Part 7

DOUBLE STARS

A large number of stars appear single, when viewed by the naked eye, but when seen through a powerful telescope, are seen to be, in reality, two stars which revolve round one another. Many thousands of such double stars are now known to exist; indeed, apparently single stars have been found, upon closer examination, to be composed of a group of four or five or more stars--so that the name “multiple stars” has been given to such groups. They are near one another in the astronomical use of that word--though they may actually be hundreds of thousands, or millions of miles apart. Many of these double stars seem to be quite separate from one another. Others appear to have some physical connection. Those which are known to form systems are known as binaries.

COLORED STARS

Many of the double stars exhibit curious and beautiful phenomena of complementary colors. In such cases, the larger star is usually more or less reddish or orange, and the smaller one bluish-green or greenish-blue. Many of the double stars, on the contrary, are of the same color. There are white, red, blue, orange, green and yellow stars. The planets also vary greatly in color--Venus, e.g., being white, Mars reddish, etc. Inasmuch as the planets only reflect light, however, this is due to quite different causes; the other colored stars are self-luminous suns which emit light of their own.

VARIABLE STARS

In addition to variations in the color of stars, they also vary greatly in brilliance, and certain stars are much brighter at times than at others. In some cases these changes in brilliance are regular; in others, irregular. “Omicron,” for example, which, Bayer recorded in his Atlas in 1603, is a regular variable; its period of change is 331 days, 8 hours; in other words, it reaches its greatest brightness about 12 times in 11 years, when it sometimes attains the brilliancy of a star of the 2nd magnitude, at which brilliancy it remains stationary for about a fortnight. It then diminishes during about three months, until it sinks down to a star of magnitude 9½, or even becomes totally invisible. It remains in this condition for about 5 months, and then gradually recovers--during the next following 3 months--its maximum brilliancy. In other words, its brilliancy is absolutely periodic. Other variables are by no means regular, however, but “come and go” at different intervals.

Various theories have been advanced by way of explanation--one of the simplest being that such stars are in reality double, one being luminous and the other not; and that, during their revolutions, the non-luminous star partially or totally eclipses the bright one, at stated intervals. The whole subject, however, is difficult, and much yet remains to be learned concerning these variable stars.

TEMPORARY STARS

From time to time, stars have suddenly appeared in the heavens, where no star existed before! Such stars have usually become increasingly brilliant for a short period of time, and then as suddenly died away again, leaving no trace of their existence behind them. These “new stars” for long puzzled astronomers. The theory often advanced to explain them is that some distant star has “exploded,” and the increasing brilliance which we see is the result. If such were the case, its sudden dimming-down and disappearance would be quite intelligible--as would be its sudden appearance. A large number of such stars have now been recorded, and their existence is no longer in doubt. In some cases, they have remained visible for weeks or months before their final disappearance.

STAR GROUPS--CLUSTERS

Here and there throughout the sky are places where the brighter stars seem to be clustered. These families of stars are of such magnificent proportions as to stagger the imagination. Among the best known are the Pleiades, the Hyades, Coma Berenices and Orion. Although they appear to us very close together, they are not really so, being usually several hundreds of thousands of miles apart. Many of these star-groups are irregular; but numbers of them constitute clusters, which are of various sizes and shapes. Perhaps the most interesting are the so-called “globular clusters,” because they present the appearance of stars having been massed together as globes. Some of them contain five or six thousand stars. Although they appear to us so close together, it has been calculated that, in a cluster containing 5,000 stars the average distance of the stars from one another would be 30,000 times the distance of the sun from the earth! The vast distances of space considered in astronomy may perhaps be realized by this fact--when it is considered that such a cluster appears to us as a single star, only capable of being separated into its component parts by means of high-powered telescopes!

ECLIPSES

The total eclipse of the Sun, January 24, 1925, brought the subject of eclipses to the public attention as never before, and many thousands of persons watched that beautiful and impressive sight through smoked glasses or strips of film.

When we speak of eclipses, we usually mean an eclipse of either the Sun or the Moon. How are such eclipses caused?

A total or partial eclipse of the sun is caused by the moon passing between the earth and the sun, the three celestial bodies forming, as it were, a straight line. The sun is then shut-off from the vision of the inhabitants of our globe over a certain, limited area of its surface. The shadow cast by the moon falls across the earth.

But how is the moon eclipsed? Certainly the sun does not pass between the moon and the earth, on such occasions! What causes the moon to be eclipsed?

