THE RIDDLE OF THE MILKY WAY
During dark but starlit nights, the gorgeous firmament is decorated
with an irregular band of light that describes a winding path across
the heavens. It continues also in the quarters hidden from our sight so
that it may be said to surround the firmament like a girdle. This band,
which is most luminous in the Northern Hemisphere, is called “The Milky
Way.”[1] It forms an angle with the equator of about 60° and divides
the firmament in two nearly equal parts--the northern, however, is
slightly larger.
[1] The literal translation of the Swedish name is “The Wintry
Way.”
The Milky Way, no less than other stellar phenomena, attracted the
early attention of the people. The Dieri Tribe in Central Australia
says that the Milky Way is the stream of heaven and the Mexicans
consider it the source of all that is. Tradition endeavoured to explain
its origin. Its milky appearance caused the Romans to call it “_Via
Lactea_,” a name that is retained in translated form in most modern
languages. This name is coupled with the legend of the Hercules-child,
who sucked the breast of Juno and when it was pushed away by the
incensed goddess, the milk was spread across the sky.
Nevertheless, the human race, until about two hundred years ago, had
little conception of the extraordinary importance of the Milky Way.
Anaxagoras and Democritos surmised, however, that it consists of a
collection of exceedingly minute and densely clustered stars each of
which has the nature of our Sun. Ptolemy described, nearly two thousand
years ago, its position on the firmament and his observations are
valid today as far as determinations with the naked eye suffice. When
Galileo introduced the telescope, the conception of the Milky Way as
made up of innumerable stars was verified. Not quite two hundred years
ago Swedenborg, in his cosmological speculations, considered our solar
system as a part of the Galaxy. Wright, Kant, and Lambert further
amplified these theories.
The first important forward step was taken by the great William
Herschel in his statistical researches. He demonstrated that the
stars lie closer to each other the nearer the Milky Way they are
located. This is mainly true about the small stars invisible to the
naked eye while the more luminous ones are more evenly distributed
over the heavens. In certain parts of the Milky Way the stars are
over one hundred times more crowded than at its poles--that is, the
points farthest removed from the Galaxy. Herschel’s investigations
were continued and elaborated by Struve, and later by numerous other
scientists.
Through these researches, it has been determined that the Milky Way
is, so to speak, the foundation upon which the star system, visible
to us, is built. All kinds of stellar bodies have been studied and
their distribution has proved to be symmetrical with reference to the
plane of the Galaxy. The majority is greatly concentrated around the
Milky Way. To these belong the new stars which occasionally blaze into
existence, as the well-known new star in Perseus, 1901, and nearly all
of which have appeared in the Milky Way or in its immediate vicinity.
We also find there the irregular nebulæ, enormous, vastly diffused
volumes of gas, among which the best known is the Orion nebula, and
which seem to constitute the primeval matter out of which the stars are
born. We might further mention the star-clusters, dense, ball-shaped
agglomerations of stars, and the so-called planetary nebulæ, which--at
least in their visible outer shell--also consist of gas accumulations
with a spherical or ellipsoid conformation. The numerous spiral nebulæ
on the other hand, those strange stellar bodies to which we later
shall have occasion to return, are beyond comparison more frequent in
the regions surrounding the poles of the Galaxy than in the rest of the
heavens.
Many astronomers have considered the Milky Way itself a nebula. The
most common theory doubtless is that it closely conforms to a spiral
nebula--an opinion that has found a particularly warm advocate in
the Dutch astronomer, Easton (see Figure 1). A few years ago Prof.
Bohlin expressed the view that it is most akin to a planetary nebula,
or more precisely to a ring nebula which is supposed to grow out of
a planetary ellipsoid nebula by the gaseous matter being driven from
its poles toward its equator. It is of a certain interest that this
theory lends itself to the support of Swedenborg’s--nevertheless
improbable--hypothesis about the origin of the planets in the solar
system. As we later shall see the Easton conception has the better
reasons in its favour.
If classified according to age the stars are again distributed with the
Milky Way as a reference point. Thus, let us consider their evolution,
which for various reasons is assumed to take the following course.
We may commence when the star-matter existed on the nebula stage. It
then radiated the light characteristic of certain incandescent gases,
principally the lightest two, hydrogen and helium, and further of an
otherwise unknown gas called nebulium (nebula-substance). These gases
were later condensed and dark spectral lines commenced to appear
beside the bright lines of the aforesaid gases. Stars on this stage,
named after their discoverer Wolf-Rayet stars, occur only in the
immediate vicinity of the Milky Way. A later stage in their evolution
is represented by the so-called helium stars in whose spectrum the dark
helium lines preponderate. They are considerably concentrated around
our Galaxy. Somewhat more evenly distributed and yet of decidedly
greater frequence in the neighbourhood of the Milky Way, the hydrogen
stars appear, characterized by strongly developed hydrogen lines and
somewhat retreating helium lines. These stars are more developed than
the helium stars and form with them the group of white stars so named
after the colour of their light. Next in evolution follow the yellow
stars, to which our Sun belongs. Dark metal lines appear in their
spectrum. They are more evenly distributed than the groups mentioned
before. Still further is this true about the red stars whose spectra
contain the characteristic bands of chemical compounds and therefore
betray comparatively advanced cooling. They are fairly uniformly spread
over the heavens but are still somewhat more numerous in the vicinity
of the Milky Way than further therefrom.
