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W. L. (William Larkin) Webb.

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really exist at these great distances. We may unhesitatingly
affirm that they do, because of the well-known whiteness of the
small stars of the Milky Way. It is true that Pickering has in-
vestigated the distribution of the helium stars in the Harvard
Annals, Vol. 56, No. II., and Campbell quotes these data in Lick
Observatory Bulletin No. 195 as showing that the helium stars are
all bright objects. Pickering believed his tabulations to indicate
"that of the bright stars, one out of four belongs to this class (B),
while of the stars of the sixth magnitude there is only one out of
twenty; and that few if any would be found fainter than the
seventh or eighth magnitude." The implication here is that no
helium stars exist at very great distances corrresponding to small
magnitudes; but of course such a view is untenable.

It probably is true that the group of helium stars at a distance
of some 540 light-years from our sun, and thus comparatively near
us, does cease after a certain faintness and distance has been



UNPARALLELED DISCOVERIES OF T. J. J. SEE 203

reached; but is equally certain that other clusters or clouds of
helium stars recur at greater distances, among the millions of small
white stars constituting the Milky Way. For as Herschel long ago
noticed the Galaxy is everywhere observed to traverse the circuit
of the heavens in a clustering stream; and our view of it from the
region of the sun is not essentially different from the view that
could be obtained from other points in this starry stratum. Add
to this consideration the fact of the well-known whiteness of the
small stars in the Milky Way, and we are authorized to conclude
that an indefinite number of clusters or groups of helium stars will
be found in the Milky Way, and thus such stars will certainly
exist at the greatest depths to which our giant telescopes can
penetrate.

We must therefore be on our guard against the superficial
view, that because the helium stars near the sun fade away as the
sixth magnitude is approached, other groups of stars of this type
do not occur at greater distances. The typical whiteness of the
millions of small stars which make up the Milky Way, and the
clustering character of that magnificent collection of stars, alike
forbid any such inference.

Herschel had the correct view of the constitution of the
Galaxy a century ago. Unfortunately his works have been very
inaccessible, and are so little used that many erroneous conceptions
have been given currency by more superficial investigators. It is
impossible to commend too highly the movement now on foot in
England to reissue the collected works of Sir William Herschel.
In all that pertains to the sidereal universe as a whole he is easily
the greatest of all modern astronomers, and will always remain
unrivaled.

4. EXPLANATION OF THE METHODS EMPLOYED BY CAMPBELL

FOR FINDING THE AVERAGE DISTANCE OF THE GROUP OF

NAKED EYE HELIUM STARS.

This is essentially a combination of the line of sight motion
as found at Lick Observatory, with the proper motions resulting
from observations with the meridian circle, by many observers, as



204 BRIEF BIOGRAPHY AND POPULAR ACCOUNT OF THE

worked up by Boss of the Dudley Observatory, Albany, New York.
By the recent study of several thousand of the brighter stars
included in his Prelimimary General Catalogue, Professor Boss
has deduced their proper motions with a high degree of accuracy.
Campbell found from 180 of these stars resembling our sun in
spectral type that their average cross proper motion in the sky,
from the values derived by Boss, was about 0.11 second of arc per
annum, while at the same time their average speed in the line of
sight shown by the spectrograph at Lick Observatory was 8.9
miles per second, or two hundred and eighty million miles a year.
Having the average motion in the line of sight, in absolute units,
and the average cross proper motion in seconds of arc, it is easy
to find how far away a base line of 280 million miles would have to
be to subtend an angle of 0.11 of a second of arc. It turns out to
be ninety-two light-years.

In this way it is possible to get the average distances of large
groups of stars. Here are some of the results found by Campbell.



Type


No.


Average Yearly
Cross-motion


Average Radial
Velocity in Miles
per Second


Average Rela-
tive Parallax


Average Dis-
tance in Light-
years


B-B 6
Bs-B 9
A
F
G
K
M


312
90
172'
180
118
346
71


0.0078

0.0182
0.0368
0.1075
0.0748
0.0516
0.0384


3.9
4.2
6.5
8.9
9.9
10.4
10.6


0.0061
0.0129
0.0166
0.0354
0.0223
0.0146
0.0106


534
253
196
92
146
223
308



This table contains the most important results of the Camp-
bell-Boss method of obtaining average distances for large groups
of stars. It need scarcely be remarked that its significance can
hardly be overrated. But whilst the average values given are quite
trustworthy, the method is of course inapplicable to the individual
stars] and if their distances are to be found, recourse would have
to be had to the standard method of parallaxes, or to the spectro-
scopic method in the case of visual binaries.



