Nebulæ and Clusters
In his interesting and valuable work on "The Stars," the late Prof.
Newcomb said--
"Great numbers of the nebulæ are therefore thousands of times the
dimensions of the earth's orbit, and most of them are thousands of
times the dimensions of the whole solar system. That they should be
completely transparent through such enormous dimensions shows their
extreme tenuity. Were our solar system placed in the midst of one of
them it is probable that we should not be able to find any evidence of
its existence"!
Prof. Perrine thinks that the total number of the nebulæ will ultimately
be found to exceed a million.[352]
Dr. Max Wolf has discovered a number of small nebulæ in the regions near
Algol and Nova Persei (the great "new star" of 1901). He says, "They
mostly lie in two bands," and are especially numerous where the two bands
meet, a region of 12 minutes of arc square containing no less than 148 of
them. They are usually "round with central condensation," and form of
Andromeda nebula.[353]
Some small nebulæ have been found in the vicinity of the globular
clusters. They are described by Prof. Perrine as very small and like an
"out of focus" image of a small star. "They appear to be most numerous
about clusters which are farthest from the galaxy." Prof. Perrine says,
"Practically all the small nebulæ about the globular clusters are
elliptical or circular. Those large enough to show structure are spirals.
Doubtless the majority of these are spirals."[354] This seems further
evidence in favour of the "spiral nebular hypothesis" of Chamberlin and
Moulton.
A great photographic nebula in Orion was discovered by Prof. Barnard in
1894. In a drawing he gives of the nebula,[355] it forms a long streak
beginning a little south of [Greek: g] Orionis (Bellatrix), passing
through the star 38 Orionis north of 51 and south of 56 and 60 Orionis.
Then turning south it sweeps round a little north of [Greek: k] Orionis;
then over 29 Orionis, and ends a little to the west of [Greek: ê] Orionis.
There is an outside patch west of Rigel. Barnard thinks that the whole
forms a vast spiral structure; probably connected with the "great nebula"
in the "sword of Orion," which it surrounds.
From calculations of the brightness of surface ("intrinsic brightness") of
several "planetary" nebulæ made by the present writer in the year 1905, he
finds that the luminosity is very small compared with that of the moon.
The brightest of those examined (_h_ 3365, in the southern hemisphere,
near the Southern Cross) has a surface luminosity of only 1/400 of that of
the moon.[356] The great nebulæ in Orion and Andromeda seem to have "still
smaller intrinsic brightness."
Arago says--
"The spaces which precede or which follow simple nebulæ, and _a
fortiori_ groups of nebulæ, contain generally few stars. Herschel
found this rule to be invariable. Thus every time that, during a short
interval, no star appeared, in virtue of the diurnal motion, to place
itself in the field of his motionless telescope, he was accustomed to
say to the secretary who assisted him (Miss Caroline Herschel),
'Prepare to write; nebulæ are about to arrive.'"[357]
Commenting on this remark of Arago, the late Herbert Spencer says--
"How does this fact consist with the hypothesis that nebulæ are remote
galaxies? If there were but one nebula, it would be a curious
coincidence were this one nebula so placed in the distant regions of
space as to agree in direction with a starless spot in our sidereal
system! If there were but two nebulæ, and both were so placed, the
coincidence would be excessively strange. What shall we say on
finding that they are habitually so placed? (the last five words
replace some that are possibly a little too strong).... When to the
fact that the general mass of nebulæ are antithetical in position to
the general mass of the stars, we add the fact that local regions of
nebulæ are regions where stars are scarce, and the further fact that
single nebulæ are habitually found in comparatively starless spots,
does not the proof of a physical connection become overwhelming?"[358]
With reference to the small elongated nebula discovered by Miss Caroline
Herschel in 1783 near the great nebula in Andromeda, Admiral Smyth says,
"It lies between two sets of stars, consisting of four each, and each
disposed like the figure 7, the preceding group being the smallest."[359]
Speaking of the "nebula" Messier 3--a globular cluster in Canes
Venatici--Admiral Smyth says, "This mass is one of those balls of compact
and wedged stars whose laws of aggregation it is so impossible to assign;
but the rotundity of the figure gives full indication of some general
attractive bond of union."[360] The terms "compact and wedged" are,
however, too strong, for we know that in the globular clusters the
component stars must be separated from each other by millions of miles!