[Illustration: Fig. 1. The Milky Way, pictured as a spiral nebula by
Easton]
These facts are demonstrated in the statistics by E. C. Pickering,
Director of the Harvard Observatory, who divided the firmament in
four equal zones, the first of which is nearest to the Milky Way (and
includes it) and the last of which contains the Galactic poles. His
table shows the percentages of different stars in each of the four
zones.
| | | _White | |
_Galactic | _Helium | _Hydrogen | Yellow | _Yellow | _Red
Latitude_ | Stars_ | Stars_ | Stars_ | Stars_ | Stars_
---------------+---------+-----------+--------+---------+-------
±8.1° | 51.2 | 37.4 | 29.7 | 29.4 | 26.7
±21.6 | 31.7 | 28.6 | 27.9 | 26.7 | 27.6
±39.8 | 11.9 | 18.3 | 21.1 | 21.9 | 23.6
±62.3 | 5.2 | 15.7 | 21.3 | 22.0 | 22.1
Number of | | | | |
stars observed | 716 | 1885 | 1329 | 1719 | 457
The difference is most pronounced in the two first groups; in the
three last it is small but unmistakable. An even distribution would
correspond to 25 per cent. in all four divisions of the heavens.
These comprehensive statistics, embracing 6106 stars, seem to indicate
that the stars in their first stage were within the Milky Way but
subsequently drifted away with increasing age. This leads us to the
thought that they originated from the irregular, nebulous accumulations
which occur in the Milky Way and in its vicinity, or more correctly
from similar formations which formerly existed in these regions but
which now have clustered into stars. This agrees very well with
another observation. With the help of the spectroscope the motion
of different stars has been determined with reference to the point
where the sun now is. The velocities have been found greater the
older the stars are as shown in the table below taken mainly from the
investigations of the renowned astronomer, Campbell.
_Mean velocity of_:
Irregular nebulæ 0 Km. ( 0 Miles) per sec.
Wolf-Rayet stars 4.5 ” ( 2.8 ” ) ” ”
Helium stars 6.5 ” ( 4.0 ” ) ” ”
Hydrogen stars 11 ” ( 6.8 ” ) ” ”
Yellow stars 15 ” ( 9.3 ” ) ” ”
Red stars 17 ” (11.5 ” ) ” ”
Planetary nebulæ 25 ” (15.5 ” ) ” ”
To these figures a few remarks founded on recent observations might be
made. The average distance between us and the stars in each group is
different and the yellow stars, to which indeed our Sun belongs, are
those nearest to us in space. They are therefore easier to observe than
stars in the other groups. Campbell’s statistics include also for this
reason a smaller number of stars in this class than in the others. It
is conceivable and by the astronomer Halm held to be true that the mean
velocity of the smaller stars is greater than that of the larger ones.
This is the condition existing in a mixture of different gas molecules,
with which the brilliant French scientist, Henri Poincaré, compared
the throng of stars, inasmuch as the heavier molecules possess the
slower motion. To confirm this W. S. Adams of the Carnegie Observatory
on Mount Wilson compared stars of equal velocity in their own orbits.
Such stars are considered to be on the average equally far removed
from us. He found the theory of Halm confirmed. The mean velocity of
the hydrogen stars was reduced from 11 km. (6.8 miles) to 7.5 km. (4.7
miles), that of the yellow stars from 15 km. (9.3 miles) to 9.2 km.
(5.8 miles), and that of the red stars from 17 km. (11.5 miles) to 14
km. (8.7 miles), while that of the helium stars remained unchanged. The
sequence of the stars arranged according to velocity in the line of
sight is evidently not modified by this new calculation.
In regard to the motion of the planetary nebulæ it should be mentioned
that Campbell in this connection has carried out a great number of new
determinations according to which the mean observed velocity of these
large bodies in the line of sight must be increased to not less than 42
km. (26 miles) per second.
Campbell and Moore contribute the following interesting data in regard
to Nebula N. G. C. 7009 (Fig. 2):
“Measures of the rotational velocity of the nebula enable us to draw
some interesting conclusions concerning its mass. On the most plausible
assumption as to the location of the axis of rotation the orbital speed
of the nebular materials lying at a distance of 9 seconds of arc from
the centre is about 6 km. (3.7 miles) per second. If we provisionally
assume the mass of the central nucleus to equal that of the Sun,
Kepler’s law connecting the periodic time with the distance from the
nucleus tells us definitely that the nebula is distant from us only 8.9
light years. This must be regarded as an improbably small value, in
view of other evidence bearing on the question. For assumed distances
of 100 and 1000 light years, which we have reason to believe are more
probable orders of nebular distance, the masses of the nebula would be
respectively 11.3 and 113 times that of the Sun, and the corresponding
periods of rotation 1371 and 13,710 years. From these considerations
it seems certain that the mass of the planetary nebula N. G. C. 7009
is several times that of the Sun. The nebula is therefore competent,
from the point of view of its mass, to develop into a system more
pretentious than is our solar system.
