us from out the depths of space.

Variable stars are found in greater numbers in some parts of the heavens than in others. Those of a white colour, and with shorter and more regular periods, are most numerous in the region of the Milky Way; those that are small, with long periods and of a reddish hue, are more widely removed from that zone. Stars of this class are all very remote, and no attempt has as yet been made to ascertain the parallax of Algol.

Several theories have been suggested in order to account for the periodical brilliancy of those stars. It has been suggested that the stars have opaque non-luminous patches on their surfaces, and that during axial rotation their light ebbs and flows according as the dark or bright portions are turned towards us. This theory is highly improbable. Another and more plausible reason, especially with regard to short period variables, is, that around those stars there revolve opaque bodies or satellites which at times intercept a portion of their light by producing a partial eclipse of their discs, similar to that caused by the dark body of the Moon when passing between the Sun and the Earth.

It is now known that in the case of variables of the Algol type, the periodical fluctuations of their light arises from this cause, and that round Algol there is a dark world or satellite travelling, which completes a revolution of its orbit in about sixty-nine hours, and that, during each circuit, it intercepts one half of the light of its primary by partially eclipsing the orb, and thereby creating a diminution in its apparent magnitude which becomes perceptible at recurring intervals.

STAR GROUPS.—These are plentifully scattered over the heavens and, by their conspicuous brilliancy, add to the grandeur and magnificence of the midnight sky. The Hyades in Taurus, of which Aldebaran is the chief, forming the eye of the Bull, attract attention.

The stars in Coma Bernices form a rich group; the sickle in Leo, the seven stars in Ursa Major, and those in Cassiopeia and Aquila are familiarly known to all observers. Besides these, there are many other groups and aggregations of stars which adorn the celestial vault and enhance the beauty of the heavens.

STAR CLUSTERS.—On observing the heavens on a clear, dark night, there can be seen in different parts of the sky closely aggregated groups of stars called clusters. In some instances the component stars are so near together that the naked eye is unable to discern the individual members of the cluster. They then assume an indistinct, hazy, cloudlike appearance. Upwards of 500 clusters are known to astronomers, the majority of which are very remote. Many of them contain thousands of stars compressed into a very small space, and others are so distant that the largest telescopes are incapable of resolving their nebulous appearance into separate stars.

Star clusters have been arranged into two classes, 'irregular' and 'globular;' but no sharp line of demarcation exists between them, though each have their distinctive peculiarities. Irregular clusters consist of aggregations of stars brought promiscuously together, and presenting an appearance devoid of any structural arrangement. They are of different shapes and sizes, possess no distinct outline, and are not condensed towards their centre, like those that are globular. On examination, they present an intricate reticulated appearance; streams and branches of stars extend outwards from the parent cluster, sometimes in rows and sinuous lines, and, in other instances, diverging from a common centre, forming sprays. Sometimes the stars are seen to follow each other on the same curve which terminates in loops and arches of symmetrical proportions.

There are three conspicuous clusters in the northern sky that are visible to the naked eye—viz. the Pleiades in Taurus, the Great Cluster in the sword-handle of Perseus, and Praesepe in Cancer, commonly called the Beehive.

The cluster which from time immemorial has had bestowed upon it the chief attention of mankind are the beautiful Pleiades or Seven Sisters, and intertwined among its stars are the legendary and mythological beliefs of ancient nations and untutored tribes inhabiting the different regions of the globe. When viewed with a telescope of moderate size the cluster appears as a scattered group, and numerous stars become visible that are imperceptible to ordinary vision.

In the sword-handle of Perseus there is a cluster which, to the naked eye, appears as a small patch of luminous cloud. This inconspicuous object when observed with an instrument of moderate power is resolved into a magnificent assemblage of stars, and presents a spectacle which creates in the mind of the beholder mingled feelings of admiration and amazement. No telescope has yet penetrated its utmost depths, or revealed all the glories of this shining region, crowded with glittering points of light comparable in number to the pebbles strewn on the shore of a troubled sea.

The cluster Praesepe in Cancer is visible on a clear night to the unaided eye as a small nebula. This object attracted the attention of Galileo, to which he applied his newly invented telescope, and was delighted to find that his glass was capable of resolving it into a group of stars thirty-six in number, and all of comparatively large magnitude. The disappearance of Praesepe in consequence of the condensation of vapour in the atmosphere was regarded by the ancients as a sure indication of approaching rain. In the same constellation, near the Crab's southern claw, there is another rich cluster, which consists of 200 stars of the ninth and tenth magnitudes.

In Sobieski's Shield there is a magnificent fan-shaped cluster of minute stars with a prominent one in its centre; and in the constellation of the Southern Cross there is a cluster which, on account of the varied colours of its component stars, has been compared by Sir John Herschel to 'a piece of rich fancy jewellery;' eight of the principal stars being coloured red, green, and blue.

GLOBULAR CLUSTERS.—These have been described by Herschel as 'the most magnificent objects that can be seen in the heavens.' They are all very remote, of a rounded form, and when viewed with a telescope present the appearance of 'a ball of stars.' In some clusters the constituent stars are distinguishable as minute points of light; in others, more remote, they are of a coarse granular texture, and in those still more distant they resemble a 'heap of golden sand.' Some clusters are situated at such a profound distance in space that it is impossible with the most powerful of telescopes to define their stellar structure; all that can be distinguished of these is a cloudy luminosity resembling in appearance an irresolvable nebula. Globular clusters usually present a radiated appearance. Rays, branches, and spiral-shaped streams of stars appear to flow from the circumference of some; and, in other instances, fantastic appendages of stars project outwards from the parent cluster. There doubtless exists much variety in the structural arrangement of these clusters, and an equal diversity in the magnitude and number of the stars which enter into their formation. The stars in some clusters may equal those of the first magnitude, and in others they may not exceed in dimensions the minor planets. In the telescope they vary in size from the eleventh to the fifteenth magnitude; the smaller stars occupy the centre of a cluster, whilst the larger ones are found near its circumference. Globular clusters are more condensed towards their centre than those of irregular shape, and some have a nucleated appearance. This apparent condensation is not altogether owing to the depth of star strata as viewed from the circumference of the cluster, but there appears to exist an attractive force (probably gravitational) which draws the stars towards its centre, and if this 'clustering power' were not opposed by some other counteracting force, those bodies would coalesce into one mass. It may be 'that a centrifugal impulse predominates by which full-grown orbs are driven from the nursery of suns in which they were reared to seek their separate fortunes and enter on an independent career elsewhere.'

It is not known how the dynamical equilibrium of a star cluster is maintained; and on account of its extreme distance no motion is perceptible among its component stars. The laws by which those stellar aggregations are produced and governed are wrapped in obscurity, and the nature of the motions of their stars, whether towards concentration or diffusion, cannot at present be ascertained. If those globular clusters could be observed sufficiently near, they would most probably expand into vast systems of suns occupying immense regions of space.

The largest and most magnificent globular cluster in the heavens is Omega Centauri, in the Southern Hemisphere. To the naked eye it resembles a round, indistinct, cometary object, about equal to a star of the fourth magnitude; but when observed with a powerful telescope it appears as a globe of considerable dimensions composed of innumerable stars of the thirteenth and fifteenth magnitudes, all exceedingly minute and gathered into small knots and groups. A remarkable cluster in Toucani is described by Sir John Herschel as 'most magnificent; very large; very bright, and very much compressed in the middle.' The interior mass consists of closely aggregated pale rose-coloured stars, surrounded by others of a pure white which embrace the remainder of the cluster. There is a fine globular cluster in Sagittarius between the Archer's head and the bow. It was observed by Hevelius in 1665. The central portion is very much compressed, and consists of excessively minute stars enclosed by others of larger size. In Aquarius there is a magnificent ball of stars of a beautiful spherical form, which Sir J. Herschel compared to a heap of fine sand. Numerous other clusters are profusely distributed over the heavens, occupying regions in the profound depths of space which can only be reached by the aid of most powerful instruments.

