M. A. Orr

“ At one end of the series we find stars like Gamma in the Sails of the Ship Argo. When its light is spread out by prisms into a coloured band it is a wonderful sight, for not only are there dark lines and dark flutings, but wide bright bands mingle with these and stand out against the fainter background. We are fortunate to possess γ Velorum in the southern hemisphere, for it is the only bright star of its class. The type is known as Wolf-Rayet Stars, from the name of their discoverer. ”

“ The Greater Cloud is much brighter to the naked eye than the Lesser, and it is much more complex and interesting in the telescope. It contains, moreover, the wonderful Looped Nebula, of which we have already spoken.
Both Clouds consist of gaseous nebulae and star-clusters on a background of vague nebulosity and crowds of almost indistinguishable stars. But the white nebulae shun the Clouds, just as they shun the Milky Way. ”

“ SOUTHERN HEMISPHERE
The brighter of the two Pointers to the Cross, Alpha Centauri, ranks very high among first-magnitude stars, though it is excelled in brightness by Sirius and Canopus; but its greatest claim to our interest is that among all the host of stars it is our nearest neighbour. Yet, should we decide to pay a visit and travel with the speed of light (11 million miles a minute) , it would be four years and three months before we could reach our destination—so great are the spaces which separate our sun with his family of planets from the nearest of his brother suns. ”

“ More beautiful and wonderful still are the clusters of stars of various tints and magnitudes, where scores, even hundreds, are gathered together in what to the eye is a tiny patch, though it may in fact take light many years to cross from one side of it to the other.
The finest example in the heavens of this kind of cluster is Kappa [Pg 60] Crucis, near β in the Southern Cross. [8] It is just visible to the naked eye as a small star, and in a binocular the main star is seen to be surrounded by a number of others; in a telescope it is a glorious sight. ”

“ Facts like these lead astronomers to believe that the Milky Way has a definite relation with all the visible universe, that even the most distant nebula is not an outlying universe apart from ours, but all are parts of one vast stellar system. ”

“ Unlike the mysterious Mira stars, the variation of Algol stars has been explained. The sudden drop in the light is a partial eclipse, caused by a dark or partly dark companion which for a time hides the bright star from us. When a source of light is coming towards us, the lines in its spectrum are shifted towards the violet, when going away they are shifted towards the red, exactly as the whistle of an engine becomes more shrill when approaching us, and falls to a lower pitch when going away. ”

“ The speed of the star in its orbit can be accurately determined (by the amount of shift in the spectrum lines) in miles per second, even when we do not know its distance from us; hence the size of its orbit can be calculated, since we know the period in which it is completed; and, [Pg 53] further, the size of the orbit gives us the mass of the stars, for their movements depend upon the attraction they exercise over one another, and this is proportional to their mass; and so we are able to picture the system, although the eclipsing star is never seen and the distance from us may never be known. ”

“ In the eighteenth century the French astronomer Lacaille, who did much excellent work in the southern hemisphere with a tiny telescope of only half an inch aperture, had the unhappy idea of filling up the spaces still left empty with scientific instruments. It is easy to make Birds of Paradise and Flying Fishes out of the stars, but such things as telescopes, easels, and sextants do not lend themselves to irregular groups, and they are very much out of place among the mythical beasts and heroes which we are accustomed to see on our star-maps. ”

“ The blue Orion [Pg 24] stars are the brightest and hottest of all those about which we have spoken, and are generally thought to have reached the climax of a star’s career. They are great globes of thin gas, much less dense than water, but glowing hot through and through. The outer layers naturally part with their heat first, and in cooling they absorb more and more light from the radiant centre. As the blue rays are always first and most powerfully affected, the blue tint is soon lost, and the colour slowly changes through white, yellow, and orange to deep-red. ”

“ It is not therefore wholly dark, but it shines with so feeble a light that, if it were brought as near to us as our own sun, it would appear only one-hundredth as bright as he is, even though it is a somewhat more massive body. Sirius itself is only about two and a half times as massive as our sun, but immensely more brilliant. It is the typical star of the “Sirian” class, to which belong many of the [Pg 20] brightest stars in the heavens, white stars in whose spectra broad hydrogen lines form the most striking feature, indicating a very extensive atmosphere of glowing hydrogen. ”

“ Thus, Wolf-Rayet and Orion stars have a low average speed, and both are very much more numerous in and near the Milky Way than elsewhere; Sirian stars are travelling a good deal faster, show a marked tendency to congregate in two streams, and move chiefly parallel with the plane of the Milky Way; solar stars, including our own sun, move more rapidly still and show less preference for the Galaxy and the two streams: Antarians have the most rapid motions of all, but these appear to be haphazard, and the stars are scattered all over the sky in every direction. ”

“ My readers will find that the southern hemisphere possesses the most beautiful part of the Milky Way, the two brightest stars in the sky, the finest coloured star-cluster and the largest globular cluster, the brightest double star, the nearest of the stars, and the brightest of the large gaseous nebulae. Let us add that the southern hemisphere has been less studied than the north, and therefore there is an even wider field for amateur workers. ”

“ We know also, from simply counting the stars in different regions of the sky, that their numbers increase regularly as we go from north or south towards the Milky Way, and stars of all magnitudes are most abundant within its course. We saw also that star-clusters and certain kinds of nebulae frequent it, while other kinds avoid it, and that blue and white stars are the most abundant near it, and tend to move through space in planes parallel with it, while the redder stars are scattered and move about in all directions. ”

