has for a long time excited the greatest attention. In almost all the
investigated instances it has been observed that two spiral branches
are coiling about the central body. This would indicate that the matter
is in a revolving movement about the central axis of the spiral, and
that it has streamed away from the axis in two opposite directions.
Sometimes the matter appears arranged as in a coil; of this type the
great nebula of Andromeda is the best-known example (Fig. 49). A closer
inspection of this nebula with more powerful instruments indicates,
however, that it is also spiral and that it appears coiled, because we
are looking at it from the side. The late famous American astronomer
Keeler, who has studied these nebulæ with greater success than any one
else, has catalogued a great many of them in all the divisions of the
heavens which were accessible to his instruments, and he has found
that these formations are predominatingly of a spiral nature.
[Illustration: Fig. 51.—Central portion of the great nebula in
Orion. Taken at the Yerkes Observatory. Scale, 1 mm. = 12 sec.
of arc]
Some nebulæ, like the so-called planetary nebulæ, offer rather the
appearance of luminous spheres. We may assume in these cases that the
explosions were less violent, and that the spirals, therefore, are
situated so closely together that they seem to merge into one another.
Possibly the inequalities in their development have become equalized
in the course of time. A few nebulæ are ring-shaped, as the well-known
nebula of Lyra (see Fig. 50). These rings may, again, have been
formed out of spiral nebulæ, and the spirals may have gradually been
obliterated by rotation, while the central nebulous matter may have
been concentrated on the planets travelling round the central star.
Schaeberle, an eminent American astronomer, has discovered traces of
spiral shape also in the Lyra nebula.
Another kind of nebula is the ordinary nebula of vast extension and
irregular shape, evidently formed out of most extremely attenuated
matter; well-known characteristic examples are found in Orion, about
the Pleiades, and in the Swan (Figs. 51, 52, and 53). In these nebulæ
portions of a spiral structure have likewise often been discerned.
[Illustration: Fig. 52.—Nebular striæ in the stars of the
Pleiades. Taken at the Yerkes Observatory on October 19, 1901.
Scale, 1 mm. = 42.2 sec. of arc]
We have said that the collision between two celestial bodies would
result in the formation of a spiral with two wings. If the impact is
such that the two centres of the celestial bodies move straight towards
each other, a disk will arise, and not a spiral; or if one star is
much smaller than the other, possibly a cone, because the gases will
uniformly be spread in all directions about the line of impact. A
perfectly central impact is obviously very rare; but there may be cases
which approach this limiting condition more or less, especially when
the relative velocity of the two bodies is small. By slow diffusion a
feebly developed spiral may also be converted into a rotating disklike
structure. The extension of these nebular structures will depend upon
the ratio between the mass of the system and the velocity of ejection
of the gases. If, for example, two extinct suns of nearly equal
dimensions and mass, like our sun, should collide, some gas masses
would travel into infinite space, being hurled out with a velocity
of more than 900 km. (550 miles) per second; while other particles,
moving at a slower rate, would remain in the neighborhood of the
central body. The nearer to that body, the smaller was their velocity.
From their position they might fall back into the central body, to be
reincorporated in it, if two circumstances did not prevent this. The
one circumstance is the enormous radiation pressure of the glowing
central mass. That pressure keeps masses of dust particles floating,
which by friction will carry the surrounding masses of gas with them.
Owing to the absorption of the radiation by the dust particles, only
the finer particles will be supported farther outside, and at the
extreme margin of the nebula even the very finest dust will no longer
be maintained in suspension by the greatly weakened radiation pressure.
Thus we arrive at an outer limit for the nebula. The other circumstance
is the violent rotation which is set up by the impact of the central
bodies. The rotation and the centrifugal forces will produce a
disk-shaped expansion of the whole central mass. Owing to molecular
collisions and to tidal effects, the angular velocity will in the
denser portions tend to become uniform, so that the whole will rotate
like a flattened-out ball filled with gas, and the spiral structure
will gradually disappear in those parts. In the more remote particles
the velocity will only increase to such an extent as to equal that of
a planet moving at the same distance—that is to say, the gravitation
towards the central body will be balanced by the centrifugal force, and
at the very greatest distances the molecular bombardments, as well as
gravitation towards the centre, will become so insignificant that any
masses collected there will retain their shape for an almost unlimited
space of time.