The finest and most remarkable object of this class visible in the northern heavens is the Great Cluster which lies between Eta and Zeta Herculis. It was discovered by Halley in 1714, who writes: 'This is but a little patch, but it shows itself to the naked eye when the sky is serene and the moon absent.' When observed with a powerful telescope its magnificence at once becomes apparent to the beholder. 'Perhaps,' says Dr. Nichol, 'no one ever saw it for the first time through a telescope without uttering a shout of wonder.' At its circumference the stars are rather scattered, but towards the centre they appear so closely aggregated that their combined effulgence forms a perfect blaze of light. Sir William Herschel estimated that there are 14,000 stars in the cluster, each a magnificent world but unaccompanied by any planetary attendants.



As a result of more recent investigations this number has been considerably reduced, and it is now generally believed that about 4,000 stars enter into the formation of the cluster. As its distance from the Earth is unknown, it follows that there must be some uncertainty attached to any conclusions that may be arrived at with regard to this superb object. Miss Agnes Clerke estimates the number of the constituent stars at 4,000, and in support of her conclusion this talented lady writes as follows: 'The apparent diameter of this object, including most of the "scattered stars in streaky masses and lines" which form a sort of "glory" round it, is 8'; that of its truly spherical portion may be put at 5'. Now, a globe subtending an angle of 5' must have (because the sine of that angle is to radius nearly as to 1 : 687) a real diameter 1/687 of its distance from the eye, which, if we assume to be such as would correspond to a parallax of 1/20 of a second, we find that the cluster, outliers apart, measures 558,000 millions of miles across. Light, in other words, occupies thirty-six days in traversing it, but sixty-five years in journeying thence hither. Its components may be regarded, on an average, as of the twelfth magnitude; for, although the divergent stars rank much higher in the scale of brightness, the central ones, there is reason to believe, are notably fainter. The sum total of their light, if concentrated into one stellar point, would at any rate very little (if at all) exceed that of a third-magnitude star. And one star of the third is equivalent to just four thousand stars of the twelfth magnitude. Hence we arrive at the conclusion that the stars in the Hercules Cluster number much more nearly four than fourteen thousand.'

For what purpose do those thousands of clustering orbs shine? Who can tell? Night is unknown in the regions illumined by their brilliant radiance. This stupendous aggregation of suns testifies to the magnificence of the starry heavens, and to the omnipotence of the Creator.

GALAXIES.—These consist of vast aggregations of stars which form separate 'island universes' floating in the depths of space; they are believed to equal in magnitude and magnificence the Milky Way—the galaxy to which our system belongs.

NEBULAE.—We now reach the last, and what are believed to be the most distant of the known contents of the heavens. They are all exceedingly remote, devoid of any perceptible motion, faintly luminous, and, with the exception of two of their number, invisible to the naked eye. Halley was the first astronomer who paid any attention to those objects. In 1716 he enumerated six of them, but of this number only two can, in a strict sense, be regarded as nebulae, the others since then have been resolved into magnificent star clusters. In 1784, Messier catalogued 103 nebulae, and the Herschels—father and son—in their survey of the stellar regions, discovered 4,000 of those objects. There are now 8,000 known nebulae in the heavens, but the majority of them are not of much interest to astronomers. Prior to the invention of the spectroscope it was believed that all nebulae were irresolvable star clusters, but the analysis of their light by this instrument indicated that their composition was not stellar but gaseous. Their spectra consist of a few bright lines revealing the presence of hydrogen, nitrogen, and other gaseous elements.

Much that is mysterious and uncertain is associated with those objects which appear to lie far beyond the limits of our sidereal system. It is now generally believed that they exhibit the earliest stage in the formation of stars and planets—inchoate worlds in process of slow evolution, which will eventually condense into systems of suns, and planetary worlds.

Nebulae present every variety of form. Some are annular, elliptic, circular, and spiral; others are fan-shaped, cylindrical, and irregular, with tufted appendages, rays, and filaments. A fancied resemblance to different animated creatures has been observed in some. In Taurus there is a nebula called the 'Crab' on account of its likeness to the crustacean; another is called the 'Owl Nebula' from its resemblance to the face of that bird. The Orion Nebula suggests the opened jaws of a fish or sea monster, hence called the Fish-Mouth Nebula. There is a Horse-Shoe Nebula, a Dumb-Bell Nebula, and many others of various shapes and forms. They are classified as follows: (1) Annular Nebulae, (2) Elliptic Nebulae, (3) Spiral Nebulae, (4) Planetary Nebulae, (5) Nebulous Stars, (6) Large Irregular Nebulae.

ANNULAR NEBULAE.—These resemble in appearance an oval-shaped luminous ring; they are comparatively few in number, and not more than a dozen have been discovered in the whole heavens. The most remarkable object of this class is the Ring Nebula, which is situated between the stars Beta and Gamma Lyrae. It is visible in a moderate-sized telescope as a well-defined, flat, oval ring; its central part is not quite dark but is occupied by a filmy haze of luminous matter which is prolonged inwards from the margin of the ring. When examined with a high power the edges of the ring have a fringed appearance, and numerous glittering stellar points become visible both within and without its circumference. This nebulous ring, though a small object in the telescope, is of enormous magnitude, and if it were not more distant than 61 Cygni, one of the nearest of the fixed stars, its diameter would not be less than 20,000 millions of miles, but it has been estimated by Herschel that it is 900 times more remote than Sirius. How stupendous, then, must be its dimensions, and how bewildering to our conception is the profound immensity of space in which it is located! An annular nebula similar to that of Lyra, but on a smaller scale, is found in Cygnus, and within it there can be seen a conspicuous star. Another exists in Scorpio which contains two stars situated within the ring at diametrically opposite points to each other.

ELLIPTICAL NEBULAE.—The most interesting object of this class is the Great Nebula in Andromeda, called 'the transcendentally beautiful queen of the nebulae'—an appellation which it scarcely merits. This object, which is plainly visible to the naked eye, is of an oval shape, of a milky white colour, and is situated near the most northern star of the three which form the girdle of Andromeda. It was known to the ancients, and Ali Sufi, a Persian astronomer who flourished in the tenth century, alludes to it; but it did not attract much attention until the seventeenth century. Simon Marius was the first to observe this object with a telescope. This he did on December 15, 1612; he describes it as shining with a pale white light resembling in appearance the flame of a candle when seen through a semi-transparent piece of horn. When examined with a high magnifying power it is seen to occupy a largely extended area measuring 4 deg. in length and 2-1/2 deg. in breadth. Its luminosity increases from the circumference to the centre, where there can be seen a small nucleus with an ill-defined boundary, which has the appearance of being granular, but its composition is not stellar. Two dark channels running almost parallel to each other and to the axis of the nebula have been observed by Bond; these, when prolonged, form into curves which terminate in two great rings. They are wide rifts which separate streams of nebulous matter, and are indicative that some formative processes may be going on within the nebula.

