Part 6

During the past century, many of these “tramps of the solar system” have been discovered and their orbits computed. The “head” may range from ten thousand to a million miles, or more, while its “tail” may stream across the heavens for millions of miles. These comets’ tails always point away from the sun; and for long the reason for this was not known. It is now believed that this is due to light-pressure; the energy of the sun’s rays press this delicate matter outwards into space. (This theory has been elaborated at considerable length by the Swedish astronomer and chemist, Arrhenius.)

Many readers of this little book will remember the excitement caused by Halley’s comet, which came relatively close to the earth in 1910, so that many persons thought there would be a collision, and were terrified accordingly! As a matter of fact, the tails of comets are usually of almost inconceivable tenuosity. Halley first observed this comet, computed its orbit and predicted the date of its return.

Some comets have tails: others do not. Not much is known concerning the origin and destination of comets; where they originated, or how. They travel at tremendous speed over many millions of miles of space, returning after a few years, or after a lapse of several centuries. They are very striking looking, even when observed by the naked eye. A number of comets have been noted. The following are a few of the more remarkable comets which were observed during the past century:

The Comet of 1811. This was visible for nearly a year and a half, and was carefully studied by William Herschel. Its tail was said to be nearly a hundred million miles long, and fifteen million miles broad.

Encke’s Comet (1819). This comet is of extreme interest because of its change of volume. Moulton says: “On October 28, 1828, it was 135,000,000 miles from the sun, and had a diameter of 312,000 miles. On December 24, its distance was 50,000,000 miles and its diameter was 14,000 miles; while at its perihelion passage of December 17, 1838, at a distance of 32,000,000 miles, its diameter was only 3,000 miles.”

Beila’s Comet (1826). This comet has a most interesting history. In 1846 it was again seen; and a month later it had divided into two parts. They traveled along parallel orbits, some 160,000 miles apart. In 1852, they were seen to be 1,500,000 miles apart. Since then they have never been seen. They have, apparently, vanished from the face of creation!

Donati’s Comet (1858). This comet was visible for more than nine months. Its tail was estimated as 54,000,000 miles long. Its period of revolution was more than 2,000 years.

The Great Comets of 1880 and 1882. The latter of these passed through some hundreds of thousands of miles of the sun’s corona. Its orbit was not appreciably changed, but, after emerging, it was seen to possess at least five nuclei--showing the effect upon the comet of the disruptive forces through which it had passed.

NEBULAE

These are of especial interest, for the reason that they have played so large a part in forming cosmic theories--the Laplacian, the Planetestimal, etc. Nebulæ are of various kinds--“Annular Nebulæ,” resembling a flat, oval, solid ring, having a dark hole in the center. Then there are “Elliptic Nebulæ,” of varying degrees of eccentricity; the Great Nebula in Andromeda being a good example. (Numbers of isolated stars may be found within its limits.) “Spiral Nebulæ” are, perhaps, the best known of all, and their name accurately describes their appearance. There are also the so-called “Planetary Nebulæ,” as well as Nebulous Stars, Irregular Nebulæ, etc. Of late years, much interest has been centered upon the so-called “Dark Nebulæ.” Herschel had long before described various “holes in the heavens,” wherein no stars could be discerned. It is now believed that such spots do not represent “holes,” as much as dark masses of matter, which seem to blot out the bright stars behind them. The interested reader may refer to Hale’s “The Depths of the Universe” for additional information upon this topic, which is relatively new to astronomy. It is also interesting to note that the spectra of Nebulæ contain the bright lines in the green of a substance called “nebulium,” because it is not found except in nebulæ.

THE MILKY WAY

This is, in a sense, one vast nebula running right round the heavens in the form of a belt, or ring; its familiar resemblance to spilt milk being the origin of its popular name. To the naked eye, it appears merely a hazy band of light, but the telescope shows that it is made up of an enormous number of stars, millions of miles apart, but which can only be distinguished from one another by telescopic aid. It constitutes the so-called “Galaxy.” It seems to be spread out in the form of a vast disk, whose diameter is many times its thickness. Our solar system appears to be near the center of this vast system, and, as we penetrate further and further into space, it becomes apparent that fewer and fewer stars, and fewer and fewer nebulæ, seem to exist. Hence the limitation of the material Universe. The Milky Way is made up of thousands of millions of suns; yet their enormous distances make them appear to constitute one vast, luminous belt encircling our globe!

THE NUMBER OF STARS

When the heavens are viewed with the naked eye, a few hundred stars may perhaps be seen--some bright, some faint. Viewed through opera glasses, many more stars may been seen; while their number is again greatly increased by the use of a telescope. The larger and more powerful the telescope employed, the greater the number of stars thus discovered in the depths of space. The interesting question thus arises: What is the total number of stars in the entire firmament? Can they be estimated? And if so, what would their approximate number be?