“A few speculations concerning this nebula may not be without interest
and value.
“The faint extensions to the east and to the west of the elliptical
figure suggest an encircling ring of materials whose principal plane,
passing through the nucleus, passes also near our (the observers’)
position in space. These extensions terminate in condensed nuclei at
equal distances from the nucleus and on exactly opposite sides of the
nucleus. The faint extensions and condensations may be and probably are
largely the effect of the edgewise projection of such a ring, as in the
case of Saturn’s rings when the observer is in the plane of the rings.
The forms of the two terminating condensations, and especially the wing
extending up and out from the east condensation, suggest that we are
not precisely in the plane of the assumed ring.
“The form of the main nebula appears to be ellipsoidal and not chiefly
elliptical.
“The space immediately surrounding the central nucleus appears to be
relatively vacuous. Aside from the nucleus, the principal mass of
visible nebulosity exists in the brilliant ring, roughly elliptical
as to its inner and outer boundaries, which occupies the region about
midway between the nucleus and the outer edge of the nebular structure.
The brilliant ring is probably in reality an ellipsoidal shell; the
projection of such a shell upon a plane at right angles to the line
of sight would naturally show a relatively dark central area, but the
projection principle may not be the only one involved.
“If this nebula is in process of development into a solar system, the
indications are for a system having certain resemblances to our solar
system. Our four outer planets have a combined mass 225 times as great
as that of the four inner planets. Similarly in N. G. C. 7009, there
is apparently a paucity of material to form planets near the nucleus
and an abundance of material for planets at greater distances from the
nucleus.”
[Illustration:
Fig. 2. Planetary nebula N.G.C. 7009 (composite drawing, from
Curtis’s photographs of the nebula made with the Crossley
reflecting telescope. The scale is in seconds of arc).
From _Proceedings of the National Academy of Sciences of U. S. A._]
Interesting observations have been presented recently also with
reference to the largest among the irregular nebulæ, namely the Orion
nebula. Three astronomers in Marseilles, Bourget, Fabry, and Buisson,
found that parts of this nebula, in the neighbourhood of the so-called
trapeze and very close to each other, moved with different velocities
and that this difference might amount to 10 km. (6.2 miles) per
second. The south-easterly part approaches us while the north-easterly
recedes from us. Consequently a violent whirl-motion undoubtedly
takes place in this region. This observation has been verified by
the well-known Chicago astronomer, Frost, who employed a different
method of investigation than his predecessors. He noted differences in
velocity amounting to 11 km. (6.8 miles) per second between points not
over two seconds of arc distant from the trapeze.
If therefore we say that the irregular nebulæ on the average possess
no motion, this statement does not preclude important local deviations
from the rule within the nebulæ, intimating a transformation which
probably leads to the concentration of the nebulous matter toward the
centre of the whirl.
Leaving out, to begin with, the planetary nebulæ, it appears that the
original matter of the stars stands still in space, that their average
velocity increases with increasing age and approaches a mean value
of about 18 km. per second or roughly 1000 times the speed of the
ordinary passenger train. Our Sun, in particular, moves with a velocity
of 20 km. (12.4 miles) per second toward a point in the constellation
Hercules 30 degrees north of the equator.
What force then shall we say it is that causes the motion of the stars?
As far as we know none but gravitation. It appears therefore as if
the gaseous primeval substance of the stars were not governed by this
force. It might prove hazardous, however, to make this assumption
as gases also possess weight and even the most rarefied strata of
the Earth’s atmosphere exert barometric pressure by virtue of their
attraction to the mass of the earth. Rather the immobility of the
nebulæ is due to the frequent collisions between the molecules in any
quantity of gas even if it be attenuated to such a high degree as in
the nebulæ. Thus, the molecules strike a balance, as it were, against
each other so that the different parts of the gas accumulations
shortly are brought to rest relative to each other. The irregular
gas mists around the Milky Way form therefore a continuous whole. A
different condition obtains in regard to condensed stellar bodies such
as the stars. They may in the densest throng move during billions of
years before they collide; but they might on the other hand enter
nebulous masses and thereby suffer gradual retardation. We now refer
to stars moving outside of the vapour clouds. They are therefore
unrestricted and the longer they have obeyed gravitation without
impeding encounters with nebulous matter, in other words the longer
the time elapsed since they emerged from the gas accumulations which
gave them birth, the swifter is their motion. Their (average) velocity
can of course not exceed a certain limit which in our parts of the
universe appears to be about 18 km. (11.2 miles) per second. Campbell’s
measurements show that for the youngest stars (all except the red) the
velocity is greatest in the plane of the Milky Way, a natural enough
condition as the attracting matter here is most abundant.