Prof. Chamberlin suggests that the secondary nebula (as it is called) in
the great spiral in Canes Venatici (Messier 51) may possibly represent
the body which collided with the other (the chief nucleus) in a grazing
collision, and is now escaping. He considers this secondary body to have
been "a dead sun"--that is, a dark body.[361] This would be very
interesting if it could be proved. But it seems to me more probable that
the secondary nucleus is simply a larger portion of the ejected matter,
which is now being gradually detached from the parent mass.
Scheiner says "the previous suspicion that the spiral nebulæ are star
clusters is now raised to a certainty," and that the spectrum of the
Andromeda nebula is very similar to that of the sun. He says there is "a
surprising agreement of the two, even in respect to the relative intensity
of the separate spectral regions."[362]
In the dynamical theory of spiral nebulæ, Dr. E. J. Wilczynski thinks that
the age of a spiral nebula may be indicated by the number of its coils;
those having the largest number of coils being the oldest, from the point
of view of evolution.[363] This seems to be very probable.
In the spectrum of the gaseous nebulæ, the F line of hydrogen (H[Greek:
b]) is visible, but not the C line (H[Greek: a]). The invisibility of the
C line is explained by Scheiner as due to a physiological cause, "the eye
being less sensitive to that part of the spectrum in which the line
appears than to the part containing the F line."[364]
An apparent paradox is found in the case of the gaseous nebulæ. The
undefined outlines of these objects render any attempt at measuring their
parallax very difficult, if not impossible. Their distance from the earth
is therefore unknown, and perhaps likely to remain so for many years to
come. It is possible that they may not be farther from us than some of the
stars visible in their vicinity. On the other hand, they may lie far
beyond them in space. But whatever their distance from the earth may be,
it may be easily shown that their attraction on the sun is directly
proportioned to their distance--that is, the greater their distance, the
greater the attraction! This is evidently a paradox, and rather a
startling one too. But it is nevertheless mathematically true, and can be
easily proved. For, _their distance being unknown_, they may be of any
dimensions. They might be comparatively small bodies relatively near the
earth, or they may be immense masses at a vast distance from us. The
latter is, of course, the more probable. In either case the _apparent_
size would be the same. Take the case of any round gaseous nebula.
Assuming it to be of a globular form, its _real_ diameter will depend on
its distance from the earth--the greater the distance, the greater the
diameter. Now, as the volumes of spheres vary as the cubes of their
diameters, it follows that the volume of the nebula will vary as the cube
of its distance from the earth. As the mass of an attracting body depends
on its volume and density, its real mass will depend on the cube of its
distance, the density (although unknown) being a fixed quantity. If at a
certain distance its mass is _m_, at double the distance (the _apparent_
diameter being the same) it would have a mass of eight times _m_ (8 being
the cube of 2), and at treble the distance its mass would be 27 _m_, and
so on, its _apparent_ size being known, but not its _real_ size. This is
obvious. Now, the attractive power of a body varies directly as its
mass--the greater the mass, the greater the attraction. Again, the
attraction varies _inversely_ as the square of the distance, according to
the well-known law of Newton. Hence if _d_ be the unknown distance of the
nebula, we have its attractive power varying as _d_{3} divided by _d_{2},
or directly as the distance _d_. We have then the curious paradox that for
a nebula whose distance from the earth is unknown, its attractive power on
the sun (or earth) will vary directly as the distance--the greater the
distance the greater the attraction, and, of course, conversely, the
smaller the distance the less the attractive power. This result seems at
first sight absurd and incredible, but a little consideration will show
that it is quite correct. Consider a small wisp of cloud in our
atmosphere. Its mass is almost infinitesimal and its attractive power on
the earth practically _nil_. But a gaseous nebula having the same
_apparent size_ would have an enormous volume, and, although probably
formed of very tenuous gas, its mass would be very great, and its
attractive power considerable. The large apparent size of the Orion nebula
shows that its volume is probably enormous, and as its attraction on the
sun is not appreciable, its density must be excessively small, less than
the density of the air remaining in the receiver of the best air-pump
after the air has been exhausted. How such a tenuous gas can shine as it
does forms another paradox. Its light is possibly due to some
phosphorescent or electrical action.
