Astronomy for Everybody/Part 6/Chapter 1

Having completed our survey of that small section of the universe in which we have our dwelling, our next task is to fly in imagination to those distant parts of space occupied by the thousands of stars which stud our sky. This is the field of astronomy in which the most wonderful discoveries have been made in recent times. We now know things about many stars which, even to such an observer as Sir William Herschel, would have seemed far beyond the possibilities of human ken. But the very vastness of the field and the minuteness of the details into which recent research has gone render it impossible to undertake anything like a comprehensive survey within the limits of the present little book. All we can do is to point out the more salient features of the universe of stars as they have been brought to light by observers and investigators of the past and present. The reader who desires further details and a wider idea of the methods and results of recent research relating to the stars may find them in a volume which the present author has recently devoted to the subject.^[1]

From the childhood of the race men have inquired: "What is a star?" To this question no answer was pos​sible until recent times. Even within the last century little more could be said than that they were shining bodies whose nature was to us a mystery. At the present time we may define the stars as immense globes of matter, generally millions of times the size of the earth, so intensely hot that they shine by their own light, and so massive that they may continue to give light and heat for unknown millions of years without cooling off. What we have said of the sun probably applies in a greater or less degree to the great majority of the stars. It is true that we cannot study their surfaces because, even in the most powerful telescopes, they appear as mere points of light. But the analogy with our sun and with other heavenly bodies leads us to believe that each of them revolves on its axis as the sun does, and that, could we see it at the proper distance, it would present much the same appearance as our sun. We have abundant evidence that rotation is the order of nature in the case of all the heavenly bodies. In the few cases where it is possible to decide whether a star does or does not rotate, the question has been answered in the affirmative.

There are innumerable differences of detail among the stars. Indeed it would seem that no two are exactly alike in their physical constitution, any more than two men are alike in their personal appearance and make-up. In the chapter on the sun we tried to give an idea of the enormous temperature of that body, which far exceeds any degree of heat we can produce on the earth. We have good reason to believe that, while the stars differ widely in temperature, the great majority of them are ​far hotter even than the sun. This is true of their surfaces and must be still more true of their vast interiors.

Stars are not the only bodies which fill these distant regions of space. Scattered over the sky are immense masses of exceedingly rare matter which, from their cloud-like appearance, are called nebulæ. In size these bodies far exceed the sun or stars. A nebula only as large as our solar system would probably be invisible in the most powerful telescope, and could never be impressed even on the most delicate photograph of the sky unless above the ordinary brightness. Those that we know have probably hundreds or thousands of times the extent of our whole solar system. We may therefore classify those bodies of the universe which shine by their own light as stars and ebulæ.

When we read of astronomical discoveries, we commonly think of them as being made by looking through a telescope. But this is no longer the case. The greatest astronomical development of recent times consists in proving the existence of dark bodies of the nature of planets, revolving around many stars. These objects are absolutely invisible in any telescope which it would be possible to construct. Such an instrument could tell us nothing about the constitution of a star. The great engine of progress has been the spectroscope, which is described in a previous chapter. From what has there ​been said the reader will see that, using words in their ordinary sense, we do not see anything by the aid of a spectroscope. What we do with it is to analyse the rays of light into their component parts, just as a chemist analyses a compound body into its simple elements. A spectroscopic analysis is more complicated from the fact that the number of elements which compose a ray of light is generally indefinite. The great advantage of spectroscopic analysis arises from the fact that it is independent of distance. The farther a star is away, the more difficult it is to see, whether we look at it with the naked eye or through a telescope. Its light diminishes as the square of the distance increases; twice as far away it gives us only one fourth the light; three times as far away, only one ninth the light, and so on. But if enough light comes from the star to enable its spectrum to be analysed, the result can be reached equally well no matter how great the distance. As the chemist could analyse a mineral brought from the planet Mars, were such a thing possible, as easily as he could if he found it on the earth, so, when a ray of light reaches the spectroscope, the fact that it may have been hundreds of years on its way, does not interfere with the drawing of conclusions from it.

When the spectrum of a star is formed it is always found to be crossed by numerous dark lines. This shows that all the stars, like our sun, are surrounded by atmospheres which are not as hot as the central body. But this does not imply that the atmosphere is cold. On the contrary, it is probably hotter than the flame of any furnace we have on earth, even in the case of the cooler stars.