UNPARALLELED DISCOVERIES OF T. J. J. SEE 205

5. SOME OF THE DISTANCES OF THE REMOTEST STARS AS
HERETOFORE CALCULATED BY ASTRONOMERS.

1. Sir William Herschel, Phil. Trans., 1802, p. 498, "almost 2,000,000 light-

years."

2. Sir John Herschel, "Outlines," edition of 1869, p. 583, "upwards of

2,000 light-years."

3. Guillemin, "The Heavens," trans, by Lockyer, 1867, p. 433, "upwards of

20,000 light-years."

4. Bartlett, "Spherical Astronomy," 1874, p. 149, "upwards of 2,437.5 light-

years."

5. Newcomb, "Popular Astronomy," edition of 1878, p. 481, "about 14,000

light-years" (for the Herschel stars).

6. Clerke, " System of the Stars," 1890, p. 314, "less than 36,000 light-years."

7. Ranyard, " Old and New Astronomy," 1892, p. 748, " less than 70,000 light-

years."

8. Young, " General Astronomy," edition of 1904, p. 563, " 10,000 to 20,000

light-years."

9. Newcomb, " The Stars," 1908, p. 319, " at least 3,000 light-years."
10. See, "Researches," Vol. II, 1910, p. 638, "4,500,000 light-years."

From this table it will be seen that there was a great falling
off in the distances following the epoch of Sir William Herschel;
and that the present writer was the first to recognize the fallacy of
the recent estimates of distance, and to restore the large values
used by that unrivaled astronomer one hundred and ten years
ago. Here we have a good illustration of the retrogradation of
opinion in astronomy, under the cultivation of inferior genius.
Sir John Herschel's preference for such small distances over the
large values used by his father is indeed remarkable and very
regrettable . Evidently the small value used by Newcomb is simply
an echo of the reduction in distance made by Sir John Herschel.
The absurdity of these small values not over five times that of
the helium stars of 4.14 magnitude investigated at Lick Observa-
tory ought to impress us with the small importance to be attach-
ed to any opinion merely because it is currently accepted. Thus
we have a clear case of misleading tradition transmitted from the
second Herschel, and the amazing spectacle of the whole world
using values about a thousand times too small, for the greater part
of a century, in times which were supposed to be very enlightened.
Strange indeed that the correct work of the great Sir William



206 BRIEF BIOGRAPHY AND POPULAR ACCOUNT OF THE

Herschel should have been neglected all this time! Will it seem
credible to future ages that such a remarkable retrogradation of
opinion could have occurred and persisted during the nineteenth
and twentieth centuries? If so, it must be attributed to the nar-
rowing effects of extreme specialization, which, with the advance
of science, has been difficult to avoid in our time, and yet is utterly
disastrous to the growth of true natural philosophy as the study
of nature in the widest sense.

6. OTHER METHODS FOR CONFIRMING THE GREAT DEPTH OF

THE MILKY WAY.

(a) The girdle of helium stars about our sun, according to the
Lick determination, has a mean distance of 540 light-years, or a
mean diameter of 1,080 light-years. If this be one- twentieth of
the average thickness of the Milky Way stratum, as one may infer
from the appearance of certain clusters in the constellation Sagit-
tarius, which are near enough to be studied intelligently, then we
have 21,600 light-years for the average thickness of the Milky
Way.

Now when we traverse the Milky Way from Centaurus to
Cepheus, over an arc of 180 in length, the central band appears
to the naked eye to have a width of 3 or 4, as was long ago re-
marked also by Herschel and Struve. This is an extension along
the circle of the Galaxy of about 60 times its thickness. If then
the thickness be 2 1,600 light-years, the double depth of the stratum
in both directions becomes about two- thirds of 21,600X60=
864,000 light-years. And if only the faint or distant telescopic
stars be considered, the width of their belt of distribution is nar-
rower, and the depth would be found several times greater yet.
Wherefore it seems certain that the profundity of the Milky Way,
considerably exceeds a million light-years, and may be several
times that depth.

(b) Accordingly if we make the very moderate hypothesis
that the width of 3 or 4, which was also noticed by Herschel and
Struve, represents chiefly the nearer portion of the Galaxy; and



UNPARALLELED DISCOVERIES OF T. J. J. SEE 207

that the remoter portion has a width not exceeding 1, we should
conclude that the depth may be found by multiplying the thick-
ness or apparent angular width of 21,600 light-years by the num-
ber of degrees in the radius, 57.3. This gives for the depth 1,237,-
680 light-years, and this value might be considerably increased by
adjustments in the data which are not improbable.