The apparent size of "the great nebula in Andromeda" shows that it must be
an object of vast dimensions. The nearest star to the earth, Alpha
Centauri, although probably equal to our sun in volume, certainly does not
exceed one-hundredth of a second in diameter as seen from the earth. But
in the case of the Andromeda nebula we have an object of considerable
apparent size, not measured by seconds of arc, but showing an area about
three times greater than that of the full moon. The nebula certainly lies
in the region of the stars--much farther off than Alpha Centauri--and its
great apparent size shows that it must be of stupendous dimensions. A
moment's consideration will show that whatever its distance may be, the
farther it is from the earth the larger it must be in actual size. The sun
is vastly larger than the moon, but its apparent size is about the same
owing to its greater distance. Sir William Herschel thought the Andromeda
nebula to be "undoubtedly the nearest of all the great nebulæ," and he
estimated its distance at 2000 times the distance of Sirius. This would
not, however, indicate a relatively near object, as it would imply a
"light journey" of over 17,000 years! (The distance of Sirius is about 88
"light years.")
It has been generally supposed that this great nebula lies at a vast
distance from the earth, possibly far beyond most of the stars seen in the
same region of the sky; but perhaps not quite so far as Herschel's
estimate would imply. Recently, however, Prof. Bohlin of Stockholm has
found from three series of measures made in recent years a parallax of
0"·17.[365]
This indicates a distance of 1,213,330 times the sun's distance from the
earth, and a "light journey" of about 19 years. This would make the
distance of the nebula more than twice the distance of Sirius, about four
times the distance of [Greek: a] Centauri, but less than that of Capella.
Prof. Bohlin's result is rather unexpected, and will require confirmation
before it can be accepted. But it will be interesting to inquire what this
parallax implies as to the real dimensions and probable mass of this vast
nebula. The extreme length of the nebula may be taken to represent its
diameter considered as circular. For, although a circle seen obliquely is
always foreshortened into an ellipse, still the longer axis of the ellipse
will always represent the real diameter of the circle. This may be seen by
holding a penny at various angles to the eye. Now, Dr. Roberts found that
the apparent length of the Andromeda nebula is 2-1/3 degrees, or 8400
seconds of arc. The diameter in seconds divided by the parallax will give
the real diameter of the nebula in terms of the sun's distance from the
earth taken as unity. Now, 8400 divided by 0"·17 gives nearly 50,000, that
is, the real diameter of the Andromeda nebula would be--on Bohlin's
parallax--nearly 50,000 times the sun's distance from the earth. As light
takes about 500 seconds to come from the sun to the earth, the above
figures imply that light would take about 290 days, or over 9 months to
cross the diameter of this vast nebula.
Elementary geometrical considerations will show that if the Andromeda
nebula lies at a greater distance from the earth than that indicated by
Bohlin's parallax, its real diameter, and therefore its volume and mass,
will be greater. If, therefore, we assume the parallax found by Bohlin,
we shall probably find a _minimum_ value for the size and mass of this
marvellous object.
Among Dr. Roberts' photographs of spiral nebulæ (and the Andromeda nebula
is undoubtedly a spiral) there are some which are apparently seen nearly
edgeways, and show that these nebulæ are very thin in proportion to their
diameter. From a consideration of these photographs we may, I think,
assume a thickness of about one-hundredth of the diameter. This would give
a thickness for the Andromeda nebulæ of about 500 times the sun's distance
from the earth. This great thickness will give some idea of the vast
proportions of the object we are dealing with. The size of the whole solar
system--large as it is--is small in comparison. The diameter and thickness
found above can easily be converted into miles, and from these dimensions
the actual volume of the nebula can be compared with that of the sun. It
is merely a question of simple mensuration, and no problem of "high
mathematics" is involved. Making the necessary calculations, I find that
the volume of the Andromeda nebula would be about 2·32 trillion times
(2·32 × 10{18}) the sun's volume! Now, assuming that the nebulous matter
fills only one-half of the apparent volume of the nebula (allowing for
spaces between the spiral branches), we have the volume = 1·16 × 10{18}.