In the centre of this system the main bulk of the matter would be
concentrated in a sun of extreme brightness, whose light intensity
would, however, owing to strong radiation, diminish with comparative
rapidity.
[Illustration: Fig. 53.—Nebular striæ in the Swan. New General
Catalogue, 6992. Taken at the Yerkes Observatory on October 5,
1901. Scale, 1 mm. = 41 sec. of arc]
Such an extensive nebular system, in which gravitation, on account
of the enormous distances, would act feebly and very slowly, would
yet, in spite of the extraordinary attenuation of matter in its outer
portions, and just on account of its vast extension, be able to stop
the movement of the particles of dust penetrating into it. If the
gases of the nebula are not to escape into space, notwithstanding the
infinitesimal gravitation, their molecules must be assumed to be almost
at a stand-still, and their temperature must not rise by more than
50° or 60° Cent. above absolute zero. At such low temperatures the
so-called adsorption plays an enormously important part (Dewar). The
small dust particles form centres about which the gases are condensed
to a remarkable degree. The extremely low density of these gases does
not prevent their condensation; for the adsorption phenomenon follows a
law according to which the mass of condensed gas will only be reduced
by about one-tenth when the density of the surrounding gas has been
decreased by one-ten-thousandth. The mass of dust particles or dust
grains will thus be augmented, and when they collide they will be
cemented together by the semiliquid films condensed upon them. There
must, hence, be a relatively energetic formation of meteorites in
the nebulæ, and especially in their interiors. Then stars and their
satellites, migrating through space, will stray into these swarms of
gases and meteorites within the nebulæ. The larger and more rapidly
moving celestial bodies will crush through this relatively less dense
matter; but thousands of years may yet be occupied in their passing
through nebulæ of vast dimensions.
[Illustration: Fig. 54.—Nebula and star rift in the Swan, in
the Milky Way. Taken by M. Wolf, Königstuhl, near Heidelberg]
[Illustration: Fig. 55.—Great nebula near Rho, in Ophiuchus.
Photograph by E. E. Barnard, Lick Observatory. There are
several empty spots and rifts near the larger stars of the
nebula]
An extraordinarily interesting photograph obtained by the celebrated
Professor Max Wolf, of Heidelberg, shows us a part of the nebula in
the Swan into which a star has penetrated from outside. The intruder
has collected about it the nebulous matter it met on its way, and
has thus left an empty channel behind it marking its track. Similar
spots of vast extent, relatively devoid of nebulous matter, occur
very frequently in the irregular nebulæ; they are frequently called
“fissures,” or by the specifically English term “rifts,” because they
have generally a long-drawn-out appearance. The presumption that these
rifts represent the tracks of large celestial bodies which have cut
their way through widely expanded nebular masses (Fig. 54) has been
entertained for a long time.
[Illustration: Fig. 56.—Star cluster in Hercules. Messier 13.
Taken at the Yerkes Observatory. Scale, 1 mm. = 9.22 sec. of
arc]
The smaller and more slowly moving immigrants, on the other hand,
are stopped by the particles of the nebulæ. We therefore see the
stars more sparsely distributed in the immediate neighborhood of
the nebulæ, while in the nebulæ themselves they appear more densely
crowded. This fact had struck Herschel in his observations of nebulæ;
in recent days it has been investigated by Courvoisier and M. Wolf. In
this way several centres of attraction are created in a nebula; they
condense the gases surrounding the nebula, and catch, so to say, any
stray meteorites and collect them especially in the inner portions of
the nebula. We frequently observe, further, how the nebular matter
appears attenuated at a certain distance from the luminous bright
stars (compare Figs. 52 and 55). Finally, the nebulæ change into star
clusters which still retain the characteristic shapes of the nebulæ;
of these the spiral is the most usual, while we also meet with conical
shapes, originating from conical nebulæ, and spherical shapes (compare
Figs. 56, 57, and 58).