(c) In addition to these general arguments, founded on the
principles of geometry, we might introduce another based on actual
measurement. The Lick helium stars, of average brightness 4.14
mag., were found to have an average distance of 540 light-years.
If they were brought near enough to us to appear of 1st magni-
tude, this distance would have to be divided by 4 = \/(2.512) 3 ,
and thus we find for the first magnitude helium stars a distance of
135 light-years.

Now in calculating the plan of the construction of the heavens
from the apparent breadth of the Milky Way, Herschel arrived at
the conclusion that the thickness of the stratum is about 80 times
greater than the diameter of the sphere including the first magni-
tude stars represented by Sirius (Phil. Trans., 1785, p. 254). And
if the average distance of these stars be taken as 135 light-years,
the mean diameter of the shell in which they are included will be
270 light-years. This would give exactly 21,600 light-years for
the thickness of the stratum of the Milky Way, as before.

It is true that Herschel classed all first magnitude stars in one
group, and took no account of the fact that the helium stars are
the more remote and the more brilliant; yet regarding the Galaxy
as a stratum of stars chiefly of the helium type, which certainly
is true of all the more distant portions of that magnificent col-
lection of stars, we may consider the reasoning of this great astron-
omer as still valid. And the argument in regard to the depth of the
Milky Way is thus the same as that given above under (a) and (b).

7. THE EFFECTS OF THE EXTINCTION OF LIGHT IN SPACE.

This problem has been treated with some detail in the 23d
chapter of my "Researches," Vol. II., 1910, but we shall here



208



BRIEF BIOGRAPHY AND POPULAR ACCOUNT OF THE



examine the subject with greater care, especially as to the most
probable average value of the coefficient of extinction. The light
was shown by Struve to be defined by the equation



-, (0.990651) x -
x



(1)



where x is the distance of the star, in units of A = \/ (2.512) n and
n is the difference in magnitude. At very great distances nearly
all the light is cut off, and it therefore becomes a question of high
importance to determine as accurately as possible the proper value
for the coefficient of extinction.

Struve's value, used in the above formula, seems to be too
small, and I have therefore calculated a new table, to show the
effect of increasing the coefficient. In justification of this course
it should be recalled that Sir William Herschel ignored extinction
entirely; but while this procedure obviously is defective, it is
pretty clear, from the aspects of the Milky Way as now made
known by modern research, that Struve's coefficient is decidedly
too small. The following table shows the effects of varying the
coefficient, upon stars 17 magnitudes fainter, corresponding to a
distance 2,512 times larger, where x 1 = 2,511.

TABLE FOR VARYING COEFFICIENT OF EXTINCTION.



;i=Ceoff. of
Extinction.


fr-i.


Fractional Part of Light Transmitted, in Spite
of Extinction,


0.990651
0.995

0.996
0.997
0.998
0.999

0.9995
1.00000


0.000,000,000,05709
0.000,003,4072

0.000,042,571
0.000,52923
0.006,5567
0.081,091

0.284,846
1.000,00


1


17514000000 (Struve's value)
1


293 490
1


23490
1


1889.5
1


152.51


12332 (Sees value)
1


3.5107
1.00000 (Herschel's value)



UNPARALLELED DISCOVERIES OF T. J. J. SEE 209

From the study of this table, we perceive that at the distance
% = 2, 5 12, corresponding to an enfeeblement of 17 magnitudes,
from mere increase of distance alone, the extinction of light varies
from almost total loss, with Struve's co-efficient, to no loss what-
ever, on Herschel's tacit hypothesis of zero extinction. This latter
view, however, certainly is extreme, and probably all modern
astronomers agree that there is extinction of light due to cosmical
dust in space. A hazy background of dust is shown on the
photographs of the Milky Way and other portions of the sky, and
proved to pervade the solar system by the universal prevalence
of meteors.

Since, however, both comets and nebulae are found to be
extremely tenuous bodies, and observed to transmit the light of
stars with but excessively slight enfeeblement, it is obvious that
the general extinction will be much smaller still, but yet appreci-
able. I have therefore adopted a co-efficient of 0.999, about one-
hundredth larger than Struve's, as best harmonizing all known
phenomena. This value, it is true, is much nearer to Herschel's
than to Struve's co-efficient, yet it admits an extinction of light
which becomes appreciable at great distances, while for moderate
distances it is nearly insensible; and I believe this to correspond
closely with all the known facts of the sidereal universe.