If the nebula had the same density as the sun, this would be its mass in
terms of the sun's mass taken as unity, a mass probably exceeding the
combined mass of all the _stars_ visible in the largest telescopes! But
this assumption is, of course, inadmissible, as the sun is evidently quite
opaque, whereas the nebula is, partially at least, more or less
transparent. Let us suppose that the nebula has a _mean_ density equal to
that of atmospheric air. As water is about 773 times heavier than air, and
the sun's density is 1·4 (water = 1) we have the mass of the nebula equal
to 1·16 × 10{18} divided by 773 × 1·4, or about 10{15} times the sun's
mass, which is still much greater than the probable combined mass of all
the _visible_ stars. As it seems unreasonable to suppose that the mass of
an individual member of our sidereal system should exceed the combined
mass of the remainder of the system, we seem compelled to further reduce
the density of the Andromeda nebula. Let us assume a mean density of, say,
a millionth of hydrogen gas (a sufficiently low estimate) which is about
14·44 times lighter than air, and we obtain a mass of about 8 × 10{7} or
80 million times the mass of the sun, which is still an enormous mass.
As possibly I may have assumed too great a thickness for the nebula, let
us take a thickness of one-tenth of that used above, or one thousandth of
the length of the nebula. This gives a mass of 8 million times the sun's
mass. This seems a more probable mass if the nebula is--as Bohlin's
parallax implies--a member of our sidereal system.
If we assume a parallax of say 0"·01--or one-hundredth of a second of
arc--which would still keep the nebula within the bounds of our sidereal
system--we have the dimensions of the nebula increased 17 times, and hence
its mass nearly 5000 times greater (17{3}) than that found above. The mass
would then be 40,000 million times the sun's mass! This result seems
highly improbable, for even this small parallax would imply a light
journey of only 326 years, whereas the distance of the Milky Way has been
estimated by Prof. Newcomb at about 3000 years' journey for light.
In Dr. Roberts' photograph many small stars are seen scattered over the
surface of the nebula; but these do not seem to be quite so numerous as in
the surrounding sky. If the nebula lies nearer to us than the fainter
stars visible on the photograph, some of them may be obscured by the
denser portions of the nebula; some may be visible through the openings
between the spiral branches; while others may be nearer to us and simply
projected on the nebula.
To add to the difficulty of solving this celestial problem, the
spectroscope shows that the Andromeda nebula is not gaseous. The spectrum
is, according to Scheiner, very similar to that of the sun, and "there is
a surprising agreement of the two, even in respect to the relative
intensities of the separate spectral regions."[366] He thinks that "the
greater part of the stars comprising the nucleus of the nebula belong to
the second spectral class" (solar), and that the nebula "is now in an
advanced stage of development. No trace of bright nebular lines are
present, so that the interstellar space in the Andromeda nebula, just as
in our stellar system, is not appreciably occupied by gaseous
matter."[366] He suggests that the inner part of the nebula [the
"nucleus"] "corresponds to the complex of those stars which do not belong
to the Milky Way, while the latter corresponds to the spirals of the
Andromeda nebula."[366] On this view of the matter we may suppose that the
component particles are small bodies widely separated, and in this way the
_mean_ density of the Andromeda nebula may be very small indeed. They
cannot be large bodies, as the largest telescopes have failed to resolve
the nebula into stars, and photographs show no sign of resolution.
It has often been suggested, and sometimes definitely stated, that the
Andromeda nebula may possibly be an "external" universe, that is an
universe entirely outside our sidereal system, and comparable with it in
size. Let us examine the probability of such hypothesis. Assuming that the
nebula has the same diameter as the Milky Way, or about 6000 "light
years," as estimated by Prof. Newcomb, I find that its distance from the
earth would be about 150,000 "light years." As this is about 8000 times
the distance indicated by Bohlin's parallax, its dimensions would be 8000
times as great, and hence its volume and mass would be 8000 cubed, or
512,000,000,000 times greater than that found above. That is, about 4
trillion (4 × 10{18}) times the sun's mass! As this appears an incredibly
large mass to be compressed into a volume even so large as that of our
sidereal system, we seem compelled to reject the hypothesis that the
nebula represents an external universe. The sun placed at the distance
corresponding to 150,000 light years would, I find, shine as a star of
less than the 23rd magnitude, a magnitude which would be invisible in the
largest telescope that man could ever construct. But the combined light of
4 trillion of stars of even the 23rd magnitude would be equal to one of
minus 23·5 magnitude, that is, 23-1/2 magnitude brighter than the zero
magnitude, or not very much inferior to the sun in brightness. As the
Andromeda nebula shines only as a star of about the 5th magnitude the
hypothesis of an external universe seems to be untenable.