The planetary nebulæ possess a greater velocity although they, as
consisting of mist vapours, are in the first stage of evolution. Faster
yet do the spiral nebulæ move according to measurements by Wolf of
Heidelberg. This shows that they are of a different nature from the
irregular nebulæ, which form the matrix of the Milky Way. A closer
examination of the few--thirteen in all--planetary nebulæ, determined
by the American astronomer Keeler, convinced me that they approach the
Galaxy from its poles with a moderate speed, and subsequently under
the influence of its attraction curve their orbit, rapidly gain in
velocity, and finally rush into the nearest part of the Milky Way with
a very high speed.
A great number of them are no doubt caught in the mists or star-throngs
of the Milky Way after exposure to numerous collisions and sweeping
away all matter in their course. Such clean-swept traces are very
common in the area of the Milky Way. One of the most beautiful examples
is the so-called Cocoon nebula in the constellation Cygnus (the
Swan). It has left in its wake a dark rift, in whose bottom, however,
exceedingly small and evidently very remote stars are visible according
to the German astronomer Wolf (see _Worlds in the Making_, page 172,
Fig. 55).
The great mean velocity of the planetary nebulæ indicates that they
originally did not belong to the Galactic system, a conclusion also
reached by Bohlin, but for other reasons. They are nevertheless more
abundant in the neighbourhood of the Milky Way than in other parts
of the heavens. This fact, if viewed superficially, might lead to
the belief that they are indigenous to the Galactic system, but is
explained by their concentration in obedience to gravitation toward the
Milky Way.
Quite recently (1917) Van Maanen determined the distance of one of
these highly interesting celestial bodies, tabulated in the New
General Catalogue as No. 7662. Its distance was found to be only about
140 light years. This is about sixteen times the distance of Sirius and
the mean distance of a star of the fifth magnitude. This circumstance
agrees very well with the idea that this nebula is captured by the
Galactic system to which it has approached from very distant parts of
the space outside of the Galactic system.
One of the most remarkable astronomical discoveries in recent years
was made by Kapteyn, who thereby as well as by other achievements has
gained perhaps the highest rank among astronomers of today. He has
shown that the stars rushing forth in the neighbourhood of the Sun
belong to two great groups, one coming from the constellation Orion,
and the other at nearly a right angle (100°) from the constellation
Scorpio. In the former, we find nearly all the helium stars hitherto
studied. We have previously seen that these stars stand almost still
with reference to the Galaxy while the irregular nebulæ possess no
motion at all relative to the same reference point--and the Galaxy is
the natural datum-line for all astronomical measurements--so that the
motion of the first-mentioned star-group toward the Sun is principally
due to the Sun’s own motion. This group, according to Kapteyn, obeys
the law of relative star velocities even better than the combined world
of all stars; thus with reference to the Sun, the helium stars are the
slowest, the yellow stars the fastest, while the hydrogen stars occupy
a middle position, all a self-evident consequence of their own velocity
with reference to the Galaxy which increases from helium stars to
yellow stars.
Kapteyn has shown another regularity in this group which is easily
explained. We have previously mentioned that the yellow stars are most,
the helium stars least removed from their place of birth in the Milky
Way. The result is that the yellow stars appear (on the average) to
come from a point farther from the Galaxy than the apparent origin of
the hydrogen stars and more remote yet than that of the helium stars.
On account of the relatively high velocity of the yellow stars in
different directions, their stream appears to be more divergent than
the stream of hydrogen stars, and helium stars move in almost parallel
paths (nearly directly opposed to the Sun’s true motion with reference
to the Galaxy).
Similar regularities have been found by Kapteyn in the second
star-drift which would lead us to think, as indeed Kapteyn assumes,
that these stars also developed from an original nebulous mist, which
arrived in our neighbourhood from the unknown distant, but is now used
up in the formation of corresponding stars. Here again the yellow stars
should have departed farther from their matrix than the white hydrogen
stars. Helium stars are very rare in this drift, so that no reliable
statistics have as yet been made for them.
It has been one of the most difficult problems of cosmogony to form
a theory to account for the origin of the Galactic system. We may,
almost yearly, witness how new stars blaze into existence only to fade
rapidly and in a few years return to their old insignificance--that is,
they become invisible to the naked eye although through powerful lenses
we may frequently discover an exceedingly faint star in their position.
As a rule, a nebula of the planetary type is formed in the course of a
few months. Somewhat later the nebula is transformed into a Wolf-Rayet
star. It is interesting to note that Wright found the central bodies in
certain planetary nebulæ to be Wolf-Rayet stars. We have good reasons
to assume this blazing forth into light to signify the collision of two
faintly luminous or possibly extinct stars. The new lights appear also
in stellar regions where the star density is very great, particularly
in the Milky Way or its vicinity.