This is, broadly, the type of evolution through which Herschel,
relying upon his observations, presumed a nebula to pass. He was,
however, under the impression that the nebulous matter would directly
be condensed into star clusters without the aid of strange celestial
immigrants.
[Illustration: Fig. 57.—Star cluster in Pegasus. Messier 15.
Taken at the Yerkes Observatory. Scale, 1 mm. = 6.4 sec. of arc]
It has been known since the most ancient times, and has been confirmed
by the observations of Herschel and others in a most convincing manner,
that the stars are strongly concentrated about the middle line of the
Milky Way. It is not improbable that there was originally a nebula of
enormous dimensions in the plane of the Milky Way, produced possibly by
the collision of two such giant suns as Arcturus. This gigantic nebula
has gathered up the smaller migrating celestial bodies which, in their
turn, have condensed upon themselves nebular matter, and have thereby
become incandescent, if they were not so before. The rotational
movement in those parts which were far removed from the centre of the
Milky Way may be neglected. At a later period collisions succeeded
between the single stars which had been gathered up, and it is for this
reason that gaseous nebulæ, as well as new stars, are comparatively
frequent phenomena in the plane of the Milky Way. This view may some
day receive confirmation, when we succeed in proving the existence of
a central body in the Milky Way, evidence of which might possibly be
deduced from the curvature of the orbits of the sun or of other stars.
[Illustration: Fig. 58.—Cone-shaped star cluster in Gemini.]
As regards the ring-shaped nebula in the Lyre (Fig. 50), the most
recent measurements made by Newkirk point to the result that the star
visible in its centre is distant from us about thirty-two light-years.
As it appears probable that this star really forms the central core
of the nebula, the distance of the nebula itself must be thirty-two
light-years. From the diameter of the ring-shaped nebula which Newkirk
estimates at one minute of arc, this astronomer has calculated that
the distance of the ring from its central body is equal to about three
hundred times the radius of the earth’s orbit—that is to say, the ring
is about ten times as far from its sun as Neptune is from our sun.
There is a faint luminescence within this ring. The nebular matter may
originally have been more concentrated at this spot than in the outer
portions of the ring itself. But this mass was probably condensed on
meteors which immigrated from outside, and when these meteors coalesced
dark planets were produced which move about the central body, and which
have gathered about them most of the gases. If that central body were
as heavy as our sun, the matter in the ring should revolve about it
in five thousand years. That rotation would suffice to wipe out the
original spiral shape, enough of which has yet been left to permit of
our distinctly discerning the two wings of the spiral. The central
body of this ring-shaped nebula gives a continuous spectrum of bright
lines which is particularly developed on the violet side. The star
would therefore appear to be much younger and much hotter than our
sun, and its radiation pressure would therefore be much more intense.
The period of rotation of the nebula may, for this reason, have to be
estimated at a considerably higher figure.
The eminent Dutch astronomer Kapteyn has deduced from the proper
motions of 168 nebulæ that their average distance from the earth is
about seven hundred light-years and equal to that of stars of the tenth
magnitude. The old idea, that the nebulæ must be infinitely farther
removed from us than the fainter stars, would therefore appear to be
erroneous. According to the measurements of Professor Bohlin, the
nebula in Andromeda may indeed be at a distance of not more than forty
light-years.
The “new stars” form a group among the peculiar celestial bodies which
on account of their variable light intensity have been designated as
“variable stars,” and of which a few typical cases should be mentioned,
because a great scientific interest attaches to these problems. The
star Eta, in Argus, may be said to illustrate the strange fate that
a star has to pass through when it has drifted into a nebula filled
with immigrated celestial bodies. It is one of the most peculiar
variable stars. The star shines through one of the largest nebular
clouds in the heavens. Whether it stands in any physical connection
with its surroundings cannot be stated without further examination.
