There are different ways of illustrating this point, but I think the simplest, as well as the most striking, is that which is founded on the velocity of light. It is a remarkable fact that the beautiful star known as Vega[3] has a distance from us so tremendous that its light must have taken somewhere about eighteen years to travel hither from thence. Notwithstanding that the light dashes along with such inconceivable speed that it will cover 185,000 miles in every second, notwithstanding that a journey at this pace will complete the entire circuit of this globe seven or eight times between two successive ticks of the clock, the light will, nevertheless, take eighteen years to reach our eye from the time it leaves Vega. We do not, therefore, see the star as it is at present; we see it as it was eighteen years ago. For the light which this evening enters our eyes has been all that time on its journey. Indeed, if Vega were actually to be blotted out from existence it would still continue to shine out as vividly as ever for eighteen years before all the light on its way had reached us.
We have been led to the belief that among the more distant stars in the universe there must be many which are fully a thousand times as far from us as is Vega, hence we arrive at the startling conception that the light they emit has been on its journey for 18,000 years before it reached us. When we look at those lights to-night we are actually viewing them as they were 18,000 years ago. In fact, those stars might have totally vanished 17,000 years ago, though we and our descendants may still see them glittering for yet another thousand years.
We shall realize a little more fully what this reasoning involves if we suppose that astronomers dwelt on such a star, and that they had eyes and telescopes sufficiently keen not only to discern our little earth, but even to scrutinize its surface with attention. Let us suppose that the stellar astronomers looked at England: do you think they would see a network of railways joining mighty and populous cities, furnished with immense manufactories and with countless institutions? Such would be the England of to-day. But from the distance at which these astronomers are situated light takes 18,000 years for its journey, and, therefore, what they would see would be England as it was 18,000 years ago. To them England would even now appear as a country mainly covered with forests inhabited by bears and wolves, and totally void of any trace of civilization. This illustration will, at all events, serve to convey some conception of the distance at which the outskirts of our visible universe are plunged in the depths of space.
FOOTNOTES:
[3] Vega is the brightest star in the Lyre and is nearly always at night directly overhead in our latitude.—E. S.
THE STARS.—Amédée Guillemin
No sight is at once so awe-inspiring and so grand as that of the heavens on a beautiful night. If care be taken to choose as a standpoint for observation an open place, such as a plain or the summit of a hill on land, or, again, the open sea, and if the atmosphere, somewhat charged with dew, possesses all its transparency and purity, we shall see thousands of luminous points twinkling in all directions, accomplishing slowly and together their silent march. The contrast of the obscurity which reigns on the surface of the earth with the brightness of that resplendent vault gives an indefinite depth to the celestial ocean that deepens over our heads. But let us here leave the magnificence of the spectacle to study it in its most minute details.
Let us commence with the appearances. A characteristic common to all the stars is an incessant and very rapid change of brightness, which has received the name of scintillation. This is accompanied by variations of color equally rapid, due to the same cause as the successive disappearances and reappearances. All stars scintillate, whatever may be their brilliancy, at least in our temperate regions. But the intensity of this luminous movement is not the same in all, and it varies, moreover, both with the degree of purity of the sky, the elevation of the stars above the horizon, and the temperature of the night.
According to Arago, scintillation is due to the difference of velocity of the various colored rays traversing the unequally warm, unequally dense, unequally humid atmospheric strata. Thus, in tropical regions, where the atmospheric strata are more homogeneous, scintillation is rarely observed in stars the elevation of which above the horizon is more than 15°, or the sixth of the distance of the horizon from the zenith. “This circumstance,” says Humboldt, “gives to the celestial vault of these countries a particularly calm and soft character.”
Another specific character of the stars is that their diameters are without appreciable dimensions. To the naked eye, this distinction would be insufficient, since, the moon and the sun excepted, the most considerable planets have not sensible diameters. But, while the magnifying power of optical instruments shows us the principal planets under the form of clearly defined disks, the most powerful glasses only show a star as a luminous point. The distance which separates us from these bodies is so great that there is nothing to astonish us in such a result.