We see therefore repeatedly how mists with enclosed central stars
originate. They remind us to a certain extent of the Galaxy with its
clouds and stars and along the road thus suggested trials have been
made to reach the solution of the riddle. The difficulty lies in the
fact that the orbs whose collision create “new stars” are small,
probably smaller than our Sun, while the mass gathered in the Milky
Way most likely is trillions[2] of times greater than that of the Sun.
It is true that we know a few unique stars, such as Arcturus, which
exceed our Sun in size several tens of thousand times, but not even two
such stars would account for the mass of the Galaxy, and furthermore
the probability that two stellar bodies of such rare dimensions would
collide is so very small that it must be left out of account.
[2] American and French numeration; billions acc. to Swedish
and English usage.
Kapteyn’s star-drifts, containing many thousands or probably millions
of stars, appear to furnish the bridge that leads to the solution
of the riddle of the Milky Way. These drifts were once enormous
gas-clouds, in mass probably several million times greater than that
of the Sun. They also had an extension equal to trillions of stars.
The probability for the meeting of two such gas-drifts is comparatively
large and should not be much smaller than for the entrance of a
star-drift into the Milky Way, an occurrence which actually has
happened as shown by Kapteyn.
When two such enormous gas-clouds meet, each with a cosmic velocity of
about 20 km. per second, a long time would not elapse before the gas
molecules in the region of interpenetration would be retarded in their
original motions. An extraordinarily strong concentration and heating
would occur in this territory, which is surrounded by the comparatively
cold and heavy masses which remain unaffected because outside of the
impact-area. A certain degree of equalization would naturally take
place in the layers adjoining the boundary between active and inactive
parts and the former would, furthermore, be set into a rapid spin
around an axis perpendicular to the plane containing the two original
motions. On account of the great viscosity of gases, particularly
at high temperature, the central part would rotate as a coherent
unit. Thus it would form a disk of gaseous matter. This disk would be
thickest in the middle and would become thinner toward the edge where
centrifugal force acts most powerfully.
[Illustration: Fig. 3. N.G.C. 4594. Exposure = 2 hours. 1mm = 6”7.
From _Proceedings of the National Academy of Sciences of U. S. A._]
Such a discous nebula has been investigated by the astronomer F. G.
Pease of the Carnegie Observatory on Mount Wilson. By means of the
spectroscope he has studied the motion of Nebula No. 4594 in the New
General Catalogue (see Fig. 3). This body is believed to be a spiral
nebula like those in Figs. 4 and 5 but viewed from the side so that
the spirals appear as a band. As the picture shows, this band is
coursed through by a thick dark line, owing, it is believed, to a cold
non-luminous dust-cloud outside of the spiral. The bright band is
broadest in the centre. The curvature of the dark middle line in the
shape of an arc, whose apex points downward, combined with the fact
that the major portion of light falls above this arc, indicates
that we do not view the nebula exactly on edge but from the upper
(north) side of a plane through the arms of the nebula. The nucleus
speeds away from us at the dizzying rate of 1180 km. (730 miles) per
second. The east edge, _i. e._ the left on the picture, departs with
the still higher velocity of 1630 km. (1100 miles) per second, while
the west (right) edge retreats at the rate of only 800 km. (495 miles)
per second. According to Pease, the nebula rotates as a solid disk so
that the difference between the velocity of any point and that of the
centre increases in the same proportion as the distance of the point in
question from the centre. It is probable that we have been prevented
from observing the outer parts, corresponding to the spiral arms
proper, by the ring of dust which encircles the nebula. The visible
portion occupies an arc of 2¼ minutes on each side of the centre. Its
spectrum corresponds to that of star-group F-5 among the yellow stars
in the Harvard classification. Therefore, it is not the light of the
coherent gas-cloud which preponderates but rather that of the stars
consolidated within the cloud, and corresponding to the stars in the
Milky Way. This star-light is so bright that it entirely suppresses the
radiation from the gas-cloud itself.
Such parts of the gas-aggregation as are most removed from the place of
collision continue in their course through space little affected by the
attraction of the central mass because of the great distance involved.
The portions nearer the point of impact receive orbits curved by this
same attraction and the curvature becomes the sharper the nearer the
axis of rotation. One result of the mutual gravitation between the
central mass and the particles in the outer sections of the nebula is
also that the velocity in the spiral arms becomes smaller the farther
the section in question is located from the centre, just as comets in
the solar system move slower the farther they are removed from the Sun.
But in all portions outside the central region the attraction is too
weak to give circular orbits to the gaseous matter. All substance in
these localities, therefore, departs ever more from the centre. As the
spiral arms stretch out into straight lines such matter finally leaves
the central disk altogether. It is possible that only the disk itself
remains in the nebula computed by Pease.