It is evident, however, that the mass of the Andromeda nebula must be
enormous; and if it belongs to our sidereal system, and if the other great
nebulæ have similar masses, it seems quite possible that the mass of the
_visible_ universe may much exceed that of the _visible_ stars, and may be
equal to 1000 million times the sun's mass--as supposed by the late Lord
Kelvin--or even much more.
With reference to the small star which suddenly blazed out near the
nucleus of the Andromeda nebula in August, 1885, Prof. Seeliger has
investigated the decrease in the light of the star on the hypothesis that
it was a cooling body which had suddenly been raised to an intense heat by
the shock of a collision, and finds a fair agreement between theory and
observation. Prof. Auwers points out the similarity between this outburst
and that of the "temporary star" of 1860, which appeared in the cluster 80
Messier, and he thinks it very probable that both phenomena were due to
physical changes in the nebulæ in which they appeared.
The appearance of this temporary star in the Andromeda nebula seems to
afford further evidence against the hypothesis of the nebula being an
external universe. For, as I have shown above, our sun, if placed at a
distance of 150,000 light years, would shine only as a star of the 23rd
magnitude, or over 15 magnitudes fainter than the temporary star. This
would imply that the star shone with a brightness of over a million times
that of the sun, and would therefore indicate a body of enormous size. But
the rapid fading of its light would, on the contrary, imply a body of
comparatively small dimensions. We must, therefore, conclude that the
nebula, whatever it may be, is not an external universe, but forms a
member of our own sidereal system.
In Sir John Herschel's catalogue of Nebulæ and Clusters of Stars,
published in 1833, in the _Philosophical Transactions_ of the Royal
Society, there are many curious objects mentioned. Of these I have
selected the following:--
No. 496 is described as "a superb cluster which fills the whole field;
stars 9, 10 ... 13 magnitude and none below, but the whole ground of the
sky on which it stands is singularly dotted over with infinitely minute
points." This is No. 22 of Sir William Herschel's 6th class, and will be
found about 3 degrees south and a little east of the triple star 29
Monocerotis.
No. 650. This object lies about 3 degrees north of the star [Greek: m]
Leonis, the most northern of the bright stars in the well-known "Sickle,"
and is thus described by Sir John Herschel: "A star 12th magnitude with an
extremely faint nebulous atmosphere about 10" to 12". It is between a star
8-9 magnitude north preceding, and one 10th magnitude south following,
neither of which are so affected. A curious object."
No. 1558. Messier 53. A little north-east of the star [Greek: a] Comæ
Berenices. Described as "a most beautiful highly compressed cluster.
Stars very small, 12th ... 20th magnitude, with scattered stars to a
considerable distance; irregularly round, but not globular. Comes up to a
blaze in the centre; indicating a round mass of pretty equable density.
Extremely compressed. A most beautiful object. A mass of close-wedged
stars 5' in diameter; a few 12th magnitude, the rest of the smallest size
and innumerable." Webb says, "Not very bright with 3-7/10 inches;
beautiful with 9 inches." This should be a magnificent object with a very
large telescope, like the Lick or Yerkes.
No. 2018. "A more than usually condensed portion of the enormous cluster
of the Milky Way. The field has 200 or 300 stars in it at once." This lies
about 2° south-west of the star 6 Aquilæ, which is near the northern edge
of the bright spot of Milky Way light in "Sobieski's Shield"--one of the
brightest spots in the sky.
No. 2093. "A most wonderful phenomenon. A very large space 20' or 30'
broad in Polar Distance, and 1{m} or 2{m} in Right Ascension, full of
nebula and stars mixed. The nebula is decidedly attached to the stars, and
is as decidedly not stellar. It forms irregular lace-work marked out by
stars, but some parts are decidedly nebulous, wherein no star can be
seen." Sir John Herschel gives a figure of this curious spot, which he
says represents its "general character, but not the minute details of
this object, which would be extremely difficult to give with any degree of
fidelity." It lies about 3 degrees west of the bright star [Greek: z]
Cygni.