Astronomers have been baffled in their attempts to discover the nature of the Andromeda Nebula. Though great telescopes have been able to render visible thousands of stars over and around it, yet the nebula itself is irresolvable and bears no trace of stellar formation; neither, according to Dr. Huggins, is its spectrum gaseous, a circumstance which deepens the mystery associated with this object. Its distance is unknown, and its dimensions cannot be ascertained.

Other elliptical nebulae are found in different regions of the heavens. In Ursa Major there is an oval nebula resembling that of Andromeda, but on a much smaller scale. It possesses a nucleus, and on the photographic plate there can be detected the presence of spiral structure, indicating the existence of streams of nebulous matter. Adjacent to this nebula is another of the same class with a double nucleus, and associated with it is a nebulous star.

SPIRAL NEBULAE.—The great reflector of Earl Rosse at Parsonstown was the successful means by which nebulae of this form were discovered. This powerful telescope was capable of defining with greater accuracy the structural formation of those objects than any other instrument in use. It was ascertained that spiral coils and convoluted whorls enter into the structure of most nebulae, indicating a similarity in the process of change which may be going on in these vast accumulations of cosmical matter. The most interesting specimen of a spiral nebula is situated in Canes Venatici. It consists of spiral coils emanating from a centre with a nucleus and surrounded by a narrow luminous ring. In appearance it resembles the coiled mainspring of a watch.

PLANETARY NEBULAE.—These have been so named on account of the resemblance which they bear to the discs of planets. They are of uniform brightness, circular in shape, with sharply-defined edges, and are frequently of a bluish colour. They are more numerous than annular nebulae; three-fourths of their number are in the Southern Hemisphere, and they are situated in or very near the Milky Way. Those objects were first described by Sir William Herschel, who was rather perplexed as to what was their real nature and how he should classify them. He remarked that they could not be planets belonging to far-off suns, nor distant comets, nor distended stars. Consequently, he concluded rightly that they were nebulae. When observed with large telescopes, they lose their planetary aspect, and their sharpness of outline is less apparent; their discs become broken up into bright and dark portions, and in some, numerous minute stars have been observed, whilst others have well-defined nuclei.

The most prominent nebula of this class is situated in the constellation Ursa Major, and is called the Owl Nebula, from its fancied resemblance to the face of that bird. Sir John Herschel describes it as 'a most extraordinary object, a large, uniform nebulous disc, quite round, very bright, not sharply defined, but yet very suddenly fading away to darkness.' When examined in 1848 with Earl Rosse's reflector, two bright stars were discovered in its interior; each was in the centre of a circular dark space surrounded by whorls of nebulous matter—hence the origin of its name. This nebula gives a bright line spectrum indicative of gaseous composition. It is believed to consist chiefly of hydrogen and other gases which form a globe of such stupendous magnitude that, if we surmise its distance from the earth to be sixty-five light years—an estimate much too low—'its diameter would exceed that of the orbit of Neptune upwards of 100 times.'[10] Within its compass the orbs of hundreds of solar systems as large as that of ours would be able to perform their revolutions, having spacious intervals existing between each system. Another interesting planetary nebula is in the constellation of the Dragon, near to the pole of the ecliptic; it is slightly oval, of a pale blue colour, and contains a star of the eleventh magnitude in its centre. It gives a gaseous spectrum. Attempts have been made to determine its parallax, but without success, and during the eighty years it has been under observation it has remained apparently motionless. Its light period, if estimated at 140 years, would indicate the existence of a globe with a diameter equal to forty-four diameters of the orbit of the planet Neptune.[11] A nebula of this class was discovered by Sir John Herschel in the Centaur. He described it as resembling Uranus, but larger; its colour was of a beautiful rich blue, and its light equalled that of a star of the seventh magnitude.

NEBULOUS STARS.—These stars are each surrounded by a luminous haze several minutes of arc in diameter and of a circular form. Sir William Herschel, by his observation of those objects, arrived at the conclusion 'that there exists in space a shining fluid of a nature totally unknown to us, and that the nebulosity about those stars was not of a starry nature.' Thirteen stars of this type have been enumerated by him and many others have since been discovered. The 'glow' which surrounds them has been observed in a few instances to have vanished without leaving any trace of nebulosity behind, but the causes which have brought about such a result are entirely unknown. The nature of those stars is involved in considerable obscurity, and one class of nebula would seem to merge into the other; nebulous stars with faint aureolae do not differ much from small nebulae interspersed with stellar points.

LARGE IRREGULAR NEBULAE.—These are found in both hemispheres, and are remarkable on account of the varied appearances which they present, and the large extent of space which many of them occupy. In some, the nebulous matter of which they are composed can be seen like masses of tufted flocculi, sometimes piled up, and at other times promiscuously scattered, resembling in appearance the foam on the crested billows of a surging ocean rendered suddenly motionless, or cirro-cumuli floating in a tranquil sky. Islands of light with intervening dark channels, promontories projecting into gulfs of deep shade, sprays of luminous matter, convoluted filaments, whorls, wreaths, and spiral streams all enter into the structural formation of a great nebula.

The Great Nebula in Argo, in the Southern Hemisphere, is one of the most remarkable objects of this class. It consists of bright irregular masses of luminous matter, streaks and branches, and occupies an area about equal to one square degree. At its eastern border is situated the variable star Eta Argus, which fluctuates between the first and seventh magnitudes in a period of about seventy years.

A rich portion of the Galaxy lies in front of the nebula, which creates an effect as if it were studded over with stars. Sir John Herschel, in describing this nebula, writes as follows:—'The whole is situated in a very rich and brilliant part of the Milky Way, so thickly strewed with stars that, in the area occupied by the nebula, not less than 1,200 have been actually counted. Yet it is obvious that these have no connection whatever with the nebula, being, in fact, only a simple continuation over it of the general ground of the Galaxy. The conclusion can hardly be avoided that, in looking at it, we see through and beyond the Milky Way, far out into space, through a starless region, disconnecting it altogether from our system. It is not easy for language to convey a full impression of the beauty and sublimity of the spectacle which this nebula offers as it enters the field of view of a telescope, fixed in right ascension, by the diurnal motion, ushered in as it is by so glorious and innumerable a procession of stars, to which it forms a sort of climax, and in a part of the heavens otherwise full of interest.' Another large bright nebula (called 30 Doradus), also in the Southern Hemisphere, is composed of a series of loops with intricate windings forming a kind of open network against the background of the sky which it adorns. Sir John Herschel describes it as one of the most extraordinary objects in the heavens.

The 'Crab' Nebula in Taurus, the 'Horse-Shoe' Nebula in Sobieski's Shield, and the 'Dumb-Bell' Nebula in Vulpecula are remarkable objects, but the assistance of a powerful telescope is required to bring out their distinctive features. The 'Crab' Nebula is partially resolvable into stars; the other two are believed to be gaseous.