What we call “stars” are, of course, in practically all cases suns--often vastly larger and hotter than our own. These stars differ from one another in order of brilliance; some are brighter than others. They are accordingly classified according to their order of brilliance, and known as stars of the “First Magnitude,” of the “Second Magnitude,” etc., up to about the Seventeenth Magnitude. Any star of a given magnitude is, roughly, about two-and-a-half times as brilliant as one of the next lower order, and this variation holds throughout--each magnitude being that much greater in brilliance.

The “magnitude” of the stars varies according to their light-giving power, and also their distance from us. One of the methods adapted to measure the magnitude is to compare its brightness with an artificial star, gradually cutting-off its light by means of neutral, tinted glass until the two are equal. The color of the star must be taken into account, in such measurements, the eye being more sensitive to some colors than to others.

Now, it is an interesting and significant fact that the number of the stars decreases as their magnitude decreases; that is to say, the greatest number of stars are found of the first magnitude; a lesser number of the second; still less of the third, and so on (broadly speaking). After reaching the ninth magnitude, the number very rapidly diminishes. It has been calculated that there are about 120,000,000 stars in the first 16 or 17 magnitudes. If the proportion were maintained throughout, however, there would be more than ten times that number. Some authorities have asserted that there are, roughly, half a billion stars of varying magnitudes in the heavens.

THE POSITION OF OUR SOLAR SYSTEM

It has been maintained that our solar system is at, or very near, the center of the whole Universe. Certain it is that the further we proceed into space, the less the number of stars encountered, which has given rise to the suspicion that their number is actually limited, and that the whole Universe consists of a sort of sphere, in which is enclosed all the stars that exist, and that, beyond this sphere, no stars whatever remain. No matter exists beyond this point! Such a view fits in rather well with Einstein’s conception of “curved space,” and a finite universe of infinite proportions! Of course, it is conceivable that, outside this vast system, another similar system may exist, and another and still another; but of such systems we know nothing, and it seems improbable that proof of their existence could ever be obtained by man. So far as we can tell, the universe is One, and the matter and energy of that one are limited.

THE MOVEMENT OF OUR SOLAR SYSTEM

It has been shown that our whole solar system is sweeping through space at the speed of about ten miles a second towards the stars in the constellation Hercules, and particularly towards Vega, one of its suns. However, Vega is likewise moving through space, so that by the time our sun reaches the spot now occupied by Vega (half a million years or so) Vega will no longer occupy that position, and no “collision” will take place in consequence! We shall not, in fact, pass very near that star.

DISTANCES OF THE STARS

Astronomical distances are so vast that they can only be measured in the mind relatively. The distances between the planets in our own solar system seem big enough; yet they shrink into insignificance when compared to the distances which separate our whole solar system from even the nearest of the stars. Alpha Centauri is the nearest star, and it is separated from us by a distance 276,000 times as great as that which separates us from our sun. It is approximately 25 billion miles away. Traveling with the speed of an express train flung into space, at 40 miles an hour, towards the nearest star, without any stoppage or any slowing down, we should not arrive at our destination until after an interrupted flight of 75 million years. Yet this is our nearest neighbour! Only a very few of the stars are within 400,000,000,000,000 miles of the sun. The great majority of them are many times this distance from us.

So vast are these distances that some simple means of expressing them on paper was sought. A “light Year” was finally decided upon as the unit of measurement--that is, the distance which light would travel in one year, speeding at the rate of 186,000 miles a second. It has been estimated that many stars are one, two, three and perhaps five hundred thousand light-years distant from us in space. The interested reader may figure-out the number of miles this represents for himself!

TEMPERATURE OF THE STARS

Measurements which have been undertaken prove that the surface temperature of our Sun is between 5,000°C. and 7,000°C. It is thought that many stars are considerably hotter than this. We can form no adequate conception of such intense heat; all matter would be vaporized; yet, under the enormous pressures which must prevail, these vapors would in turn be converted into thick, semi-fluid substances--especially in the interior.

FIXED STARS

The so-called “fixed” stars are those which do not appear to change their positions in the heavens for long periods of time together. There are, of course, no “fixed” stars at all since every celestial body is moving with greater or lesser rapidity through space; but these stars are so far distant from us that such movements are inappreciable, even after long periods of time, and in spite of the most careful observations. In comparison with the more rapidly moving heavenly bodies, they do not appear to “move,” and have been denominated “fixed stars” in consequence.