Another astronomer on Mount Wilson, A. Van Maanen, has investigated
a nebula, No. 101 in Messier’s catalogue (Fig. 4). This nebula lies
nearly at a right angle to the line of vision which consequently almost
coincides with the axis of rotation. The motion of the different parts
of this nebula has been calculated with the help of photographs taken
in the years 1899, 1908, and 1914, whereby its changed position with
reference to surrounding fixed stars has been recorded. Out of 87
points in the spirals only 9 moved in the direction of the hands on a
clock while the other 78 moved in the opposite sense. The mean angular
velocity is 0.022 seconds of arc per year, corresponding to 85,000
years for one complete revolution at 5 minutes’ distance from the
centre. The absolute velocity 2 minutes of arc from the centre is 1.5
times as great as at 7.5 minutes’ distance.
Fig. 4 is reproduced from Van Maanen’s original. It shows clearly the
general regularity in the motion of the component parts as well as
the numerous exceptions to the rule. Such exceptions may be caused by
perturbations due to invading masses, which impart their own motion to
the entangling matter. These foreign bodies have probably condensed
surrounding vapours and this created the bright knots which stud the
nebular spirals. The upward motion amounts on the average to 0.007
seconds per year. While the points of condensation describe half a
revolution around the centre they depart therefrom to about twice their
original distance. More than a million years is therefore likely to
elapse before the outer portions of the nebula are so far removed from
the nucleus that the spiral form of the nebula is no more apparent.
[Illustration:
Fig. 4. Internal motions in Messier 101. The arrows indicate the
direction and magnitude of the mean annual motions. Their
scale (0”1) is indicated on the plate. The scale of the
nebula is 1mm = 10”5. The comparison stars are enclosed in
circles.
From _Proceedings of the National Academy of Sciences of U. S. A._]
[Illustration:
Fig. 5. Spiral nebula No. 51 in Messier’s Catalogue; situated in
Canes Venatici, and photographed February 7 and 8, 1910,
from Mount Wilson Observatory in California. Scale: 1
millimeter = 5 sec. arc.
Lyran Herkules
Ormbäraren Örnen Skölden
Svanen Räven
Pilen
Ormbäraren Skytten
Skorpionen
Södra Kronan Kolsäck
Syd-Korset
Skeppet Argo Kölen
Skeppet Argo Styre Segel
N
Enhörningen
Enhörningen
Kusken
Cepheus
Perseus
Kassiopeja
Svanen
]
It is clear that the Milky Way may have been formed through the
collision of two immense nebulous gas-aggregations in the manner just
described. Subsequently and by virtue of the magnitude of the Galaxy
great quantities of wandering cosmic matter and minor stellar bodies
have been gathered in occasionally accompanied by larger clusterings
such as the planetary nebulæ referred to.
How well justified we are in looking upon the Milky Way as a spiral
nebula is apparent from the picture (Fig. 5) reproducing a photograph
of the familiar regular nebula in the Dogs (Canes Venatici). It shows
a wealth of detail hitherto not surmised. The feat was accomplished in
the Carnegie Observatory on Mount Wilson in Southern California with
the help of optical resources vastly superior to all earlier means.
The Milky Way has previously been compared to this nebula, but due to
deficient enlargement their striking similarity has not been fully
recognized until now.
Assume the Sun located in the point marked “S” in Fig. 5 and some
distance above the plane of the picture, then the nebula if viewed
from this point would appear somewhat as the Milky Way appears to
us. In the middle we behold the substantial nucleus and on its left
side a cleft between the two branches of the inner spiral. Farther
to the left, we see only the outer spiral, first broadening toward
the left where it approaches “S,” then narrowing only to spread again
on account of the great clustering in the lower right part of the
spiral. The axis of the nebula corresponds to the densest parts of
the Galaxy in Cygnus (the Swan), the loop in the inner spiral again
to the empty space between Cepheus and Cassiopeia, the narrow part
of the outer spiral branch resembles the constriction at Algenib,
the following diffusion corresponds to the broad section in Auriga
(Charioteer) and Monoceros (Unicorn). At the subsequent narrow place
we see the outer nebula-clump corresponding in certain respects to the
Magellanic clouds on our firmament although these are farther removed,
and apparently not indigenous to the Milky Way. There follows in the
nebula a massive section in our system represented by the well known,
far less compact, yet brightly luminous tract containing the Southern
Cross. Here, from the star Alpha in Centaurus--the nearest bright fixed
star[3] to our Sun, “only” 4.5 light years or about 25 trillion[4]
miles distant--commences a bi-section of the spiral and strangely
enough the nebula is similarly forked. Now the outer spiral stretching
in a faint line upward from the “clump” begins to show as a weak band,
while the inner spiral stands forth powerfully above “S” corresponding
to the brilliant section of the Galaxy in Scutum (Shield) and Aquila
(Eagle). The partition in the nebula between these two branches is
the counterpart of the 110° long “prong” in the Milky Way between the
constellations Norma and Lyra. Numerous faint bridges join the two
branches in the nebula as well as in the Galaxy, according to Wolf.
