The story of the universe, told by great scientists and popular authors. Volume 1 (of 4), The starry skies.

by Unknown

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.

Wollaston affirms that the apparent diameter of the most brilliant star in the heavens, Sirius, is not more than the fiftieth part of a second of an arc. But let us hasten to say that this result still leaves a good margin as to the real dimensions of the star, since, at the distance of Sirius, an apparent diameter would represent a real diameter of 11,000,000 miles; that is, twelve times the diameter of our sun.

Let us add, lastly, that the absence of appreciable dimensions does not suffice to distinguish absolutely the stars from the planets, since a certain number of the latter, as we have before seen, appear in telescopes only as simple luminous points. Let us come, then, to a permanent specific characteristic, the knowledge of which will always prevent us from confounding a star with one of the known or unknown bodies which form part of our solar group. This characteristic is as follows:

The stars, properly so called, preserve among themselves—nearly enough for our present purpose—the same relative distances. They form, then, on the celestial vault apparent groups, the configuration of which is nearly invariable. Centuries must elapse to show a change of form, unless we employ extremely delicate measures. A planet, on the contrary, moves rapidly across these groups, to such a degree that, in the interval of a night, or at most of a few nights, this displacement is very perceptible; hence the old denomination of fixed stars, in opposition to the wandering ones, or planets.

We must be careful, however, to guard against assigning to this word a rigidity which it does not possess, for the stars really move with a velocity not inferior to that which animates the members of our system. Their immense distance is the only cause of their apparent immobility, which vanishes when precise observations, embracing a sufficient interval of time—some years, for example—are made.

A fact which strikes every one is the great diversity of brightness in the stars which people the heavens. All degrees of intensity are remarked, from the resplendent light of Sirius to the scarcely perceptible glimmer of those hardly visible to the naked eye.

Whence arises this difference of brightness? This question we can not answer for any star in particular, but it is easy to imagine that it may result from various circumstances, such as their less or greater distance, the real and various dimensions of the bodies, and, lastly, the intrinsic brightness of the light peculiar to each. However this may be, astronomers without regard to the unknown causes which may influence the intensity of the stellar light, have divided stars into classes or magnitudes; and when we speak of a star of the first, second, or fifth magnitude, it is understood that this way of speaking refers only to the apparent brightness, and that nothing is affirmed either as to the real dimensions or distance, or even intrinsic brightness.

Besides, as the stars, arranged in the order of their brightness, would form a progression decreasing by imperceptible degrees, the classes adopted are themselves conventional and arbitrary. The first six magnitudes comprise all stars visible to the naked eye. But the use of the most powerful telescopes brings to view stars of feebler light, descending to the sixteenth and seventeenth magnitudes. In truth, the progression has no inferior limit: it extends more and more in proportion as the progress of the optician’s art increases the penetrating power of our instruments.

To gain an idea of the respective intensities of the light emitted by the stars of the first six magnitudes, following the scale adopted by astronomers, the accompanying illustration (Fig. 1), should be inspected; in it the stars are figured by disks, the surfaces of which are in proportion to their brilliancy.

But, we repeat, it must not be thought that the stars ranked in the same class are, on that account, of the same brightness. Thus the light of Sirius is estimated at four times the star Alpha Centauri; but both, nevertheless, are included by astronomers in the number of the stars of the first magnitude.

Fig. 1.—Relative Brilliancy of Stars of the first Six Magnitudes

We here give the names of the twenty most brilliant stars of the two hemispheres which it is usual to consider as forming the first class. They are here arranged in the order of their brightness:

1. Sirius	11. Achernar
2. Eta Argus	12. Aldebaran
3. Canopus	13. Beta Centauri
4. Alpha Centauri	14. Alpha Crucis
5. Arcturus	15. Antares
6. Rigel	16. Altair
7. Capella	17. Spica
8. Vega	18. Fomalhaut
9. Procyon	19. Beta Crucis
10. Betelgeuse	20. Pollux

Lastly, Regulus, a bright star in the constellation of the Lion, is also ranked by some astronomers in the first magnitude, while others only admit in this class the first seventeen stars in the above list. These divergences are of no importance.

In proportion as the scale of brilliancy or magnitude is descended, the number of the stars contained in each class rapidly increases. The number of second magnitude stars in the heavens is about 65; of the third, about 200; of the fifth, 1,100; and of the sixth magnitude, 3,200. Adding these numbers together, we obtain a few over 5,000 stars of the first six magnitudes, and these comprise very nearly all those that can be seen with the naked eye.

The smallness of this number nearly always astonishes those who have not tried to form an exact estimate of the number of stars which shine in the celestial vault on the most favorable nights.

The aspect of the multitude of sparkling points which are scattered over the sky makes us disposed to believe that they are innumerable, and to be counted, if not by millions, at all events by hundreds of thousands. This is, nevertheless, an illusion. All observers who have taken the trouble to make an exact enumeration of the stars visible to the naked eye have arrived at a maximum of 3,000 as the mean number which can be observed in every part of the heavens, visible at the same time, at the same place; this, of course, is but half of the entire heavens.

Argelander has published an exact catalogue of the stars visible on the horizon of Berlin during the course of the year. This catalogue comprises 3,256 stars. According to Humboldt, there are 4,146 visible on the horizon of Paris in the whole course of the year; and as this number increases in proportion as we approach the Equator, that is to say, in proportion as the double movement of the earth unfolds to us during a year a more extensive portion of the heavens, 4,638 stars are already visible to the naked eye on the horizon of Alexandria.

We repeat, the maximum number is comprised between 5,000 and 6,000 stars for the entire heavens, including those seen by the most piercing and most accustomed eyes in the best nights for observation. When the atmosphere is lit up by the moon, or by twilight, or, as happens in the great centres of population, by the illumination of the houses and streets, the lowest magnitude stars are effaced altogether, and the number of those visible is consequently much more limited. We may add in conclusion, that the more the scintillation, the more easy it is to distinguish very faint stars.

A word now on the number of stars that can be seen with the help of the telescope. Here we shall find the numbers which our imagination had erroneously led us to believe are visible to the naked eye.

According to the illustrious director of the Observatory of Bonn—Argelander—the seventh magnitude comprises nearly 13,000 stars; the eighth, 40,000; and, lastly, the ninth, 142,000. The calculations of Struve give the total number of stars visible in the entire heavens by the aid of Sir William Herschel’s 20-foot reflector as more than 20,000,000. But, without doubt, these approximate numbers are much below the real ones. It will be seen, besides, that the richness of the heavens in stars is very unequal. The bright zone known under the name of the Milky Way alone contains, according to Herschel, 18,000,000.