“ It was one of the first doubles known, being discovered as such in 1689; and Herschel calls it a “superb [Pg 16] double star, beyond all comparison the most striking object of the kind in the heavens.” The two stars he describes as “both of a high ruddy or orange colour, though that of the smaller is of a somewhat more sombre and brownish cast.”
All stars are suns, but they vary so much in brightness and size and quality of light that it is as difficult to match two stars in the sky as two trees in a wood. Yet one of this pair is an almost exact counterpart of our sun. ”

“ This immense red star is accompanied by a little green satellite, and there is also a very close companion which can never be seen, but is known to exist through the shifting of lines in the spectrum, like that of Spica.
What an amazing variety among these ten stars! Though the eye can only decide that all are much brighter than the average, and that they differ somewhat in colour among themselves, science tells us that they vary enormously in many ways. Half belong to the class of blue Orion stars, others to the white Sirian, yellow solar, red Antarian classes ”

“ But whereas the stars in the north are fairly evenly distributed, there is a more strongly marked tendency in the south to congregate on the Milky Way, so that, while the tract through Argo and the Cross to Scorpio and Sagittarius is extraordinarily rich, the part between Orion and Fomalhaut is comparatively dark and bare, and the regions round the south pole and north of Argo are also very barren of bright stars. Some regions, however, which look dull to the eye abound in marvels for the telescope and camera. ”

“ It can scarcely be due to the near approach of a satellite, because of the irregularity in the time of maximum. There is a certain resemblance between the way in which the light waxes and wanes and the waxing and waning in the number of sunspots on the sun, and spectra of these Mira stars also somewhat resemble sunspot spectra: can it be that our sun is an incipient variable star with a period of about eleven years? It is true that sunspots seem to be cooler rather than hotter regions on the sun, but a time of maximum spots is also a time of maximum activity ”

“ In some regions of the sky we find vast spaces thinly veiled by nebulosity so faint and transparent that it seems to have reached the very limit at which matter can exist and be recognised as such. Thus in the constellation of Orion nearly all the bright stars are connected together by the vast convolutions of an exceedingly faint diffused nebula in spiral form, the innermost [Pg 81] curve of which ends in the Great Nebula of the Sword, and the whole region within is filled with faint light. ”

“ Southern Irregular and Suspected Variables
[Pg 59]
XII
STAR-CLUSTERS
Endlessly varied is the grouping of the stars. We find solitary stars, twin stars, large stars with small companions, bright stars with faint companions, stars alike in colour and type, stars in strongly contrasted colours, one young, the other aged, stars with invisible companions, couples close enough to touch one another, and couples so distant that the satellite takes more than a century to revolve round its primary. ”

“ In this way it has been discovered that an Algol star is revolving round an invisible companion, for it alternately approaches and recedes, and these movements correspond with its light-changes. It is, in fact, a spectroscopic binary which happens to have an orbit whose plane lies just in our line of sight, so that at every revolution one star passes behind the other. ”

“ But what are the early stages which precede the brilliance of an Orion star? Arguments have been brought forward to show that young stars also are comparatively cool, but even less dense than Orion stars, that as they grow denser and smaller through gravity, which causes all the particles of a star to condense towards its centre, they must at first grow gradually hotter and brighter until a certain point is reached, after which they will grow cooler and fainter again until they become quite cold and dark ”

“ As it moves through the cloud this atmosphere is continually brushed back from its advancing face, and therefore we shall see it when coming straight towards us through a less thickness of atmosphere, and it will look brighter than when it is retreating from us. ”

“ It looks [Pg 31] as if the Milky Way were the birthplace of the stars, and that as they develop they gradually scatter through space; but how are we to explain the fact that speed and direction of movement differ for different types? There seems to be no reason why a cooler star should move more quickly than a hotter one, and none of the theories yet advanced can be considered final. ”

“ The Cross seems to have been first so named by the Spanish explorer Amerigo Vespucci, at the beginning of the sixteenth century, and it was also described in letters by the Florentine Andrea Corsali, who says it [Pg 2] is so beautiful that in his opinion no other constellation in the sky is worthy to be compared with it. Some think, however, that the stars had already been recognised as forming a Cross as early as the thirteenth century, because Dante in his Purgatory speaks of four stars which glorified the southern sky ”

“ Crater, the Cup, has no bright stars; Corvus, the Crow, is an irregular little square which sailors call [Pg 9] Spica’s Spanker, a spanker being a sail of this shape, and two stars of the four point to Spica, the bright star of the Virgin.
Virgo, the Virgin, is one of the constellations of the Zodiac, that zone of stars which marks the apparent pathway of sun, moon, and planets in the sky. All the twelve zodiacal constellations can be seen in both hemispheres, but those south of the equator, which are least well seen in northern countries, are of course the best seen in the south. ”

“ Two observers comparing one at the same time with the same star will often come to opposite conclusions, showing how difficult it is to compare stars which differ much in colour. Whenever possible, red comparison stars should be selected to avoid this uncertainty. ”

“ So with the stars. Here the process of development is far more difficult to understand, and we are still ignorant concerning the birth and death of stars; but it is clear that we see a series of stages which pass gradually into one another, and that the cause of a star’s growing old is a gradual loss of heat by radiation. ”

“ But the most hopeful theory takes into account a very strange fact lately established, viz. that the variable is at its brightest not when approaching its fellow but when coming directly toward us, and at its faintest when receding directly from us. ”