[3] Up to a short time ago Alpha Centauri was considered the
fixed star nearest to the Sun. By comparing old photographs
of the firmament with such of recent date the renowned
astronomer Barnard found that a very small fixed star--of
the magnitude 10.5 and therefore far from visible to the
naked eye--in the constellation Ophiuchus (Right ascension
17 h. 58 m. 44 s., North declination 4° 27´.4 January 1,
1917) possesses a very large proper motion. It traverses in
a year 10.3 seconds on the firmament. The distance to this
star, which has the largest proper motion so far known,
was later determined to be 3.3 light years or 3/4 of the
distance from Alpha Centauri to our Sun. Hence its velocity
at right angle to the line of vision is computed to be
49 km. (32 miles) per second. Spectroscopic measurements
show that it approaches us with a velocity of 91 km. (56.5
miles) per second along the line of vision. The combined
velocity, therefore, is 103 km. (63 miles) per second, an
unusually high value. The value of 3.3 light years used in
this calculation was determined by the French astronomer
Gonnessiat, who found it by the study of old photographs
from Algeria. He also calculated the parallax of this star
to be 1 second. According to later measurements, given in
the Harvard Bulletins 616 and 617, its parallax is only
0.7 seconds and consequently its distance 4.6 light years
and its speed perpendicularly to the line of vision 70 km.
(43.5 miles) per second. Campbell, in the Lick Observatory,
had determined its radial velocity and found that it
approaches the Sun at the rate of 128 km. (79 miles) per
second. Its total velocity is, according to these two last
determinations, 146 km. (91 miles) per second.
It is by no means improbable that similar discoveries will
be made in the future, so that the Sun will be found to have
more stars in its “immediate” vicinity than previously
assumed.
[4] American and French numeration; the Swedish and English
usage is billion.
The correspondence is indeed surprisingly good. Proportions are, of
course, somewhat different--in particular is the central part of the
Galaxy not so predominating, which fact has been troublesome to the
adherents of the nebula theory. Probably it was originally denser but
has become attenuated through star-formation, explaining, for instance,
the great gap between the constellations Lyra and Vulpecula.
To give a better idea of the structure of the Milky Way, two
photographs are here reproduced as taken by Wolf, the German astronomer
in Heidelberg, who has done particularly meritorious work in this
department. One (Fig. 6) shows a section of the Galaxy in Cygnus
(the Swan) with the star Deneb in the centre and to the left the
“Northamerican-nebula” so named from its shape. Above Deneb is the dark
“hole” in Cygnus and below another chasm not quite so black. Left of
the “hole” is the winding canal enclosing the so-called Cocoon nebula.
(See _Worlds in the Making_, page 172.)
The following picture (Fig. 7) contains, in the upper left, the bright
star Altair in Aquila (the Eagle) located close to the powerful arm of
the Galaxy in this constellation. Farther to the right is the fainter
arm in Ophiuchus (the Serpent-holder). The lower half contains the
most brilliant part of the Milky Way in the constellations Scutum (the
Shield) and Sagittarius (the Archer). Bright stars are infrequent but
the fainter ones are innumerable: “They are crowded into dense clusters
and between them the most delicate star dust is scattered.”--“We behold
how the star-ribbon dissolves into detached tufts which intertwine into
the strangest patterns. These clouds of stars reach their greatest
splendour in the lower part of the map.”
[Illustration:
Fig. 6. The Milky Way between constellations Cassiopeia and Swan
from photo by M. Wolf of Heidelberg. A little to the left
of the middle the beautiful nebula America appears.
]
[Illustration:
Fig. 7. The Milky Way in constellations Eagle (upper half) and
Archer (lower half). In the upper left the bright star
Altair appears. Photo from M. Wolf of Heidelberg.
]
We also reproduce (Fig. 8) from M. Wolf a photograph in larger scale
from the region of Gamma (lower part of Fig. 7) in the Eagle
with its “trident-hole,” so called from its peculiar shape, and in
whose vicinity mists and star-clouds abound. This picture is a more
complicated counterpart of the flatter photograph by Wolf of the
“Cocoon nebula.” It appears as if three or four stellar bodies here
had stepped in from without, swept away the stars in their way, and
left clean “streets” behind. Probably other “empty” spots in the
neighbourhood have been formed in a similar manner. Another theory is
that such dark places are caused by opaque mist-formations which shield
the light of the stars behind from our sight.
[Illustration: Fig. 8. The trisected hole (Trifid-hole) in Eagle. Photo
from M. Wolf of Heidelberg.]
Through these pictures we gain a conception of the manner in which the
present stars in the Milky Way have clustered out of the original misty
chaos. We cannot avoid the idea of great exterior similarity between
the lumps formed in curdling or souring milk and those which we observe
in the Milky Way. The renowned French scientist Duclaux says in his
micro-biology: “In milk, commencing to sour, but yet entirely liquid,
we observe under the microscope a precipitation of tiny particles.
