THE STARS FOR OCTOBER.

THE PLOUGH, OR DIPPER.

I PROPOSE now to give a brie. account of the seven bright stars of the Plough, or Dipper, as they really are, not merely as they appear in the sky. I take them as the most convenient, and in several respects also as the best, illustration of what applies in reality (with changes in matters of detail) to all the thousands of stars we see, and to thousands of times as many stars which only the telescope reveals to us.

When you look during the evenings of this month at the stars of this group, seen low down toward the north, in the position shown in Map I. for the month, you see seven small points of brilliant light,—each of them seems like the "little star" in the familiar nursery rhyme. If the eye were a perfect optical instrument, and the air were perfectly transparent and still, and if, also, light instead of travelling to us in waves of many lengths, gave us an exactly truthful account of what is out yonder in space, even the seven little. stars we see would be very much reduced in seeming size. They would appear as mere points. The most powerful telescope men have yet made, and probably the most powerful telescope men ever will make, would not show these seven stars larger than points, such that the human eye could perceive no breadth in those minute disks. Such are the stars, even the leading ones, to the natural eye. In the mind's eye, however, these seven stars are very different

objects. I am not going to draw on my imagination in what I am about to tell you. I am not going to show what these stars may be, but to describe what science assures us that they are.

SIZES OF THE STARS OF THE PLOUGH, OR DIPPER.

In the first place, then, every one of these seven points of light is an enormous globe, not only larger than the earth on which we live, but thousands or rather hundreds of thousands of times larger. How large they really are we do not know; we do not even know how far away they are; but we do know they are so far away that our sun removed and set beside the nearest of them would not look so bright as the faintest of the seven. They may be so far away that our sun removed to their distance would scarce be seen at all, or would even require a powerful telescope to show him; but that he would not be so bright as Delta, the middle one, and the faintest of the seven, is certain. In considering what this means, you should remember that the sun himself looks only a small body. We might well believe, so far as appearances are concerned, that he is no larger than the moon, and the moon no larger than yonder hill that hides her from our view as she sets. But the sun is in reality a globe exceeding our earth one million and a quarter times in volume. If such a globe as our earth, only, were set aglow with a brightness so great that every part of her surface shone more resplendently than the piece of lime used in the calcium lantern (and one cannot easily look at that piece of lime so glowing), and this enormous mass of white-hot fire were set travelling away toward the nearest star of the Plough, it would be utterly lost to view before it had traversed a fiftieth part of the distance!

THEIR COMPOSITION.

Secondly, every one of the seven stars consists of matter like that in our sun, glowing with intense lustre.

When

we use the instrument called the spectroscope, distance does not prevent us from recognizing vapours of various kinds in the atmosphere o. a luminous body so long as the light reaches us in sufficient amount. In the case of the stars, distant though they are, we get the same sort of information. And thus we learn that iron, sodium, magnesium, calcium, hydrogen, and others of our familiar elements exist in the atmospheres of the stars, just as we have found that they exist in the atmosphere of our own sun. These seven stars, like our sun and their fellow-suns, are great masses of intensely hot matter, all around which there lies a deep atmosphere of glowing gases, including in the vapourous form many of those elements, such as our metals, which the greatest heat we can use serves only to melt, not to turn into vapour. You know that at a certain low degree of heat water is solid, at ordinary heat it becomes fluid, and at a great heat-much hotter than the greatest the hand can bear-water turns into steam or vapour. Iron only becomes fluid at a heat far greater than that at which water boils. You can imagine, then, how intense the heat must be at which molten iron. turns into iron-steam. But in the sun and in his fellow-suns the stars, iron, and substances still more stubborn in their resistan to heat, are turned into the form of vapour. The air of every star is a mixture of iron-steam, zinc-steam, calciumsteam, and many other such fiery vapours, besides hydrogen; and all these vapours are so hot that they shine with their own inherent lustre. Imagine an atmosphere such as this, where the clouds which form are metallic drops, and the rains which fall are sheets of molten metals!

* I must mention—without explaining, however—that by means of electricity, the most stubborn metals can be vapourized in small quantities, and for a brief space of time. But I am speaking above of such heat as we obtain in iurnaces.

THEIR MOTION.

But thirdly,-and this is the point to which I want chiefly to direct your attention,-every one of these seven suns is in swift motion. It was formerly supposed that the fixed stars really were at rest, because year after year, and century after century, passed without showing any change in their position. But gradually-even before the telescope was much used in observing the places of stars-it began to be suspected that they are slowly shifting in position on the vault of heaven. Later, very close attention was paid to the point, the telescope being used to determine the exact positions of a great number of stars, and now about 2,000 have had their slow motions on the star-vaults measured, and set down in tables for the use of astronomers employed in observatories. It occurred to me, seven or eight years

Fig. 28.-Seven stars of the Plough.

ago, that it would be interesting to picture these star-motions in maps; for tables, after all, though very pleasant in their way, are not very clear in their teachings. I made, therefore, two charts, one of all the northern stars, the other of all the southern stars, whose motions have been ascertained. These charts are given in a book of mine called "The Universe" but a sufficient idea of the method I employed may be derived from Fig. 28, above, showing the movements of the seven stars of the Plough. The little arrows attached to the seven stars show the courses along which

these stars are moving. But the length of each arrow has a meaning, too, for it is made proportional to the rate at which the star is changing its place. I have said above that the stars are in swift motion; and I have also spoken of the stars as slowly shifting in position. I think you will presently admit that both these descriptions are correct. For, first, each arrow in the figure has a length corresponding to the distance its star travels during thirty-six thousand years. After this enormous period, the stars will have moved from their present positions to the points of their respective arrows, so that the shape of the Plough will then be as in Fig. 29.

Fig. 29.-The same stars 36,000 years hence.*

It will be easy for the young student now to find the shape of the Plough at any time, past or to come. Fig. 30 shows the shape it will have 100,000 years hence; Fig. 31 shows the shape it had 100,000 years ago.

Comparing Fig. 29 with Fig. 28, it cannot but be admitted that the change is small for an interval so long as 36,000

* It may be well for me, perhaps, to explain that my charts of the motions of stars in the Great Bear, etc., were published before M. Flammarion wrote a paper called "The Past and Future of a Constellation," in which he made use of my charts, as I have myself done above. I do not in the least mind any one's borrowing from me without acknowledging the obligation,-an omission which can easily result from carelessness,but I do not wish it to be thought that I have myself borrowed without acknowledgment, where, in reality, I am only using my own material, gathered, by the way, at the cost of some labour.