The largest and most remarkable of all the nebulae is that known as the Great Nebula in Orion, which was discovered and delineated by Huygens in the middle of the seventeenth century. It is perceptible to the naked eye, and when viewed with a glass of low power can be seen as a circular luminous haze surrounding the multiple star Theta Orionis—one of the stars in the Giant's Sword, and which is of itself a remarkable object. The most conspicuous part of the nebula bears a slight resemblance to the wing of a bird; it consists of flocculent masses of nebulous matter possessing a faint greenish tinge. Sir John Herschel compared it to a surface studded over with flocks of wool, or to the breaking up of a mackerel sky when the clouds of which it consists begin to assume a cirrous appearance. Its brightest portion is occupied by four conspicuous stars, which form a trapezium; around each there is a dark space free from nebulosity, a circumstance which would seem to indicate that the stars possess the power either of absorbing or of repelling the nebulous matter in their immediate vicinity. When observed with a powerful telescope, this nebula appears to be of vast dimensions, and, with its effluents, occupies an area of 4 deg. by 5-1/2 deg.. Irregular branching masses, streams, sprays, filaments, and curved spiral wreaths project outward from the parent mass, and become gradually lost in the surrounding space. This object remained for long a profound mystery; no telescope was capable of resolving it, nor was it known what this 'unformed fiery mist, the chaotic material of future suns,' was, until the spectroscope revealed that it consists of a stupendous mass of incandescent gases—nitrogen, hydrogen, and other elementary substances, occupying a region of space believed by some to equal in extent the whole stellar system to which our Sun belongs.

In the Southern Hemisphere, near to the pole of the equator, are two nebulous clouds of unequal size; the larger having an area about four times that of the smaller. They are known as the Magellanic Clouds, having been called after the navigator Magellan. Both are visible on a moonless night, but in bright moonlight the smaller disappears. Sir John Herschel, when at the Cape of Good Hope, examined those objects with his powerful telescope. He described them 'as consisting of swarms of stars, globular clusters, and nebulae of various kinds, some portions of them being quite irresolvable, and presenting the same milky appearance in the telescope that the nebulae themselves do to the naked eye.' These are believed to be other universes of stars sunk in the profound depths of space, our knowledge of their existence being dependent upon the faint nebulous light which left them, perhaps, several thousand years ago.



The description of the various kinds of nebulae leads us to consider what is called the Nebular Hypothesis. That the stars and solar system had at some time in the past a beginning, is as much a matter of certainty as that they will at some future time cease to be. Stars, like organic beings, have their birth, grow and arrive at maturity, then decline into a state of decrepitude, and finally die out. The duration of the life of a star, which may be reckoned by millions of years, depends upon the length of time during which it can maintain a temperature that renders it capable of emitting light. By the constant radiation of its heat into space, a condition of its constituent particles consequent upon the gradual contraction of its mass will ultimately occur, which will result in the exhaustion of its stores of thermal energy, the extinction of its light, and the reduction of what was once a brilliant orb to the condition of a mass of cold, opaque, inert matter. Inquiries as to the origin of the stars have led scientific men to conclude that they have been evolved from gaseous nebulae, and these have therefore been regarded as indicating the earliest stage in the formation of suns and planets. It is believed that the condensation of those attenuated masses of luminous matter into stars is capable of accounting for the generation and formation of all the shining orbs which enter into the structure of the starry heavens. In the evolution of a 'cosmos out of a chaos' we should expect to find stars presenting every stage of development—some in an embryo state and others more advanced; stars in full vigour and activity, stars that have passed the meridian of life, and stars in a condition of decay and on the verge of extinction. The observations of astronomers have led them to conclude that this condition of 'youth and age' exists among the stellar multitude; but the characteristics by which it is distinguished are neither very obvious nor reliable.

The nebular theory is incapable of proof or demonstration; but modern discoveries tend to support the accuracy of its conclusions, and its principles have now been adopted by the majority of philosophic thinkers. The physical changes which are going on in the nebulae towards stellar evolution, or in fully formed stars towards dissolution, are so slow that the life of an individual, or even the historical records of the past, are incapable of furnishing any evidence of alteration in their condition. A period of time infinitely greater than what has elapsed since the birth of science must pass before anything can be known of the life history of the stars; indeed, the allotted span of man's existence on this planet may have terminated ere the evolution of a large nebula into a star cluster can have taken place.

The nebular hypothesis was first propounded by Kant, who suggested that the sun and planets originated from a vast and diffused mass of cosmical matter. This theory was afterwards supported by Herschel and by the great French astronomer Laplace. As a result of close and continued observation of the different classes of nebulae, Herschel arrived at the conclusion that there exists in space a widely diffused 'shining fluid,' of a nature totally unknown to us, and that the nebulosity which he perceived to surround some stars was not of a starry nature. He further adds that this self-luminous matter 'seemed more fit to produce a star by its condensation than to depend on the star for its existence.' His sagacious conclusion with regard to the non-stellar nature of this nebulous matter was afterwards confirmed by the spectroscope; for at that time it was believed that even the faintest nebulae were irresolvable star clusters.

In 1811 Herschel read a paper before the Royal Society in which he propounded his famous nebular hypothesis, and stated his reasons for believing that nebulae, by their gradual condensation, were transformed into stars. Having assumed that there exists a highly attenuated self-luminous substance diffused over vast regions of space, he endeavoured to show that by the law of attraction its particles would have a tendency to coalesce and form aggregations of nebulous matter, and that each of these, by the continued action of the same force, would gradually condense and ultimately acquire the consistence of a star. In the case of large irregular nebulae, numerous centres of attraction would originate in the mass, round which the nebulous particles of matter would arrange themselves; each nucleus, when condensation had been completed, would become a star, and the entire nebula would in this manner be transformed into a cluster of stars. Herschel believed that he could trace the different stages of nebular condensation which result in the evolution of a star. In large, faintly luminous nebulae the process of condensation had only commenced; in others that were smaller and brighter it was in a more advanced stage; in those that contained nuclei there was evidence of nascent stars; and, finally, there could be seen in some nebulae minute stellar points—new-born suns—interspersed among the haze of the transforming mass. By this theory Herschel was able to account for the phenomena associated with nebulous stars and the supposed changes which were observed in some nebulae. The nebular hypothesis as described by Herschel was not received with much favour, nor did it unsettle much the belief that all nebulae were vast stellar aggregations, and that their cloudy luminosity was a consequence of the inadequacy of telescopic power to resolve them into their component stars. Laplace, who was highly gifted as a geometrician, demonstrated how the solar system could have been evolved in accordance with dynamical principles from a slowly rotating and slowly contracting spheroidal nebula. The rotatory motion of a nebula, in obedience to a well-known mechanical law, increases as its density becomes greater, and this goes on until the tangential force at the equator overcomes the gravitational attraction at its centre. When this occurs, a revolving ring of nebulous matter is thrown off from the parent mass, and by this means equilibrium is restored between the two forces. As the rotatory velocity of the nebula continues to increase with its contraction, another ring is cast off, and in this manner a succession of revolving rings may be detached from the condensing spheroid; each newly-formed ring being nearer to the centre of the contracting mass and revolving in a shorter period than its predecessor. In the evolution of our system, the central mass of the nebula became the Sun and each of the revolving rings, by their condensation into one mass, formed a planet. In a similar manner, though on a diminished scale, the elementary planets, whilst in a nebulous state, parted with annular portions of their substance, out of which were evolved their systems of satellites. This theory furnished a plausible reason, which was capable of explaining how the orbs which constitute the solar system came into existence, and, though hypothetical, yet the manner in which it accounted for the orderly and symmetrical genesis of the system rendered it attractive and fascinating to scientific minds.

The evidence in support of the nebulous origin of the solar system, if not conclusive, is of much weight and importance. The remarkable harmony with which the orbs of the system perform their motions is strongly indicative of their common origin and that their evolution occurred in subordination to the law of universal gravitation. The following are the characteristic points in favour of this theory:—

1. All the planets revolve round the Sun in the same direction, and they all occupy nearly the same plane.

2. Their satellites, with the exception of those of Uranus and Neptune, perform their revolutions in obedience to the same law.

