Part 40

These results are confirmed by the fact that the amount of motion indicated is about what we should expect the stars to have, from their observed proper motions, combined with their probable distances. Again: the stars in the neighborhood of Hercules are mostly found to be approaching the earth, and those which lie in the opposite direction to be receding from it; which is exactly the effect which would result from the sun's motion through space. The five stars in the Dipper, which have a common proper motion, are also found to have a common motion in the line of sight. But the displacement of the spectral lines is so slight, and its measurement so difficult, that the velocities in the above table are to be accepted as only an approximation to the true values.

Chemical and Physical Constitution of the Stars.

368. The Constitution of the Stars Similar to that of the Sun.--The stellar spectra bear a general resemblance to that of the sun, with characteristic differences. These spectra all show Fraunhofer's lines, which indicate that their luminous surfaces are surrounded by atmospheres containing absorbent vapors, as in the case of the sun. The positions of these lines indicate that the stellar atmospheres contain elements which are also found in the sun's, and on the earth.

369. Four Types of Stellar Spectra.--The spectra of the stars have been carefully observed by Secchi and Huggins. They have found that stellar spectra may be reduced to four types, which are shown in Fig. 428. In the spectrum of Sirius, a representative of Type I., very few lines are represented; but the lines are very thick.

Next we have the solar spectrum, which is a representative of Type II., one in which more lines are represented. In Type III. fluted spaces begin to appear, and in Type IV., which is that of the red stars, nothing but fluted spaces is visible; and this spectrum shows that something is at work in the atmosphere of those red stars different from what there is in the simpler atmosphere of Type I.

Lockyer holds that these differences of spectra are due simply to differences of temperature. According to him, the red stars, which give the fluted spectra, are of the lowest temperature; and the temperature of the stars of the different types gradually rises till we reach the first type, in which the temperature is so high that the dissociation (161) of the elements is nearly if not quite complete.

III. NEBULÆ.

Classification of Nebulæ.

370. Planetary Nebulæ.--Many nebulæ (328) present a well-defined circular disk, like that of a planet, and are therefore called planetary nebulæ. Specimens of planetary nebulæ are shown in Fig. 429.

371. Circular and Elliptical Nebulæ.--While many nebulæ are circular in form, others are elliptical. The former are called circular nebulæ, and the latter elliptical nebulæ. Elliptical nebulæ have been discovered of every degree of eccentricity. Examples of various circular and elliptical nebulæ are given in Fig. 430.

372. Annular Nebulæ.--Occasionally ring-shaped nebulæ have been observed, sometimes with, and sometimes without, nebulous matter within the ring. They are called annular nebulæ. They are both circular and elliptical in form. Several specimens of this class of nebulæ are given in Fig. 431.

373. Nebulous Stars.--Sometimes one or more minute stars are enveloped in a nebulous haze, and are hence called nebulous stars. Several of these nebulæ are shown in Fig. 432.

374. Spiral Nebulæ.--Very many nebulæ disclose a more or less spiral structure, and are known as spiral nebulæ. They are illustrated in Fig. 433. There are, however, a great variety of spiral forms. We shall have occasion to speak of these nebulæ again (381-383).

375. Double and Multiple Nebulæ.--Many double and multiple nebulæ have been observed, some of which are represented in Fig. 434.

Fig. 435 shows what appears to be a double annular nebula. Fig. 436 gives two views of a double nebula. The change of position in the components of this double nebula indicates a motion of revolution similar to that of the components of double stars.

Irregular Nebulæ.

376. Irregular Forms.--Besides the more or less regular forms of nebulæ which have been classified as indicated above, there are many of very irregular shapes, and some of these are the most remarkable nebulæ in the heavens. Fig. 437 shows a curiously shaped nebula, seen by Sir John Herschel in the southern heavens; and Fig. 438, one in Taurus, known as the Crab nebula.