The star might, for instance, be at a considerable distance in front
of the nebula, between the latter and ourselves. Its frequent change
in light intensity suggests, however, a series of collisions, which do
not appear unnatural to us when we suppose that the star is within a
nebula into which many celestial bodies have drifted.
As this star belongs to the southern hemisphere, it was not observed
before our astronomers commenced to visit that hemisphere. In 1677
it was classed as a star of the fourth magnitude; ten years later it
was of the second magnitude; the same in 1751. In 1827 it was of the
first magnitude, and it was found to be variable—that is to say, it
shone with variable brightness. Herschel observed that it fluctuated
between the first and second magnitudes, and that it increased in
brightness after 1837, so that it was by 1838 of magnitude 0.2. After
that it began to decrease in intensity up to April, 1839, when it had
the magnitude 1.1. It remained for four years approximately at this
intensity; then it increased rapidly again in 1843, and surpassed all
stars except Sirius (magnitude -1.7).[16] Afterwards its intensity
slowly diminished once more, so that it remained just visible to the
naked eye (sixth magnitude); by 1869 it had become invisible. Since
then it has been fluctuating between the sixth and seventh magnitudes.
[Footnote 16: This figure, -1.7, signifies that the brightness
of Sirius is 2.52^{2.7} = 12 times greater than that of a
star of magnitude 1. Next to Sirius comes Canopus, with
magnitude -1.0, being 6.3 times brighter than a star of
magnitude 1.]
The last changes in the intensity of this star strongly recall the
behavior of the new star in Perseus, only that the latter has been
passing through its phases at a much more rapid rate. It appears to
be certain, however, that Eta, in Argus, was from the very beginning
far brighter than Nova Persei, and that at least once before the
great collision in 1843 (after which it was surrounded by obscuring
clouds of increasing opacity)—namely, in January, 1838, it had been
exposed to a slight collision of quickly vanishing effect. This lesser
collision was probably of the kind which Mayer imagined for the earth
and sun. It would give rise to heat development corresponding to the
heat expenditure of the sun in about a hundred years. As it had been
observed that the star was variable in an irregular manner before
that, we may, perhaps, presume that it had already undergone another
collision.
According to the observations of Borisiak, a student in Kief, the new
star in Perseus would have been, on the evening of February 21, 1901,
of 1.5 magnitude, while a few hours previously it had been of magnitude
12, and the following evening of magnitude 2.7; afterwards its
intensity increased up to the following evening, when it outshone all
the other stars in the northern sky. If this statement is not based on
erroneous observations, the new star must have been in collision with
another celestial body two days before its great collision, perhaps
with a small planet in the neighborhood of the sun, with which it later
collided. That would account for its temporary brilliancy.
New stars are by no means so rare as one might perhaps assume. Almost
every year some new star is discovered. By far most of these are seen
in the neighborhood of the Milky Way, where the visible stars are
unusually crowded, so that a collision which would become visible to us
may easily occur.
For similar reasons we find there also most of the gaseous nebulæ.
Most of the star clusters are also in the neighborhood of the Milky
Way. This is in consequence of the facts just alluded to. The nebulæ
which are produced by collisions between two suns are soon crossed by
migrating celestial bodies such as meteorites or comets, which there
occur in large numbers; by the condensing action of these intruders
they are then transformed into star clusters. In parts of the heavens
where stars are relatively sparse (as at a great distance from the
Milky Way), most of the nebulæ observed exhibit stellar spectra. They
are nothing but star clusters, so far removed from us that the separate
stars can no longer be distinguished. That single stars and gaseous
nebulæ are so rarely perceived in these regions is, no doubt, due to
their great distance.
Among the variable stars we find quite a number which display
considerable irregularity in their fluctuations of brightness,
and which remind us of the new stars. To this class belongs the
just-mentioned star Eta, in Argus. Another example (the first one
which was recognized as “variable”) is Mira Ceti, which may be
translated, “The Wonderful Star in the Constellation of the Whale.”