3. The rotation on their axes of the Sun, planets, and satellites is in the same direction as their orbital motion.

Between the orbits of Mars and Jupiter there revolves a remarkable group of small planets or planetoids. On account of the absence of a planet in this region, where, according to the laws of planetary distances, one ought to be found, the existence of those small bodies was suspected for some years prior to their discovery. The first was detected by Piazzi at Palermo in 1801; two others were discovered by Olbers in 1802 and 1807, and one by Harding in 1804. For some time it was believed that no more planetoids existed, but in 1845 a fifth was detected by Hencke, and from that year until now upwards of 300 of those small bodies have been discovered. Their magnitudes are of varied extent; the diameter of the largest is believed not to exceed 450 miles, and that of the smaller ones from twenty to thirty miles. It was surmised at one time, when only a few of those bodies were known, that they were the fragments of a planet which met with some terrible catastrophe; but since the discovery of so many other planetoids this theory cannot be maintained. According to the nebular hypothesis, these bodies are the consolidated portions of a nebulous ring which remained separate instead of having coalesced into one mass so as to form a planet. The uniform condensation of the ring would result in the formation of a multitude of small planets similar to what are found between the orbits of Mars and Jupiter. In Saturn's ring we have a remarkable instance of annular consolidation in which the form of the ring has been preserved. The ring is believed to consist of myriads of minute bodies, each of which travels in an orbit of its own as it pursues its path round the planet; the close approximation and exceeding minuteness of those moving objects create the appearance of a solid continuous ring.

Though, by means of the nebular hypothesis, it is impossible to explain all the phenomena associated with the motions of the orbs which enter into the structure of the solar system, yet this does not detract much from the merits of the theory, the fundamental principles of which are based upon the evolution of the solar system from a rotating nebula. The retrograde motions of the satellites of Uranus and Neptune, the velocity of the inner Martian moon, and other abnormalities in the system, have not as yet been explained, but doubtless there are reasons by which those peculiarities can be accounted for if they were only known, 'felix qui potuit cognoscere causas omnium rerum.'

No attempt has been made to supplant the nebular hypothesis by any other theory of cosmical evolution. Modern investigations and discoveries have strengthened its position, and at present it is the only means by which we can account for the existence of the visible material universe by which we are surrounded.

In the days when Milton lived—three hundred years ago—the nocturnal heavens presented the same appearance to an observer as they do at the present time. The stars pursued their identical paths, and looked down upon the Earth with the same aspect of serene tranquillity, regardless of the vicissitudes which affect the inhabitants of this terrestrial sphere. The constellations that adorn the celestial vault duly appeared in their seasons,

and in the ascending scale Of Heaven the stars that usher evening rose.—iv. 354-55.

The winter glories of Orion, the scintillating brilliancy of Sirius, and the spangled firmament, bearing no impress of change or variation which would lead one to conclude that the heavens were other than eternal, attracted then, as now, the admiration of beholders.

Apart from the orbs which constitute the solar system, little was known of the sidereal heavens beyond the visual effect created by the nocturnal aspect of the star-lit sky. Though ancient philosophers hazarded an opinion that the stars were suns, they received but scant attention from early astronomers, by whom they were merely regarded as convenient fixed points which enabled them to determine with greater accuracy the positions of the planets and the paths traced out by them in the heavens. The Ptolemaists, who believed in the diurnal revolution of the spheres, assigned to the stars a very subordinate place in their cosmology, which was the one adopted by Milton; and although Copernicus relegated them to their proper location in space, yet he had no clear conception of a universe of stars. Tycho Brahe, who declined to accept the Copernican theory, disbelieved that the stars were suns, and Galileo, who discovered the stellar nature of the Milky Way, remarked that the stars were not illumined by the Sun's rays in the same manner that the planets are, but expressed no opinion with regard to their physical constitution. It is only within the past fifty years that proof has been obtained of the real nature of the stars. By the spectroscopic analysis of their light it has been ascertained that the elements of matter which enter into their composition exist in a condition similar to what is found in the Sun. The stars are therefore suns, many of them surpassing in magnitude and brilliancy the great luminary of our system.

Though Milton makes frequent allusion to the magnificence of the starry heavens, we have no evidence that he regarded the stars as suns, nor does he refer to them as such in any part of his poem.[12] What impressed him most was their number and brilliancy, to which reference is made in the following passages:

About him all the Sanctities of Heaven Stood thick as stars.—iii. 60-61.

And sowed with stars the Heavens thick as a field.—vii. 358.

Amongst innumerable stars, that shone Stars distant, but nigh hand seemed other worlds.—iii. 564-65.

her reign With thousand lesser lights dividual holds, With thousand thousand stars, that then appeared Spangling the hemisphere.—vii. 381-84.

Milton describes the number of the fallen angels as

an host Innumerable as the stars of night.—v. 744-45,

and the attention of Satan is directed by the archangel Uriel to the multitude of stars formed from the chaotic elements of matter:

Numberless as thou seest, and how they move; Each had his place appointed, each his course; The rest in circuit walls this universe.—iii. 719-21.



Though Milton was doubtless familiar with the leading orbs of the firmament and knew their names, and the constellations in which they are situated, yet he makes no direct allusion to any of them in his poem. Neither Arcturus, which is mentioned in the Book of Job, nor Sirius, which attracted the attention of Homer, who compared the brightness of Achilles' armour to the dazzling brilliancy of the dog-star, finds a place in 'Paradise Lost.' And yet the superior magnitude and brilliancy of some stars when compared with those of others did not escape Milton's observation when, in describing the lofty eminence of Satan in heaven, prior to his fall, he represents him as

brighter once amidst the host Of angels than that star the stars among.—vii. 132-33.

There is but one star to which Milton makes individual allusion, and, though not of any conspicuous brilliancy, yet it is one of much importance to astronomers—

the fleecy star that bears Andromeda far off Atlantic seas Beyond the horizon.—iii. 558-60.

This is Alpha Arietis, the first point in the constellation of that name, which signifies the Ram, and from which the right ascensions of the stars are measured on the celestial sphere. In the time of Hipparchus the ecliptic intersected the celestial equator in Aries, which indicated the commencement of the astronomical year and the occurrence of the vernal equinox; but, owing to precession, this point is now 30 deg. westward of Aries and in the constellation Pisces. The star was called Hamal by the Arabs, signifying a sheep, and the animal is represented as looking backwards. Manilius writes:—

First Aries, glorious in his golden wool, Looks back and wonders at the mighty Bull.

Aries is associated with the legend of the Golden Fleece, in quest of which Jason and his valiant crew sailed in the ship 'Argo.' In the autumn, Andromeda is situated above Aries, and would seem to be borne by the latter, which accounts for Milton's description of the relative positions of those two constellations.

Milton alludes to the starry sphere in several passages in his poem, and also mentions the starry pole above which he soared in imagination up to the Empyrean or Heaven of Heavens. His contemplation of the Galaxy must have impressed his mind with the magnitude and extent of the sidereal universe, for he was aware that this luminous zone which encircles the heavens consists of myriads of stars, so remote as to be incapable of definition by unaided vision. Milton's description of this vast assemblage of stars is worthy of its magnificence, and the purpose with which he poetically associates this glorified highway testifies to the sublimity of his thoughts and to the originality of his genius. In those parts of his poem in which he describes the glories of the celestial regions, and instances the beautiful phenomena associated with the individual orbs of the firmament, we are able to perceive with what exquisite delight he beheld them all.