The answer is as follows: Inasmuch as both the earth and the moon are illuminated by the sun, they both cast long shadows into space, as any solid body does, when held in front of a strong light. The earth’s shadow trails away for thousands of miles into space. Into this shadow the moon enters, and when it does so, it becomes eclipsed--totally or partially, as the case may be. Total eclipses are instances when the whole surface of the celestial body is apparently covered; partial eclipses are those in which only a portion of the body is dark--the remainder being still visible.

In addition to eclipses, two other astronomical phenomena of interest should here be mentioned: Transits, and Occultations. By “transit” is meant the passage of some other heavenly body between ourselves and the sun. Thus, Mercury and Venus, both lying nearer the sun than the earth, occasionally pass in front of it. We then have a transit of Venus, or a transit of Mercury, as the case may be.

By “Occultation” is meant the hiding of one heavenly body by another--as when the moon hides some other planet or star, or one planet hides another planet or star. The three bodies are then “in line” as before. Of course, all eclipses represent instances of Occultation.

TELESCOPES

Telescopes are of relatively recent origin; the ancients were forced to make their observations without them, which makes some of their conclusions all the more remarkable. There is considerable evidence that the builders of the Great Pyramid employed the “Grand Gallery” for astronomical observations (see “The Great Pyramid of Egypt,” in the present series), and other devices were employed. But no telescopes of any great power of magnification existed before the last century, while our present marvelous instruments of precision are the evolution of the present century.

Telescopes are of two kinds: refracting and reflecting. Any small telescope exemplifies the former; the incoming light-rays are focussed by a series of lenses, and directly observed by the eye. In the employment of reflecting telescopes, however, another principle is employed: the incoming light-rays are caught and reflected by means of a curved mirror, and focussed on a lens, which in turn is inserted in an elaborate eye-piece, in which the light-rays are magnified and measured. Some of the modern instruments have a forty or more inch aperture, and are capable of enormous powers of magnification.

THE SPECTROSCOPE: SPECTRUM ANALYSIS

For more than two thousand years, astronomy remained a purely mechanical and mathematical science, being limited to observations and deductions therefrom; but in 1860 the method of spectrum-analysis was discovered. This was a most revolutionary discovery, inaugurating, as it did, the whole science of astro-physics; and enabling us to know as much of the physics and chemistry of distant stars and nebulæ--their nature, constitution, and temperature--as we know of the planets of our own system! Even the existence of otherwise invisible stars has been demonstrated in this manner--their orbits, rate of motion, and mass. The science of astro-physics is now one of the most exact in the whole realm of science; and has only been rendered possible by the invention of the spectroscope. As this instrument plays such an important part in all astronomical research, a brief explanation of the instrument becomes necessary.

If a ray of sunlight be passed through a glass prism, the ray is split up into its primary colors; so that, instead of a single spot of white light being visible a narrow band of brilliant colors is seen--ranging from red to violet. But this is not the most important part of the discovery. When this spectrum was closely examined, it was found to be crossed by numerous black bands of various thicknesses. Sometimes these occurred in groups, sometimes singly. By enlarging the spectrum by passing it through several prisms, as many as 3,000 of these bands could be counted. The nature and explanation of these strange bands of blackness remained long uninterpreted, however. It remained for Kirchoff, in 1860, to discover their uses and significance.

Briefly, it is this. The chemical elements, when heated to a state of incandescence, present each one its own characteristic spectrum; each one has its own peculiar markings, or band of lines. No two elements are exactly like in their bands, as shown in the spectrum. Hence, whenever that particular marking is observed, it becomes certain that that element, and none other, is present. These spectra are very varied; iron, for example, has more than 2,000 such bands, while lead and potassium have but one each.

In this way--all the chemical elements having been studied, and their characteristic bands known--it became possible to explore the stars, planets and suns, and discover their chemical composition. For, no matter where an element was discovered--on this earth or on the remotest star--it would always cast its particular spectrum, when thus examined. The effect of all this upon astronomy can be perceived at once. Not only the heavenly bodies known to us, but those which have never been seen by human eye--even when aided by the most powerful telescopes--can be studied and their chemical composition and structure accurately determined. Here is progress indeed!

All this becomes the more remarkable when we stop to consider the immense distances of space, and how widely separated the heavenly bodies are from one another. This may, perhaps, be shown by one or two illustrations. We are, roughly, about 93,000,000 miles from our own sun. Now, the majority of the stars we see are suns, like ours. The sun next removed from us in space is about 275,000 times as far from us as we are from our sun. The orbit of Halley’s comet, of which so much has been written lately, is some 3,280,000,000 miles in length; and this sporadic body, coursing through space at a speed 50 times greater than a rifle bullet, takes 75 years to complete its circuit. The nearest star has been calculated to be nearly 25 trillion miles away; while some of the stars are 40 times as far from us as that!