An enfeeblement to one-twelfth at a distance appropriate to
stars 17 magnitudes fainter, could easily be compensated for by a
corresponding abnormal brilliancy of the remotest stars, which on
several grounds seems to be highly probable. Thus our procedure
involves no extravagant assumptions as to the great brightness of
the most distant stars, or as to large extinction of light, while on
the other hand it avoids Herschel's tacit hypothesis of zero extinc-
tion, which certainly is unjustifiable.*

* In an important paper read to the Bavarian Academy of Sciences, June 10,
1911, p. 459, Professor H. von Seeliger likewise reaches the conclusion that the
absorption is very small, amounting to 0.34 of a magnitude at 780 times the dis-
tance of Sirius, which Seeliger takes for the border of the sidereal system.



210 BRIEF BIOGRAPHY AND POPULAR ACCOUNT OF THE

8. A GRAPHICAL METHOD FOR DETERMINING THE DEPTH OF
THE GALAXY, BASED ON THE STUDY OF CLUSTERS.

1. Make a diagram of ten or twenty concentric circles, sepa-
rated by equal intervals, each corresponding to one hundred thou-
sand light-years. In this scheme no clusters will be included
within the central circle, because the actual measurements for
parallax have excluded this possibility. But the various clusters
of the N.G.C. may be plotted within the outer circles, or beyond
them all, according to the results given by Herschel's rule of bright-
ness.

2. It is required therefore to locate the clusters, and to indi-
cate their apparent angular diameters by dots of appropriate size.
Some allowance must of course be made for the varying stages of
development of the different clusters, but if there is a decreasing
angular diameter with distance it may be held that the method of
estimating distance devised by Herschel is essentially valid, and
in fact our only method of fathoming these immense distances,
and thus determining the depth or profundity of the Milky Way.

3. A careful attempt has been made to apply this method
using the data of the N.G.C., and the results of the Crossley photo-
graphs recently obtained at Lick Observatory. The results of
this investigation are shown to confirm the present theory.

9. FINAL TEST OF THE INDEFINITE EXTENSION OF THE MILKY

WAY DESIRABLE.

This should be made by the graphical method just outlined,
but by means of more powerful instruments than any yet systemat-
ically employed in this work. To feel satisfied that the universe
extends on indefinitely, we must have proof of additional clusters
of stars of smaller magnitude, and more compressed constitution,
as from the narrowing effect of perspective, at great distances.
Probably we shall not know what the sidereal heavens contain in
the way of vanishing clusters till the Milky Way is systematically
photographed for just such objects, and this very likely will require
a long campaign of photographic research with a large instrument.




THE HERSCHEL-SEE THEORY OF THE GALAXY.

The Sun and all the Stars visible to the naked eye are included within the small white speck,
below the center. The vacant lane extending to the right from this speck may produce the dark
areas of the Coal Sacks near the Southern Cross. When we look outward from our eccentric situation
the Milky Way appears brighest and broadest towards Sagittarius, and faintest and narrowest towards
Monoceros. The diameter of the whole Sidereal -Universe as here shown is so great that light could
not traverse it in less than five million years. This Herschel-See Theory of the Galaxy gives the
reader a good idea of the real nature of the magnificent collection of millions of stars which appears
to us as the clustering stream of the Milky Way.



UNPARALLELED DISCOVERIES OF T. J. J. SEE 211

But as many large reflectors are now coming into use, we may hope
for it before many years elapse. This would be completing on a
modern scale the sidereal soundings left somewhat incomplete by
the systematic explorations of the Herschels.

In a private letter, written in response to my recent inquiry
regarding the power of the 60-inch reflector of the Solar Observa-
tory at Mt. Wilson, Professor W. S. Adams, the acting director,
informs me that this fine instrument probably will show visually
stars as faint as 18th magnitude. He points out, however, that
the magnitude scale is not well defined for such faint objects, and
that very few astronomers have enough experience to fix it at the
present time.

Adams also informs me that from a photograph of the region
of the northern celestial pole of four hours' duration, Professor
E. C. Pickering has derived a value of 21.0 magnitude for the
faintest stars, by the system of photographic magnitudes in use at
the Harvard College Observatory. Obviously there is some un-
certainity in this value, but it probably is not extreme.