The invention of the telescope, and the important discoveries made by Kepler, Galileo, and Newton in the seventeenth century, were the means of effecting a rapid advance in the science of astronomy; but that branch of it known as sidereal astronomy was not then in existence. The star depths, owing to inadequate telescopic power, remained unexplored, and the secrets associated with those distant regions were inviolable, and lay beyond the reach of human knowledge. The physical constitution of the stars was unknown, nor was it ascertained with any degree of certainty that they were suns. The knowledge possessed by astronomers in those days was but meagre compared with what is now known of the sidereal heavens. Milton's astronomical knowledge, we find, was commensurate with what was known of the stellar universe, and this he has conspicuously displayed in his poem.



CHAPTER VIII

DESCRIPTION OF CELESTIAL OBJECTS MENTIONED IN 'PARADISE LOST'

THE SUN

The surpassing splendour of the Sun, as compared with that of any of the other orbs of the firmament, is not more impressive than his stupendous magnitude, and the important functions which it is his prerogative to fulfil. Situated at the centre of our system—of which he may be regarded as 'both eye and soul'—the orb has a diameter approaching 1,000,000 miles, and a mass 750 times greater than that of all the planets combined. These, by his attractive power, he retains in their several paths and orbits, and even far distant Neptune acknowledges his potent sway. With prodigal liberality he dispenses his vast stores of light and heat, which illumine and vivify the worlds circling around him, and upon the constant supply of which all animated beings depend for their existence. Deprived of the light of the Sun, this world would be enveloped in perpetual darkness, and we should all miserably perish.

The Sun is distant from the Earth about 93,000,000 miles. His diameter is 867,000 miles, or nearly four times the extent of the radius of the Moon's orbit. The mass of the orb exceeds that of the Earth 330,000 times, and in volume 1,305,000 times. The Sun is a sphere, and rotates on his axis from west to east in 25 days 8 hours. The velocity of a point at the solar equator is 4,407 miles an hour. The density of the Sun is only one-fourth that of the Earth, or, in other words, bulk for bulk, the Earth is four times heavier than the Sun. The force of gravity at the Sun's surface is twenty-seven times greater than it is on the Earth; it would therefore be impossible for beings constituted as we are to exist on the solar surface.

The dazzling luminous envelope which indicates to the naked eye the boundary of the solar disc is called the PHOTOSPHERE. It is most brilliant at the centre of the Sun, and diminishes in brightness towards the circumference, where its luminosity is but one-fourth that of the central portion of the disc. The photosphere consists of gaseous vapours or clouds, of irregular form and size, separated by less brilliant interstices, and glowing white with the heat derived from the interior of the Sun. In the telescope the photosphere is not of uniform brilliancy, but presents a mottled or granular appearance, an effect created by the intermixture of spaces of unequal brightness. Small nodules of intense brilliance, resembling 'rice-grains,' but which, according to Nasmyth, are of a willow-leaf shape with pointed extremities, which form a network over portions of the photosphere, are sprinkled profusely over a more faintly luminous background. These 'grains' consist of irregular rounded masses, having an area of several hundred miles. By the application of a high magnifying power they can be resolved into 'granules'—minute luminous dots which constitute one-fifth of the Sun's surface and emit three-fourths of the light. This granulation is not uniform over the surface of the photosphere; in some parts it is indistinct, and appears to be replaced by interlacing filamentous bands, which are most apparent in the penumbrae of the spots and around the spots themselves. The 'granules' are the tops of ascending masses of intensely luminous vapour; the comparatively dark 'pores' consist of similar descending masses, which, having radiated their energy, are returning to be again heated underneath the surface of the photosphere.

In certain regions of the photosphere several dark patches are usually visible, which are called 'sun-spots.' At occasional times they are almost entirely absent from the solar disc. It has been observed that they occupy a zone extending from 10 deg. to 35 deg. north and south of the solar equator, but are not found in the equatorial and polar regions of the Sun. A sun-spot is usually described as consisting of an irregular dark central portion, called the umbra; surrounding it is an edging or fringe less dark, consisting of filaments radiating inwards called the penumbra. Within the umbra there is sometimes seen a still darker spot, called the nucleus. The umbra is generally uniformly dark, but at times filmy luminous clouds have been observed floating over it. The nucleus is believed to be the orifice of a tubular depression in the floor of the umbra, prolonged downwards to an unknown depth. The penumbra is brightest at its inner edge, where the filaments present a marked contrast when compared with the dark cavity of the umbra which they surround and overhang. Sometimes lengthened processes unite with those of the opposite side and form bands and 'bridges' across the umbra. The darkest portion of the penumbra is its external edge, which stands out conspicuously against the adjoining bright surface of the Sun. One penumbra will sometimes enclose several umbrae whilst the nuclei may be entirely wanting.



Sun-spots usually appear in groups; large isolated spots are of rare occurrence, and are generally accompanied by several smaller ones of less perfect formation. The exact moment of the origin of a sun-spot cannot be ascertained, because it arises from an imperceptible point; it grows very rapidly, and often attains its full size in a day.

Prior to its appearance there is an unusual disturbance of the solar surface over the site of the spot: luminous ridges, called faculae, and dark 'pores' become conspicuous, between which greyish patches appear, that seem to lie underneath a thin layer of the photosphere; this is rapidly dispelled and a fully formed spot comes into view. When a sun-spot has completed its period of existence, the photospheric matter overwhelms the penumbra, and rushes into the umbra, which it obliterates, causing the spot to disappear. The duration of sun-spots is subject to considerable variation; some last for weeks or months, and others for a few days or hours. A spot when once fully formed maintains its shape, which is usually rounded, until the period of its breaking up. Spots of long duration rotate with the Sun. Those which become visible at the edge of the Sun's limb have been observed to travel across his disc in less than a fortnight, disappearing at the margin of the opposite limb; afterwards, if sufficiently long-lived, they have reappeared in twelve or thirteen days on the surface of the orb where first observed. It was by observation of the spots that the period of the axial rotation of the Sun became known.

Sun-spots vary very much in size—some are only a few hundred miles in width, whilst others have a diameter of 40,000 or 50,000 miles or upwards. In some instances the umbra alone has a breadth of 20,000 or 30,000 miles—three times the extent of the diameter of the Earth. Spots of this size are visible to the naked eye when the Sun is partially obscured by fog, or when his brilliancy is diminished by vapours near the horizon. A year seldom passes without the occurrence of several of such spots being recorded. The largest sun-spot ever observed had a diameter of about 150,000 miles. A group of spots, including their penumbrae, will occupy an area of many millions of square miles.

By long observation it has been ascertained that sun-spots increase and diminish in number with periodical regularity, and that a maximum sun-spot period occurs at the end of each eleven years. When spots are numerous on the Sun's disc there is great disturbance of the solar surface, accompanied by fierce rushes of intensely heated gases. This solar activity is known to influence terrestrial magnetism by causing a marked oscillation of the magnetic needle, and giving rise to so-called 'magnetic storms,' accompanied by magnificent displays of aurorae, with variations in electrical earth-currents. It would therefore appear that sun-spots have a pronounced effect upon magnetic terrestrial phenomena, but how this is produced remains unknown.

Besides sun-spots, there are seen on the solar disc bright flocculent streaks or ridges of luminous matter called faculae; they are found over the whole surface of the Sun, but are most numerous near the limb and in the immediate vicinity of the spots. They have been compared to immense waves—vast upheavals of photospheric matter, indicative of enormous pressure, and often extending in length for many thousands of miles.