To begin with they are seen with difficulty and are discovered only
by slightly displacing the plane of vision. Later they develop into
distinct grains, characterized by Brownian movements, just like small
particles of clay.... Still later the phenomenon appears as a steady
molecular agglomeration. The grains have the tendency of the clay
particles to lump and precipitate.”
The first condensation-nuclei in the mist-clouds are no doubt cosmic
dust entering from without and perhaps also larger clusterings such as
meteorites and comets. At the existing low temperature, surrounding
gases condense into fluid state on the dust particles which by
virtue of these moist shells are cemented into aggregations of such
size that gravitation overcomes the repelling radiation pressure.
Gravitation assisted by the retarding vapours further mass these
aggregates together. This process of coalescence is accompanied by
heat production. Finally, small stars are formed, then groups of such
stars, while the spaces between, now comparatively devoid of matter,
appear dark much as the whey between flocks of curd. As yet, the small
stellar bodies are surrounded by quantities of dust and gas, which,
however, with continued condensation become ever more rarefied. Even
yet the big stars in Pleiades, belonging to the helium group, appear on
the photographic plates interspaced with great patches of dust-clouds.
These are now, however, so unsubstantial that they offer little
impediment to the procession of the mighty stars. The condensation
process may be greatly accelerated through the invasion of voluminous
gas nebulæ similar to the Cocoon nebula. At last all gases in the new
star condense, that is to say the shell of tenuous vapours and dust
contracts to such an insignificant thickness that it cannot be seen
except possibly from the immediate vicinity. Small bodies ingathered
through friction against the remnants of the original extended
wrapping wander as planets around the new sun, sweeping away the last
traces of unattached matter. The condensation on the new orb leaves a
“hole” in the nebulosity which in this way is transformed into stars
and their satellites which emerge from the mist and scatter on the
firmament.
The Milky Way appears to be in a rather advanced stage of this
evolution.
The “infinitely small” presents occasionally surprising likeness to the
“infinitely large.”
In this manner we can form a conception of the growth of the wonderful
structure which has brought forth the majority of the stellar bodies
that we discern. The spiral nebulæ visible at the Galactic poles are
similar formations but probably of far more modest dimensions. They may
compare to the Milky Way as the smaller planets to the Sun. According
to recent investigations the spiral nebulæ seem also to possess an
enormous velocity and they have probably invaded the Milky Way from
without.
As previously stated, an exceedingly remarkable conception of the
Milky Way exists among the Mexicans. To them it is the Matrix of all
and gave birth to the stars, the most important of which are the Sun,
the Moon, and Venus. This idea evidently agrees very well with the
results of investigations in the last few years.
Finally, a few words about the extent of the Milky Way. So far we have
not been able to measure it; only rather uncertain approximations are
possible. Wolf estimates the diameter, that is the distance between
the two spirals at the point where the Sun now is, to be about 10,000
times the distance from the Sun to the nearest fixed star, Alpha in
Centaurus, which distance on the other hand is about 10,000 times the
distance from the Sun to the remotest known planet, Neptune, or 300,000
times that from Sun to Earth. Expressed in the usual units, we arrive
at 40,000 light years or 400,000 trillion kilometres (240,000 trillion
miles). Lord Kelvin makes another estimate of 6000 light years, that is
seven times smaller. The mean diameter of the nebula proper might be
about five times larger, in round figures one hundred thousand light
years, or one billion billion kilometres (600 million billion miles).
Like a monstrous octopus, the Milky Way swims in the fathomless
ether-sea. Its dimensions are about as many times larger than those
of the earth as that globe is larger than an atom. This has caused
the gifted Irish physicist Fournier D’Albe to consider the celestial
globes as atoms, out of which systems of the order of the Milky Way
are constructed in the same manner as the earth and other stellar
bodies are upbuilt with atoms, invisible to us and yet measured with an
incredible degree of accuracy.
Fournier D’Albe goes further still. In poetic flight he does not
hesitate to endow the Milky Way organism with life. Its evolution
cannot be denied a certain similarity to the processes of life. The
great nebula owes its origin to the union of two individuals, two
nebulosities, who met in their course through boundless space. There
floated the newborn extending its tentacles in the cool ether-waves
and gained substance and strength through the smaller beings which
the surging billows brought within its reach. It has now attained
the zenith of its evolution, and is breaking up into molecules, or
solar systems, which again are composed of stellar bodies, or atoms
within the molecule. In violent exuberance of youth these rush through
space in fulfilment of their individual life. Many will in due time
undoubtedly become dust again and then serve to nourish a new youthful
nebula. Others succumb to a freezing-death but will be restored to life
in collision with a nebulosity or some other stellar body and give
form to “new stars” or planetary clouds. Again and again shall the
starry mists go through the cycle of existence and after a lifetime,
whose duration stands in proportion to their dimensions, _i. e._ may be
estimated to billions of billions of years, give rise to new celestial
beings. Thus shall it for ever continue in an eternal rhythm.