Among the numerous curious objects observed by Sir John Herschel during
his visit to the Cape of Good Hope, the following may be mentioned:--
_h_ 2534 (H iv. 77). Near [Greek: t]{4} Eridani. Sir John Herschel says,
"Attached cometically to a 9th magnitude star which forms its head. It is
an exact resemblance to Halley's comet as seen in a night glass."... "A
complete telescopic comet; a perfect miniature of Halley's comet, only the
tail is rather broader in proportion."[367]
_h_ 3075. Between [Greek: g] Monocerotis and [Greek: g] Canis Majoris. "A
very singular nebula, and much like the profile of a bust (head, neck, and
shoulders) or a silhouette portrait, very large, pretty well defined,
light nearly uniform, about 12' diameter. In a crowded field of Milky Way
stars, many of which are projected on it."[368]
_h_ 3315 (Dunlop 323). In the Milky Way; about 3° east of the Eta Argûs
nebula. Sir John Herschel says, "A glorious cluster of immense magnitude,
being at least 2 fields in extent every way. The stars are 8, 9, 10, and
11th magnitudes, but chiefly 10th magnitude, of which there must be at
least 200. It is the most brilliant object of the kind I have ever seen"
... "has several elegant double stars, and many orange-coloured
stars."[369] This should form a fine object in even a comparatively small
telescope, and may be recommended to observers in the southern hemisphere.
A telescope of 3-inches aperture should show it well.
Among astronomical curiosities may be counted "clusters within clusters."
A cluster in Gemini (N.G.C. 2331) has a small group of "six or seven stars
close together and well isolated from the rest."
Lord Rosse describes No. 4511 of Sir John Herschel's General Catalogue of
Nebulæ and Clusters (_Phil. Trans._, 1864) as "a most gorgeous cluster,
stars 12-15 magnitude, full of holes."[370] His sketch of this cluster
shows 3 rings of stars in a line, each ring touching the next on the
outside. Sir John Herschel described it as "Cluster; very large; very
rich; stars 11-15 magnitude (Harding, 1827)," but says nothing about the
rings. This cluster lies about 5 degrees south of [Greek: d] Cygni.
Dr. See, observing with the large telescope of the Lowell Observatory,
found that when the sky is clear, the moon absent, and the seeing perfect,
"the sky appeared in patches to be of a brownish colour," and suggests
that this colour owes its existence to immense cosmical clouds, which are
shining by excessively feeble light! Dr. See found that these brown
patches seem to cluster in certain regions of the Milky Way.[371]
From a comparison of Trouvelot's drawing of the small elongated nebula
near the great nebula in Andromeda with recent photographs, Mr. Easton
infers that this small nebula has probably rotated through an angle of
about 15° in 25 years. An examination I have made of photographs taken in
different years seems to me to confirm this suspicion, which, if true, is
evidently a most interesting phenomenon.
Dr. Max Wolf of Heidelberg finds, by spectrum photography, that the
well-known "ring nebula" in Lyra consists of four rings composed of four
different gases. Calling the inner ring A, the next B, the next C, and the
outer D, he finds that A is the smallest ring, and is composed of an
unknown gas; the next largest, B, is composed of hydrogen gas; the next,
C, consists of helium gas; and the outer and largest ring, D, is
composed--like A--of an unknown gas. As the molecular weight of hydrogen
is 2·016, and that of helium is 3·96, Prof. Bohuslav Brauner suggests that
the molecular weight of the gas composing the inner ring A is smaller than
that of hydrogen, and the molecular weight of the gas forming the outer
ring D is greater than that of helium. He also suggests that the gas of
ring A may possibly be identical with the "coronium" of the solar corona,
for which Mendelief found a hypothetical atomic and molecular weight of
0·4.[372]
With reference to the nebular hypothesis of Laplace, Dr. A. R. Wallace
argues that "if there exists a sun in a state of expansion in which our
sun was when it extended to the orbit of Neptune, it would, even with a
parallax of 1/60th of a second, show a disc of half a second, which could
be seen with the Lick telescope." My reply to this objection is, that with
such an expansion there would probably be very little "intrinsic
brightness," and if luminous enough to be visible the spectrum would be
that of a gaseous nebula, and no known _star_ gives such a spectrum. But
some planetary nebulæ look like small stars, and with high powers on large
telescopes would probably show a disc. On these considerations, Dr.
Wallace's objection does not seem to be valid.