This mysterious body was discovered by the Frisian priest Fabricius,
on August 12, 1596, as a star of the second magnitude. The priest, an
experienced astronomer, had not previously noticed this star, and he
looked for it in vain in October, 1597. In the years 1638 and 1639 the
variability of the star was recognized, and it was soon ascertained
to be irregular. The period has a length of about eleven months, but
it fluctuates irregularly about this figure as a mean value. At its
greatest intensity the star ranks with those of the first or second
order. Sometimes it is weaker, but it is always of more than the fifth
magnitude. Ten weeks after a maximum the star is no longer visible,
and its brightness may diminish to magnitude 9.5. In other words, its
intensity varies about in the ratio of 1: 1000 (or possibly more).
After a minimum the brightness once more increases, the star becomes
visible again—that is to say, it attains the sixth magnitude—and
after another six weeks it will once more be at its maximum. We have
evidently to deal with several superposed periods.
The spectrum of this star is rather peculiar. It belongs to the red
stars with a band spectrum which is crossed by bright hydrogen lines.
The star is receding from us with a velocity of not less than 63 km.
(39 miles) per second. The bright hydrogen lines which correspond
to the spectrum of the nebula may sometimes be resolved into three
components, of which the middle one corresponds to a mean velocity of
60 km., and the two others have variable receding velocities of 35 and
82 km.—that is to say, velocities of 25 or 22 km. less or more than
the mean velocity. Evidently the star is surrounded by three nebulæ;
one is concentrated about its centre; the two others lie on a ring
the matter of which has been concentrated on two opposite sides. The
ring, which recalls the ring nebula in the Lyre, seems to move about
the star with a velocity of 23.5 km. per second. As this revolution
is accomplished within eleven—or, more correctly, within twenty-two
months, since there must be two maxima and two minima during one
revolution—the total circumference of the ring will be 23.5 × 86,400 ×
670—1361 millions, and the radius of its orbit 217 million km., which
is 1.45 times greater than the radius of the earth’s orbit. Now the
velocity of the earth in its orbit is 29.5 km. (18.3 miles) per second.
A planet at 1.45 times that distance from the sun would have the (1.203
times smaller) velocity of 24.5 km. per second, which is approximately
that of the hypothetical ring of Mira Ceti. We conclude, therefore,
that the mass of the central sun in Mira Ceti will nearly equal the
mass of our sun. The calculation really suggests that Mira would be
about eight per cent. smaller; but the difference lies within the range
of the probable error.
Chandler has directed attention to a striking regularity in these
stars. The longer the period of their variation, the redder in general
their color. This is easily comprehended. The denser the original
atmosphere, the more widely the gases will have extended outward from
the star, and the more dust will have been caught or secreted by it. We
have seen that the limb of the sun has a reddish light because of the
quantities of dust in the solar atmosphere. The effect is chiefly to
be ascribed to the absorption of the blue rays by the dust; but it may
partly be explained on the assumption that the solar radiations render
the dust incandescent, though its temperature may be lower than that
of the photosphere, because the dust lies outside the sun, and that it
will therefore emit a relatively reddish light. The more dust there is
in a nebula, the redder will be its luminescence; and as the quantity
of dust increases in general with the extension of the nebula, that
star which is surrounded by wider rings of nebulæ will in general be
more red; but the greater the radius of the ring, the longer also will
in general be its period.
The so-called red stars show, in addition to the bright hydrogen lines,
banded spectra which indicate the presence of chemical compounds.
On this account such stars were formerly credited with a lower
temperature. But the same peculiarity is also observed in sun-spots,
although the latter, on account of their position, must have a higher
temperature than the surrounding photosphere. The presence of bands in
the spectrum certainly suggests high pressure, however. The red stars
are evidently surrounded by a very extensive atmosphere of gases, in
the inner portions of which the pressure is so high that the atoms
enter into combination. The spectra of the red stars display, on the
whole, a striking resemblance to those of the sun-spots. The violet
portion of the spectrum is weakened, because the masses of dust have
extinguished this light. Owing to the large masses of dust which lie
in our line of sight, the spectrum lines are in both cases markedly
widened and sometimes accompanied by bright lines.