In answer to an inquiry as to the possibility of getting still
fainter stars by prolonging the exposure, Professor Adams assures
me that it can be easily done, the only limit being the brightness of
the background of the sky; but that with the clear air of Mt. Wil-
son this would not be reached till the exposure had extended over
many hours. He adds that it takes about three times the expos-
ure to obtain a star one magnitude fainter. From the data here
supplied it seems certain that stars as faint as 21.0 magnitude may
be photographed at Mt. Wilson, with the 60-inch reflector, and
that by prolonging the exposure several additional hours or through
whole nights, stars of 22.0 magnitude probably could be obtained.

It is therefore well established that stars 17 magnitudes fainter
than the 225 helium stars, with average magnitude of 4.14, recently
investigated at Lick Observatory, may now be photographed with
more than one instrument; and the value of A = 2,512 used in our
calculations is amply justified. In fact it seems probable that
instead of 2,512 as our distance multiplier for stars 17 magnitudes



212 BRIEF BIOGRAPHY AND POPULAR ACCOUNT OF THE

fainter, we might have used the larger value 3,981, corresponding
to stars 18 magnitudes fainter than our 225 helium stars with
average magnitude of 4.14. This would almost have doubled the
calculated depths of the Milky Way throughout the foregoing
discussion, and given us over two million light-years, exceeding
the profundity originally concluded by Herschel in 1802. In the
Phil. Trans, for 1800, pp. 83-4, Herschel finds by a different pro-
cess that a cluster of 5,000 stars visible in his 40-foot telescope is
distant 11,765,475,948,678,678,769 miles, "a number which ex-
ceeds the distance of the nearest fixed star at least three hundred
thousand times." With modern data this proves to be 460,355
times the distance of Alpha Centauri, or 2,001,120 light-years.



10. SUMMARY OF THE CHIEF RESULTS OF THE DETERMINATION
OF THE DEPTH OF THE MILKY WAY.

From the several independent and mutually confirmatory
arguments here adduced it follows that the depth of the Milky
Way decidedly exceeds a million light-years, and substantially
accords with the profundity concluded by the illustrious Herschel
one hundred and ten years ago.

1. Herschel concluded that with his forty-foot reflector he
perceived stars whose light had occupied two million years in
reaching the earth; and he justly remarked that he had seen fur-
ther into space than any human being before him. The visual
power or light grasp of Herschel's telescope is somewhat surpassed
by modern instruments; and much additional power is given to
the modern instrument by the use of photography.

2. But if, on the one hand, the modern instruments surpass
Herschel' s in power, there is on the other some increased need for
this in that we now attempt to take account of the extinction of
light by cosmical dust in space. Neglecting this loss of light,
Herschel may have slightly overestimated the distances to which
his telescope could penetrate, but the error was scarcely of sensible
importance.



UNPARALLELED DISCOVERIES OF T. J. J. SEE 213

3. With our greatest modern instruments and the use of
photography it is certain that we can observe stars* at a distance
of over two million light-years, and it is very probable that we can
penetrate to a depth of about five million light-years. A modern
silver-on-glass reflector of twelve feet aperture would give about
six times as much light as the 60-inch reflector at Pasadena, and
with this gain of two magnitudes in light power it is probable that
we could penetrate into space at least twice this distance (theoret-
ically 2.512 is the factor) or to a depth from which the light would
take ten million years to reach the earth.

At the present time a 12-foot reflector is possible, and the
depth to which we can penetrate is simply a question of telescopic
power, which can be vastly but not indefinitely increased. And
this is true in spite of the extinction of light by cosmical dust in
space. There is a limit to the distance to which any given tele-
scope can penetrate, but it increases steadily with the aperture,
since the only question involved is one of enormous light grasp.

It is to be hoped that a telescope of not less than 12 feet aper-
ture may be built for use on the Milky Way. With such a giant
instrument discoveries of the highest order might confidently be
anticipated. A modern expansion of our views of the sidereal
universe analogous to that which marked the great epoch of Her-
schel would follow, with the most beneficial effects upon every
branch of astronomical science. Recent developments in many
lines show that the epoch of great discoveries has not passed, but
is in fact just beginning: and the estimates here laid down, as to
the depth and magnificent extent of the Milky Way, convey to us
but a dim outline of the discoveries which await the builders of the


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Online LibraryW. L. (William Larkin) WebbBrief biography and popular account of the unparalleled discoveries of T.J.J. See .. → online text (page 19 of 28)