It is usually stated in popular works on astronomy that the spectra of
gaseous nebulæ show only three or four bright lines on a faint continuous
background. But this is quite incorrect. No less than forty bright lines
have been seen and measured in the spectra of gaseous nebulæ.[373] This
includes 2 lines of "nebulium," 11 of hydrogen, 5 of helium, 1 of oxygen
(?), 3 of nitrogen (?), 1 of silicon (?), and 17 of an unknown substance.
In the great nebulæ in Orion 30 bright lines have been photographed.[374]
D'Arrest found that "gaseous nebulæ are rarely met with outside the Milky
Way, and never at a considerable distance from it."[375]
Mr. A. E. Fath thinks that "no spiral nebula investigated has a truly
continuous spectrum." He finds that so feeble is the intensity of the
light of the spiral nebulæ that, while a spectrogram of Arcturus can be
secured with the Mills spectrograph "in less than two minutes," "an
exposure of about 500 hours would be required for the great nebula in
Andromeda, which is of the same spectral type."[376] Mr. Fath thinks that
in the case of the Andromeda nebula, the "star cluster" theory "seems to
be the only one that can at all adequately explain the spectrum
obtained."[377]
Prof. Barnard finds that the great cluster in Hercules (Messier 13) is
"composed of stars of different spectral types." This result was confirmed
by Mr. Fath.[378]
From observations with the great 40-inch telescope of the Yerkes
Observatory (U.S.A.), Prof. Barnard finds that the nucleus of the
planetary nebula H. iv. 18 in Andromeda is variable to the extent of at
least 3 magnitudes. At its brightest it is about the 12th magnitude; and
the period seems to be about 28 days. Barnard says, "I think this is the
first case in which the nucleus of a planetary or other nebula has been
shown to be certainly variable." "The normal condition seems to be
faint--the nucleus remaining bright for a few days only. In an ordinary
telescope it looks like a small round disc of a bluish green colour." He
estimated the brightness of the nebula as that of a star of 8·2
magnitude.[379] Even in a telescope of 4 inches aperture, this would be a
fairly bright object. It lies about 3-1/2 degrees south-west of the star
[Greek: i] Andromedæ.
The so-called "globular clusters" usually include stars of different
brightness; comparatively bright telescopic stars of the 10th to 13th
magnitude with faint stars of the 15th to 17th magnitude. Prof. Perrine of
the Lick Observatory finds that (_a_) "the division of the stars in
globular clusters into groups, differing widely in brightness, is
characteristic of these objects"; (_b_) "the globular clusters are devoid
of true nebulosity"; and (_c_) "stars fainter than 15th magnitude
predominate in the Milky Way and globular clusters, but elsewhere are
relatively scarce." He found that "exposures of one hour or thereabouts
showed as many stars as exposures four to six times as long; the only
effect of the longer exposures being in the matter of density." This last
result confirms the late Dr. Roberts' conclusions. Perrine finds that for
clusters in the Milky Way, the faint stars (15th to 17th magnitude) "are
about as numerous in proportion to the bright stars (10th to 13th
magnitude) as in the globular clusters themselves." This is, however, not
the case with globular clusters at a distance from the Milky Way. In these
latter clusters he found that "in the regions outside the limits of the
cluster there are usually very few faint stars, hardly more than
one-fourth or one-tenth as many as there are bright stars"; and he thinks
that "this paucity of faint stars" in the vicinity of these clusters
"gives rise to the suspicion that all regions at a distance from the
Galaxy may be almost devoid of these very faint stars." The late Prof.
Keeler's series of nebular photographs "in or near the Milky Way" tend to
confirm the above conclusions. Perrine finds the northernmost region of
the Milky Way "to be almost, if not entirely, devoid of globular
clusters."[380]
According to Sir John Herschel, "the sublimity of the spectacle afforded"
by Lord Rosse's great telescope of 6 feet in diameter of some of the
"larger globular and other clusters" "is declared by all who have
witnessed it, to be such that no words can express."[381]
In his address to the British Association at Leicester in 1907, Sir David
Gill said--
"Evidence upon evidence has accumulated to show that nebulæ consist of
the matter out of which stars have been and are being evolved.... The
fact of such an evolution with the evidence before us, can hardly be
doubted. I most fully believe that, when the modifications of
terrestrial spectra under sufficiently varied conditions of
temperature, pressure, and environment, have been further studied,
this connection will be greatly strengthened."