Popular Science Monthly/Volume 42/February 1893/The New Star in the Milky Way
|THE NEW STAR IN THE MILKY WAY.|
A "NEW STAR" is a representative of a class of phenomena so rare that the number recorded during the last few centuries may be counted on the fingers. Hence we readily conceive that, since they are very striking in themselves as breaking the monotony of the starry heavens, and since also their nature was considered till quite recently to be shrouded in mystery, a most lively interest has been stirred up by the recent new arrival, not only among astronomers, but among that large class who are always on the qui vive for celestial wonders.
When tortured by the many instruments which modern science places at the observer's disposal, a new star is quite a thing per se; while at times their brilliancy is extraordinary, some of these "new stars" having rivaled both Mars and Jupiter in brightness, and even sometimes Venus.
The time that they take to wax and to wane varies very considerably; some have lasted at their greatest brightness only for days, others have remained visible for months or occasionally for years. It generally happens that a "new star" when first seen is brightest, and many have thought that this is simply because the star is at the stage most likely to be noticed by us; but this may not be the entire truth, as can be gathered from a consideration of the various views which have been put forward as to their nature.
Among the many hypotheses that have been suggested to explain how it is that these strange bodies make their appearance from time to time, we may first of all mention that which supposed them due to the sudden colliding of a comet with a star; another theory assumed that a star at some period of its existence became enveloped in a kind of crust or slag, which by some cause or other became disrupted, and revealed the glowing mass within.
Both these hypotheses, although they might to a certain degree explain the sudden brightness of the star, would not hold good with regard to the rapid diminution of its light, because, if large bodies are dealt with, the cooling must take a very long time.
The latest view put forward is, that these bodies are produced by the sudden meeting in space of two swarms or streams of meteoritic matter, each traveling with a considerable velocity, the sudden bright light being due to the collisions of the particles composing the swarms; and this hypothesis explains very well not only the sudden outburst, but the rapid decrease in brightness, due to the fact that only small particles are dealt with, and these must cool and dim quickly.
The appearance of the present new star, or "Nova," in the constellation of Auriga, was first announced by an anonymous post-card received at the Royal Observatory, Edinburgh. Why the post-card was sent anonymously remains a mystery; but the extraordinary reticence of the writer does not make any difference to the immortality of the discoverer; for while, on the one hand, newly discovered comets, which are also of an apparently temporary nature, are always associated with the names of those who first observe them, new stars, on the other hand, are always referred to by the name of the constellation in which they appear.
The instrument now used to obtain observations of these strange visitors consists of a combination of an object-glass; a prism, which is placed outside the object-glass, and a camera. The function of the prism is to separate the million strands of colored light which go to make white light; that of the object-glass is to collect each color, concentrating it at the same time, so that finally we get a fine line of rainbow color.
This method of obtaining a spectrum is by no means modern, but was suggested and used by the German optician Fraunhofer about the year 1814. He placed a prism before the object-glass of a theodolite, and in this way was the first to observe the spectra of some of the stars. By the use of this method, whether the eye or the photographic plate is used, the so-called "spectrum" of the body under observation can be studied without any difficulty. The length of the exposures required when photography is employed for stars of different magnitude varies very considerably; for the brightest a few minutes are generally ample, but for those of much smaller magnitude a space of two or three hours is by no means too long.
The spectra that are thus obtained are of various kinds, as various classes of so-called stars are observed. Some consist of bright lines on a dark background, others of dark lines on a bright background while a mixture of both these is met with. These variations in spectra depend upon the fact that any substance that is heated sufficiently to emit light whether in the heavens or on the earth, will give a spectrum. If it be a solid or liquid body, we shall have what is called a continuous spectrum—that is, a colored band bright from end to end, with no sign of any dark or bright lines about it. By continuing to heat this body until it becomes a mass of incandescent gas, the spectrum will become entirely changed, and will consist of a series of bright lines on a dark background, the number and position of the lines depending on the substance heated. But suppose, now, that the light from an incandescent solid or liquid body passes through a gas, what kind of a spectrum should we have? Experiment shows that in this case we get a continuous spectrum crossed by dark lines, these dark lines being produced by the peculiar power that a gas possesses of absorbing those particular rays of light which it emits. Thus we see that if we are dealing with incandescent solid or liquid bodies, we obtain continuous spectra; if with incandescent gases, bright-line spectra; and if with absorption, dark-line spectra; the position of the lines in all cases revealing the chemical nature of the substances.
