Abbildungen der Seite
PDF
EPUB

1

30

that the light of the star emanates from a point,-that is to say, the star has no sensible magnitude; that the image of the star has to be kept steady upon a slit only the part of an inch in breadth; and, moreover, that the effect of the earth's motion has to be counteracted. When you add to this, that the amount of light which even the brightest stars give is excessively feeble, that this line of light must be still further weakened by being spread out by a cylindrical lens (a) into a band, and when you remember that in our climate on a few only of those nights in which the stars appear to the naked eye to shine brilliantly is the air steady enough to prevent the flickering and confusion of the spectra, fatal to these extremely delicate observations, I think you will easily understand how exceeding difficult these researches must have been, and I am sure you will acknowledge the debt of gratitude which the world owes to those gentlemen who, by devoted labours, have brought the subject to this interesting issue.

In order to get a knowledge of the chemical composition of the stars, or to ascertain what chemical elements are present in them, it is necessary to use excessively delicate arrangements, by which not only the light from the star is allowed to pass through the prisms and to be received on the retina, but also that emitted by the various substances, the presence or absence of which in the stellar atmosphere it is desired to ascertain. These rays must pass together with the beam of starlight, or rather over or under the starlight, into the eyepiece, through the same prism, so that we may be able to compare the position of the dark lines in the stellar spectrum with that of the bright lines in the spectrum of

the body under examination. For this purpose a very ingenious arrangement is attached to a part of the telescope-spectroscope. It consists of a moveable mirror (ƒ), placed above the slit of the spectroscope, by means of which the light of the spark passing from the metallic poles, held between metal holders, is reflected by the small prism (e) placed on the slit, into the optical arrangement, and is received into the eye, the metal spectrum being ranged close above that derived from the star; so that the coincidence or otherwise of the two sets of lines can be accurately observed. In this way alone. is it possible to arrive at any trustworthy conclusion respecting the composition of the stars, and the existence of certain metals in the stellar atmosphere. The first result which we have to notice, then, is that the spectra of various stars differ very widely indeed from one another. As I mentioned to you, Fraunhofer in the year 1814 showed that the stellar spectra were not the same, and that they did not contain the same lines as the spectrum of the sun. I have here coloured drawings which will indicate to you, to begin with, the different nature of these stellar spectra: but these drawings do not pretend to give the exact positions of the various lines in the spectra, but only approximately to represent their general appearance.

Here (see Nos. 1 and 2 on the Chromolith. Plate facing this lecture), for example, is a picture of the spectra of the two stars composing B Cygni, in each of which, as you see, the arrangement of the lines is totally different; and moreover the arrangement of the lines here is quite different from that of the lines in the solar spectrum.

Mr. Huggins specially describes the spectra of two par

[merged small][merged small][ocr errors][merged small][merged small][merged small]

(Fig. 61). The upper drawing represents the spectrum

D

C

1100

[merged small][merged small][merged small][ocr errors]
[merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][ocr errors]

LaNa

Bi

Bi

SnCo Pb

Ag Ba Sn Pb

N Bi

In Ba

[ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small]

Li

Sn

Fe Fe

[ocr errors]
[ocr errors]

CANA

[ocr errors][merged small]

Ag

Ba Sn Pb N Bi Sn Ba

Li

FIG. 61.

Bi

Mg

Bi

IT

ticular stars, of which we have here an exact diagram

us

of Aldebaran, and the lower of Betelgeux, the star known as a in the constellation of Orion. This drawing is made on a similar plan to Kirchhoff's diagrams of the dark lines in the solar spectrum. The longer lines represent the dark bands in the stellar spectrum, the shorter ones beneath represent the bright lines of the metals with which the star spectrum was compared, the symbols of the elements thus examined being added. In the first place, then, the result at which we have arrived is that the constitution of the starlight, although not identical with the light given off by the sun, is yet similar; that is to say, the light of a fixed star gives off a continuous spectrum, interspersed by dark shadows or bands; and hence the conclusion we come to is that the physical constitution of the fixed stars is similar to that of our sun, that their light also emanates from intensely white-hot matter, and passes through an atmosphere of absorbent vapours-in fact, that the stars are suns of different systems. We find, for instance, in these two particular stars to which I am now referring, the D line caused by sodium exists: the three lines which we know as bare produced by luminous vapour of magnesium. The lines of these substances exactly agree in position with the dark stellar lines: hence both sodium and magnesium are present in the atmosphere of these far distant stars. We also find in Aldebaran that two hydrogen lines, C and F, are present; but if we look at the spectrum of a Orionis, we find that the hydrogen lines C and F are wanting. Hence we come to the conclusion that hydrogen is present in the atmosphere of the sun and in that of Aldebaran, but that it is wanting in that of Betelgeux. And so I might show you that silver is not present in Aldebaran, nor seen in a Orionis,

but that four bright lines of calcium, also seen in the sun's spectrum, are present in both stars. The lines observed in these two stars are at least seventy in number, and Mr. Huggins and Dr. Miller have found that in Aldebaran we have evidence of the presence of no less. than nine elements: namely (1) hydrogen, giving the lines c and F; (2) the metal sodium, giving the double line D; (3) magnesium, giving the lines b; (4) calcium, giving four lines; (5) iron, giving four lines, and E; (6) bismuth, giving four lines (bismuth is not found in the sun); (7) tellurium, four lines; (8) antimony is also found, three lines; and (9) mercury, four lines. Thus the element tellurium, whose name implies a purely earthly origin, is found in the star, although it does not exist in the sun, and is very rare on this earth. There are only two stars-Betelgeux, to which I have just referred, and another star called ẞ Pegasi-in which the hydrogen lines are wanting: all the other stars contain hydrogen.

We have, then, now arrived at a distinct understanding of the physical constitution of the fixed stars: they consist of a white-hot nucleus, giving off a continuous spectrum, surrounded by an incandescent atmosphere, in which exist the absorbent vapours of the particular metals. These results are interesting, as bearing on Laplace's nebular theory, because they show that the visible universe is mainly composed of the same elementary constituents, although certain of the stars differ from one another widely in their chemical constitution.

The next question to which the attention of the observers was directed was the different character of the light produced by the stars. It is well known that the stars are variously coloured: some shine with a bright

« ZurückWeiter »