​When the spectra of stars are carefully compared, it is always found that hardly any two are exactly alike. This shows that their atmospheres all differ in their physical constitution, or in the temperature of the substances which compose them. A great number of the dark lines of their spectra are found to be identical with those produced by known substances on earth. This shows that the substances of which the stars are made up are identical, in at least a great part, with those on the earth.

One of the most abundant of these substances is hydrogen. Several lines of hydrogen are found in nearly all the stars. Another substance which seems to be almost universal throughout the universe is iron. Yet another is calcium, the metallic base of lime. We all know that this substance abounds on the earth, and we have, in its diffusion among the stars, an example of the unity of nature in its widest extent.

Yet, variety is also the rule. Besides lines due to known substances, many stars show lines which have not yet been identified with those of any element that we know of. This is especially the case in the class known as Orion stars, because many of them are found in the constellation Orion. These stars are mostly very white or even blue in colour, and show a number of fine dark lines which are to a greater or less extent the same in all Orion stars, but are not those produced by any known chemical element. We therefore have reason to believe that there are in the stars other chemical elements than those with which we are acquainted.

There is a very curious case in which an element first ​excited interest through its being found in the sun and stars. For some time after the study of the sun's spectrum had been commenced, it was known that certain well-marked lines in it were not produced by any substance then known. But continued research led to the discovery that this substance existed in a Norwegian mineral, cleveite, and perhaps elsewhere on the earth. From its existence on the sun it was called helium. Its spectrum was no sooner made known than it was found that helium existed in many stars which are, for that reason, called "helium stars."

In many cases some idea can be obtained of the density of a star, or, in ordinary language, of its specific gravity. It is very remarkable that, in nearly all such cases the density is found to be far less than that of our ordinary solid or liquid substances; frequently no greater than that of air, sometimes even less. In this respect our sun, although its density is so small, seems to be an exception, and it is likely that only a very small proportion of the stars are as dense as the sun. This affords one proof of the high temperature of these bodies, which must be such that all liquid or solid substances exposed to it would boil away as water boils when put on a fire, thus changing its substance into a vapour. We have reason to believe that the stars are for the most part masses of this intensely hot vapour, surrounded perhaps by a somewhat colder surface. Possibly many of the stars ​may be of the nature of bubbles, but this is far from being established.

A star, like the sun, must be hotter in the interior than at its surface. From the latter alone can heat be radiated; hence the surface is continually cooling off, and if the matter composing the body were at rest, the cooling would soon go so far that a crust would form, as it does on a mass of molten iron. The only way in which this can be prevented is that, as the superficial portions cool, the greater density which they thus acquire causes them to sink down into the seething mass below, portions of which arise to take their place, cool off, and sink in their turn. Thus there is a continual interchange of matter between the inside and the surface, much as in a boiling pot the water at the bottom is continually being forced up to the top, while that on the top continually sinks down.

It follows from this that there must be a limit to the smallness of a star. If such a body were no larger than the moon, it would, in a few thousand years, so far cool off that a crust would form over its surface. This would cut off the currents by which the hot matter is brought to the surface and the star would soon cease to shine. As there can be little doubt that the age of most of the stars is to be reckoned by millions of years, it follows that they must be so large that they can lose heat for millions of years and yet a cool crust not form on their surface.

We have said that our sun is among the colder of the stars and also that it is among the smaller. These two facts fit well together. The smaller a star is the more ​rapidly it cools off, just as a cup of water cools off faster than a pot full.

The revelations of the spectroscope makes it very probable that every star has a life history. It began as a nebula, which, in the course of ages, slowly condensed into an intensely hot, blue-coloured star. The condensation going on, the star becomes yet hotter, until it reaches its highest temperature. Then, cooling off, its colour changes to white, yellow, and red, and the lines in its spectrum become darker and more numerous. Finally, its light dies away, as a fire flickers out when the supply of fuel is exhausted, and the star becomes a dark opaque body,—its life has ended. The greater the mass of the star the longer its life. Thus it is that the stars we observe seem to be of all ages, from the infantile nebula to the star dying of old age.