So much, then, for the general idea of the nature of a spectrum. There are some additional points to be considered when we are dealing with stars. If we observe the spectrum of a star at rest, we shall obtain lines, whether bright or dark, in their normal place in the spectrum. These lines will be peculiar to certain substances, and, in fact, their presence in the star is determined simply by them. If we deal with the light from a body which is not an apparent point, the lines will still keep the same positions, for the same reason, but each one of them will be broadened equally.
Let us now suppose the star no longer stationary, but moving with a considerable velocity. In this case the wave-length of each line will be no longer the same; but the line will have altered its position in the spectrum to an extent depending on the movement of the star toward or from the earth. The result produced in the spectrum will be the same with regard to the number of lines as was the case when the star was assumed to be motionless, but the lines will all have received a slight shift, either to one side or the other of their initial positions, according as the star is approaching or receding. If instead of one we now deal with two stars of the same chemical and physical structure, traveling with different velocities, either toward or away from us, the spectrum would show each line doubled, and the more rapid the relative motion the coarser will be the doubling. If the stars were so physically constituted that the same chemical substances were present in both, but giving bright lines in one and dark lines in the other, the spectrum would present a series of bright lines, each accompanied by a dark one, on one side or the other, according as the body which contained dark lines in its spectrum was approaching the earth or receding from it.
After this very brief statement of general principles, we can now refer to the observations that have already been made with regard to the spectrum of the present new star, observations unique in astronomical history, and of the highest importance and interest. It has been found to consist of both light and dark lines. The fact that pairs of bright and dark lines are seen proves that two bodies are in question. If we suppose two swarms of meteors colliding in space, the spectrum can be easily explained on this assumption in the light of the general principles referred to above. Further, the thickness of the lines tends to show that each one is produced by a large number of small incandescent masses moving at different velocities, rather than by one large one. The motion necessary to produce the doubling of these lines has been estimated, and the relative velocity of the two swarms has been put down as more than five hundred miles per second!
If the photographs should continue to show the same relative positions of the bright and dark lines, the observations would prove that this relative motion is not produced by the revolution of one body round another, but that a dense swarm of meteorites is moving toward the earth with a high velocity, and passing through another receding one of less density.
It will be seen that the observations harmonize well with the hypothesis that has been advanced on much less definite evidence; but this is not the only instance we can give of the grip that modern science has on large classes of phenomena which were supposed to be beyond the reach of man. The lines that have been photographed in the spectrum of this star are all such as could have been predicted with our knowledge of new stars.
As an instance of the advanced stage at which astro-physical science has arrived, we may say that, if we had no observations of new stars other than those already recorded of the present one, their whole theory could be obtained by induction. This may seem a "sweeping statement," but it is nevertheless true, for since many so-called "stars" are now known not to be "stars" like our sun, but simply clouds of meteoritic bodies clashing together, and since we know approximately the sequence of changes through which the spectra of these stars pass as their temperature is first increased and then reduced, each spectrum indicates the complexity of each swarm.
We have already seen that the doubling of the bright and dark lines indicates that we are dealing with two swarms in the present instance, one approaching and the other receding; we now learn that the condensation at which each of these swarms exists can be approximately determined; that which gives us the dark lines is denser than the one which gives us the bright ones.
In conclusion, it may be well to point out a difference of some importance between comets and these new stars. A comet, as is generally conceded, consists of a cloud of meteoritic dust traveling round the sun, sometimes in elliptic but more often in a parabolic or hyperbolic orbit; in other words, those traveling in elliptic orbits have been captured by the sun and return to it periodically, while those pursuing a parabolic or hyperbolic orbit after one passage near the sun are forever lost to us.
Thus a comet with an elliptic orbit may be said to be a member of the solar system, and on this account can approach very near to our earth; and in fact our earth has even passed through one, giving rise to the phenomena of a great number of shooting-stars.
A new star, on the other hand, never approaches our system, but is formed at very great distances from us, distances probably as great as that of the nearest star, so that light, which travels one hundred and eighty-six thousand miles per second, takes about thirty years to complete its journey to us. Our new star, then, is already old.—Saturday Review.