377. The Great Nebula of Andromeda.--This is one of the few nebulæ that are visible to the naked eye. We see at a glance that it is not a star, but a mass of diffused light. Indeed, it has sometimes been very naturally mistaken for a comet. It was first described by Marius in 1614, who compared its light to that of a candle shining through horn. This gives a very good idea of the impression it produces, which is that of a translucent object illuminated by a brilliant light behind it. With a small telescope it is easy to imagine it to be a solid like horn; but with a large one the effect is more like fog or mist with a bright body in its midst. Unlike most of the nebulæ, its spectrum is a continuous one, similar to that from a heated solid, indicating that the light emanates, not from a glowing gas, but from matter in the solid or liquid state. This would suggest that it is really an immense star-cluster, so distant that the highest telescopic power cannot resolve it; yet in the largest telescopes it looks less resolvable, and more like a gas, than in those of moderate size. If it is really a gas, and if the spectrum is continuous throughout the whole extent of the nebula, either it must shine by reflected light, or the gas must be subjected to a great pressure almost to its outer limit, which is hardly possible. If the light is reflected, we cannot determine whether it comes from a single bright star, or a number of small ones scattered through the nebula.

With a small telescope this nebula appears elliptical, as in Fig. 439. Fig. 440 shows it as it appeared to Bond, in the Cambridge refractor.

378. The Great Nebula of Orion.--The nebula which, above all others, has occupied the attention of astronomers, and excited the wonder of observers, is the great nebula of Orion, which surrounds the middle star of the three which form the sword of Orion. A good eye will perceive that this star, instead of looking like a bright point, has a hazy appearance, due to the surrounding nebula. This object was first described by Huyghens in 1659, as follows:--

"There is one phenomenon among the fixed stars worthy of mention, which, so far as I know, has hitherto been noticed by no one, and indeed cannot be well observed except with large telescopes. In the sword of Orion are three stars quite close together. In 1656, as I chanced to be viewing the middle one of these with the telescope, instead of a single star, twelve showed themselves (a not uncommon circumstance). Three of these almost touched each other, and with four others shone through a nebula, so that the space around them seemed far brighter than the rest of the heavens, which was entirely clear, and appeared quite black; the effect being that of an opening in the sky, through which a brighter region was visible."

The representation of this nebula in Fig. 441 is from a drawing made by Bond. In brilliancy and variety of detail it exceeds any other nebula visible in the northern hemisphere. In its centre are four stars, easily distinguished by a small telescope with a magnifying power of forty or fifty, together with two smaller ones, requiring a nine-inch telescope to be well seen. Besides these, the whole nebula is dotted with stars.

In the winter of 1864-65 the spectrum of this nebula was examined independently by Secchi and Huggins, who found that it consisted of three bright lines, and hence concluded that the nebula was composed, not of stars, but of glowing gas. The position of one of the lines was near that of a line of nitrogen, while another seemed to coincide with a hydrogen line. This would suggest that the nebula is a mixture of hydrogen and nitrogen gas; but of this we cannot be certain.

379. The Nebula in Argus.--There is a nebula (Fig. 442) surrounding the variable star Eta Argus (355), which is remarkable as exhibiting variations of brightness and of outline.

In many other nebulæ, changes have been suspected; but the indistinctness of outline which characterizes most of these objects, and the very different aspect they present in telescopes of different powers, render it difficult to prove a change beyond a doubt.

380. The Dumb-Bell Nebula.--This nebula was named from its peculiar shape. It is a good illustration of the change in the appearance of a nebula when viewed with different magnifying powers. Fig. 443 shows it as it appeared in Herschel's telescope, and Fig. 444 as it appears in the great Parsonstown reflector (20).

Spiral Nebulæ.

381. The Spiral Nebula in Canes Venatici.--The great spiral nebula in the constellation Canes Venatici, or the Hunting-Dogs, is one of the most remarkable of its class. Fig. 445 shows this nebula as it appeared in Herschel's telescope, and Fig. 446 shows it as it appears in the Parsonstown reflector.