Nearly all observers have arrived at the conclusion that sun-spots are depressions or cavities in the photosphere, but considerable difference of opinion exists as to how they are formed. The most commonly accepted theory is that they are caused by the pressure of descending masses of vapour having a reduced temperature, which absorb the light and prevent it reaching us. Our knowledge of the Sun is insufficient to admit of any accurate conclusion on this point; though we are able to perceive that the surface of the orb is in a state of violent agitation and perpetual change, yet his great distance and intense luminosity prevent our capability of perceiving the ultimate minuter details which go to form the texture of the solar surface. 'Bearing in mind that a second of arc on the Sun represents 455 miles, it follows that an object 150 miles in diameter is about the minimum visible even as a mere mathematical point, and that anything that is sufficiently large to give the slightest impression of shape and extension of surface must have an area of at least a quarter of a million square miles; ordinarily speaking, we shall not gather much information about any object that covers less than a million.'[13] Since the British Islands have only an area of 120,700 square miles, it is evident that on the surface of the Sun there are many phenomena and physical changes occurring which escape our observation. Though the changes which occur in the spots and faculae appear to be slow when observed through the telescope, yet in reality they are not so. Tremendous storms and cyclones of intensely heated gases, which may be compared to the flames arising from a great furnace, sweep over different areas of the Sun with a velocity of hundreds of miles an hour. Vast ridges and crests of incandescent vapour are upheaved by the action of internal heat, which exceeds in intensity the temperature at which the most refractory of terrestrial substances can be volatilised; and downrushes of the same photospheric matter take place after it has parted with some of its stores of thermal energy. Sun-spots of considerable magnitude have been observed to grow rapidly and then disappear in a very short period of time; occasionally a spot is seen to divide into two or more portions, the fragments flying asunder with a velocity of not less than 1,000 miles an hour. It is by these upheavals and convulsions of the solar atmosphere that the light and heat are maintained which illumine and vivify the worlds that gravitate round the Sun.

During total eclipses of the Sun, several phenomena become visible which have enabled astronomers to gain some further knowledge of the nature of the solar appendages. The most important of these is the CHROMOSPHERE, which consists of layers of incandescent gases that envelop the photosphere and completely surround the Sun. Its average depth is from 5,000 to 6,000 miles, and when seen during an eclipse is of a beautiful rose colour, resembling a sheet of flame. As seen in profile at the edge of the Sun's disc, it presents an irregular serrated appearance, an effect created by the protuberance of luminous ridges and processes—masses of flame which arise from over its entire surface. The chromosphere consists chiefly of glowing hydrogen, and an element called helium, which has been recently discovered in a terrestrial substance called cleveite; there are also present the vapours of iron, calcium, cerium, titanium, barium, and magnesium. From the surface of this ocean of fire, jets and pointed spires of flaming hydrogen shoot up with amazing velocity, and attain an altitude of ten, twenty, fifty, and even one hundred thousand miles in a very short period of time. They are, however, of an evanescent nature, change rapidly in form and appearance, and often in the course of an hour or two die down so as not to be recognisable. These prominences, as they are called, have been divided into two classes. Some are in masses that float like clouds in the atmosphere, which they resemble in form and appearance; they are usually attached to the chromosphere by a single stem, or by slender columns; occasionally they are entirely free. These are called quiescent prominences; they consist of clouds of hydrogen, and are of more lasting duration than the other variety, called eruptive or metallic prominences. The latter are usually found in the vicinity of sun-spots, and, besides hydrogen, contain the vapours of various metals. They are of different forms, and present the appearance of filaments, spikes, and jets of liquid fire; others are pyramidal, convoluted, and parabolic.

These outbursts, bending over like the jets from a fountain, and descending in graceful curves of flame, ascend from the surface of the chromosphere with a velocity often exceeding 100 miles in a second, and frequently reach an enormous height, but are of transient duration. They are closely connected with sun-spots, and are evidence of the tremendous forces that are in action on the surface of the Sun.

The CORONA is an aureole of light which is seen to surround the Sun during a total eclipse. It is an impressive and beautiful phenomenon, and is only visible when the Sun is concealed behind the dark body of the Moon. Professor Young gives the following graphic description of the corona: 'From behind it [the Moon] stream out on all sides radiant filaments, beams, and sheets of pearly light, which reach to a distance sometimes of several degrees from the solar surface, forming an irregular stellate halo, with the black globe of the Moon in its apparent centre. The portion nearest the Sun is of dazzling brightness, but still less brilliant than the prominences, which blaze through it like carbuncles. Generally this inner corona has a pretty uniform height, forming a ring three or four minutes of arc in width, separated by a somewhat definite outline from the outer corona, which reaches to a much greater distance and is far more irregular in form. Usually there are several "rifts," as they have been called, like narrow beams of darkness, extending from the very edge of the Sun to the outer night, and much resembling the cloud shadows which radiate from the Sun before a thundershower. But the edges of these rifts are frequently curved, showing them to be something else than real shadows. Sometimes there are narrow bright streamers as long as the rifts, or longer. These are often inclined, or occasionally even nearly tangential to the solar surface, and frequently are curved. On the whole, the corona is usually less extensive and brilliant over the solar poles, and there is a recognisable tendency to accumulation above the middle latitudes, or spot zones; so that, speaking roughly, the corona shows a disposition to assume the form of a quadrilateral or four-rayed star, though in almost every individual case this form is greatly modified by abnormal streamers at some point or other.' The corona surrounds the Sun and its other envelopes to a depth of many thousands of miles. It consists of various elements which exist in a condition of extreme tenuity; hydrogen, helium, and a substance called coronium appear to predominate, whilst finely divided shining particles of matter and electrical discharges resembling those of an aurora assist in its illumination.



We possess no knowledge of the physical structure of the interior of the Sun, nor have we any terrestrial analogy to guide us as to how matter would behave when subjected to such conditions of extreme temperature and pressure as exist in the interior of the orb. Yet we are justified in concluding that the Sun is mainly a gaseous sphere which is slowly contracting, and that the energy expended in this process is being transformed into heat so extreme as to render the orb a great fountain of light.

Milton in his poem makes more frequent allusion to the Sun than to any of the other orbs of the firmament, and, in all his references to the great luminary, describes him in a manner worthy of his unrivalled splendour, and of his supreme importance in the system which he upholds and governs. After having alighted on Mount Niphates, Satan is described as looking

Sometimes towards Heaven and the full-blazing Sun, Which now sat high in his meridian tower.—iv. 29-30.

He then addresses him thus:—

O thou that with surpassing glory crowned, Look'st from thy sole dominion like the god Of this new World—at whose sight all the stars Hide their diminished heads—to thee I call, But with no friendly voice, and add thy name, O Sun, to tell thee how I hate thy beams, That bring to my remembrance from what state I fell, how glorious once above thy sphere.—iv. 32-39.

On another occasion:—

The golden Sun in splendour likest Heaven Allured his eye.—iii. 572-73.

In describing the different periods of the day, Milton seldom fails to associate the Sun with these times, and rightly so, since they are brought about by the apparent diurnal journey of the orb across the heavens. Commencing with morning, he says:—

Meanwhile, To re-salute the world with sacred light, Leucothea waked, and with fresh dews embalmed The Earth.—xi. 133-36.

Soon as they forth were come to open sight Of day-spring, and the Sun—who, scarce up-risen, With wheels yet hovering o'er the ocean-brim, Shot parallel to the Earth his dewy ray, Discovering in wide landskip all the east Of Paradise and Eden's happy plains.—v. 138-43

or some renowned metropolis With glistering spires and pinnacles adorned, Which now the rising Sun gilds with his beams.—iii. 549-51.

while now the mounted Sun Shot down direct his fervid rays, to warm Earth's inmost womb.—v. 300-302.

for scarce the Sun Hath finished half his journey, and scarce begins His other half in the great zone of Heaven.—v. 558-60.

To sit and taste, till this meridian heat Be over, and the Sun more cool decline.—v. 369-70.

And the great Light of Day yet wants to run Much of his race, though steep. Suspense in Heaven, Held by thy voice, thy potent voice he hears, And longer will delay, to hear thee tell His generation, and the rising birth Of Nature from the unapparent deep.—vii. 98-103.

The declining day and approach of evening are described as follows:—

Meanwhile in utmost longitude, where Heaven With Earth and Ocean meets, the setting Sun Slowly descended, and with right aspect Against the eastern gate of Paradise Levelled his evening rays.—iv. 539-43.

the Sun now fallen Beneath the Azores; whether the Prime Orb, Incredible how swift, had thither rolled Diurnal, or this less volubil Earth, By shorter flight to the east, had left him there Arraying with reflected purple and gold The clouds that on his western throne attend.—iv. 591-97.

the parting Sun Beyond the Earth's green Cape and verdant Isles Hesperian sets, my signal to depart.—viii. 630-32.

Now was the Sun in western cadence low From noon, and gentle airs due at their hour To fan the Earth now waked, and usher in The evening cool.—x. 92-95.

for the Sun, Declined, was hasting now with prone career To the Ocean Isles, and in the ascending scale Of Heaven the stars that usher evening rose.—iv. 352-55.

In the combat between Michael and Satan, which ended in the overthrow of the rebel angels, Milton, in his description of their armour, says:—

two broad suns their shields Blazed opposite.—vi. 305-306,

and in describing the faded splendour of the ruined Archangel, the poet compares him to the Sun when seen under conditions which temporarily deprive him of his dazzling brilliancy and glory:—

as when the Sun new-risen Looks through the horizontal misty air Shorn of his beams, or, from behind the Moon In dim eclipse, disastrous twilight sheds On half the nations, and with fear of change Perplexes monarchs.—i. 594-99.

This passage affords us an example of the sublimity of Milton's imagination and of his skill in adapting the grandest phenomena in Nature to the illustration of his subject.

THE MOON

The Moon is the Earth's satellite, and next to the Sun is the most important of the celestial orbs so far as its relations with our globe are concerned. Besides affording us light by night, the Moon is the principal cause of the ebb and flow of the tide—a phenomenon of much importance to navigators. The Moon is almost a perfect sphere, and is 2,160 miles in diameter. The form of its orbit is that of an ellipse with the Earth in the lower focus. It revolves round its primary in 27 days 7 hours, at a mean distance of 237,000 miles, and with a velocity of 2,273 miles an hour. Its equatorial velocity of rotation is 10 miles an hour. The density of the Moon is 3.57 that of water, or 0.63 that of the Earth; eighty globes, each of the weight of the Moon, would be required to counterbalance the weight of the Earth, and fifty globes of a similar size to equal it in dimensions. The orb rotates on its axis in the same period of time in which it accomplishes a revolution of its orbit; consequently the same illumined surface of the Moon is always directed towards the Earth. To the naked eye the Moon appears as large as the Sun, and it very rapidly changes its form and position in the sky. Its motions, which are of a very complex character, have been for many ages the subject of investigation by mathematicians and astronomers, but their difficulties may now be regarded as having been finally overcome.

The phases of the Moon are always interesting and very beautiful. The orb is first seen in the west, after sunset, as a delicate slender crescent of pale light; each night it increases in size, whilst it travels eastward, until it attains the figure of a half moon; still growing larger as it pursues its course, it finally becomes a full resplendent globe, rising about the time that the Sun sets and situated directly opposite to him. Then, in a reverse manner, after full moon, it goes through the same phases, until, as a slender crescent, it becomes invisible in the solar rays; afterwards to re-appear in a few days, and, in its monthly round, to undergo the same cycle of changes. The phases of the Moon depend upon the changing position of the orb with regard to the Sun. The Moon shines by reflected light derived from the Sun, and as one half of its surface is always illumined and the other half totally dark, the crescent increases or diminishes when, by the Moon's change of position, we see more or less of the bright side. Visible at first as a slender crescent near the setting Sun, the angular distance from the orb and the width of the crescent increase daily, until, at the expiration of seven days, the Moon is distant one quarter of the circumference of the heavens from the Sun. The Moon is then a semi-circle, or in quadrature. At the end of other seven days, the distance of the Moon from the Sun is at its greatest—half the circumference of its orbit. It is then visible as a circular disc and we behold the orb as full moon. The waning Moon, as it gradually decreases, presents the same aspects reversed, and, finally, its slender crescent disappears in the Sun's rays. The convex edge of the crescent is always turned towards the Sun. The rising of the Moon in the east and its setting in the west is an effect due to the diurnal rotation of the Earth on her axis, but the orb can be perceived to have two motions besides: one from west to east, which carries it round the heavens in 29.53 days, and another from north to south. The west to east motion is steady and continuous, but, owing to the Sun's attractive force, the Moon is made to swerve from its path, giving rise to irregularities of its motion called PERTURBATIONS. The most important of these is the annual equation, discovered by Tycho Brahe—a yearly effect produced by the Sun's disturbing influence as the Earth approaches or recedes from him in her orbit; another irregularity, called the evection, is a change in the eccentricity of the lunar orbit, by which the mean longitude of the Moon is increased or diminished. Elliptic inequality, parallactic inequality, the variation, and secular acceleration, are other perturbations of the lunar motion, which depend directly or indirectly on the attractive influence of the Sun and the motion of the Earth in her orbit.

As the plane of the Moon's orbit is inclined at an angle of rather more than 5 deg. to the ecliptic, it follows that the orb, in its journey round the Earth, intersects this great circle at two points called the 'Nodes.' When crossing the ecliptic from south to north the Moon is in its ascending node, and when crossing from north to south in its descending node. In December the Moon reaches the most northern point of its course, and in June the southernmost. Consequently we have during the winter nights the greatest amount of moonlight, and in summer the least. In the evenings the moonlight is least in March and greatest in September, when we have what is called the Harvest Moon.

The telescopic appearance of the Moon is very interesting and beautiful, especially if the orb is observed when waxing and waning. As no aqueous vapour or cloud obscures the lunar surface, all its details can be perceived with great clearness and distinctness. Indeed, the topography of the Moon is better known than that of the Earth, for the whole of its surface has been mapped and delineated with great accuracy and precision. The Moon is in no sense a duplicate of its primary, and no analogy exists between the Earth and her satellite. Evidence is wanting of the existence of an atmosphere surrounding the Moon; no clouds or exhalations can be perceived, and no water is believed to exist on the lunar surface. Consequently there are no oceans, seas, rivers, or lakes; no fertile plains or forest-clad mountains, such as are found upon the Earth. Indeed, all the conditions essential for the support and maintenance of organic life by which we are surrounded appear to be nonexistent on the Moon. Our satellite has no seasons; its axial rotation is so slow that one lunar day is equal in length to fourteen of our days; this period of sunshine is succeeded by a night of similar duration. The alternation of such lengthened days and nights subjects the lunar surface to great extremes of heat and cold.