The mass corresponds to 3.5×10^{15} normal nebulae.
The distance to which the 100-inch reflector should detect the normal
nebula was found to be of the order of 4.4×10^{75} parsecs, or about
1⁄600 the radius of curvature. Unusually bright nebulae, such as M
31, could be photographed at several times this distance, and with
reasonable increases in the speed of plates and size of telescopes it
may become possible to observe an appreciable fraction of the Einstein
universe.
Mount Wilson Observatory
September 1926
[1] _Contributions from the Mount Wilson Observatory_, No. 324.
[2] These are the two Magellanic Clouds, M 31, and M 33.
[3] Bailey, _Harvard Annals_, ❰60❱, 1908.
[4] Hardcastle, _Monthly Notices_, ❰74❱, 699, 1914.
[5] This estimate by Seares is based on a revision of Fath’s counts of
nebulae in Selected Areas (_Mt. Wilson Contr._, No. 297; _Astrophysical
Journal_, ❰62❱, 168, 1925).
[6] “A General Study of Diffuse Galactic Nebulae,” _Mt. Wilson Contr._,
No. 241; _Astrophysical Journal_, ❰56❱, 162, 1922.
[7] The classification was presented in the form of a memorandum to
the Commission on Nebulae of the International Astronomical Union in
1923. Copies of the memorandum were distributed by the chairman to
all members of the Commission. The classification was discussed at
the Cambridge meeting in 1925, and has been published in an account
of the meeting by Mrs. Roberts in _L’Astronomie_, ❰40❱, 169, 1926.
Further consideration of the matter was left to a subcommittee, with
a resolution that the adopted system should be as purely descriptive
as possible, and free from any terms suggesting order of physical
development (_Transactions of the I.A.U._, 2, 1925). Mrs. Roberts’
report also indicates the preference of the Commission for the term
“extra-galactic” in place of the original, and then necessarily
non-committal, “non-galactic.”
Meanwhile K. Lundmark, who was present at the Cambridge meeting and
has since been appointed a member of the Commission, has recently
published (_Arkiv für Matematik, Astronomi och Fysik, Band 19B_,
No. 8, 1926) a classification, which, except for nomenclature, is
practically identical with that submitted by me. Dr. Lundmark makes
no acknowledgments or references to the discussions of the Commission
other than those for the use of the term “galactic.”
[8] _Problems of Cosmogony and Stellar Dynamics_, 1919.
[9] N.G.C. 4486 (M 87) may be an exception. On the best photographs
made with the 100-inch reflector, numerous exceedingly faint images,
apparently of stars, are found around the periphery. It was among these
that Belanowsky’s nova of 1919 appeared. The observations are described
in _Publications of the Astronomical Society of the Pacific_, ❰35❱, 261,
1923.
[10] “Early” and “late,” in spite of their temporal connotations,
appear to be the most convenient adjectives available for describing
relative positions in the sequence. This sequence of structural forms
is an observed phenomenon. As will be shown later in the discussion,
it exhibits a smooth progression in nuclear luminosity, surface
brightness, degree of flattening, major diameters, resolution, and
complexity. An antithetical pair of adjectives denoting relative
positions in the sequence is desirable for many reasons, but none of
the progressive characteristics are well adapted for the purpose.
Terms which apply to series in general are available, however, and of
these “early” and “late” are the most suitable. They can be assumed to
express a progression from simple to complex forms.
An accepted precedent for this usage is found in the series of stellar
spectral types. There also the progression is assumed to be from the
simple to the complex, and in view of the great convenience of the
terms “early” and “late,” the temporal connotations, after a full
consideration of their possible consequences, have been deliberately
disregarded.
[11] _Publications of the Lick Observatory_, ❰13❱, 12, 1918.
[12] _H_{β}_ is brighter than N_{2}. Patches with similar spectra are
often found in the arms of late-type spirals—N.G.C. 253, M 33, M 101.
The typical planetary spectrum, where _H_{β}_ is fainter than N_{2},
is found in the rare cases of apparently stellar nuclei of spirals;
for instance, in N.G.C. 1068, 4051, and 4151. Here also the emission
spectra are localized and do not extend over the nebulae.
[13] _Monthly Notices_. ❰74❱, 699, 1914.
[14] _Annalen der Wiener Sternwarte_, ❰20❱, 1907.
[15] _Astronomische Nachrichten_, ❰214❱, 425, 1921.
[16] _Publications of the Lick Observatory_, ❰13❱, 1918.
[17] Since C is constant for all nebulae in a given class, the linear
relation between Δ log d and C for the different classes is something
more than a mere geometrical relation arising from the observed
equality of the mean m_{T} in the various classes.
[18] This is apparent even among the observed classes. Referring
to formula (3), m_{T} + 5 log b will be constant in so far as
C_{e} + 5 log (1 – e) is constant. The following table indicates that
the latter term is approximately constant throughout the sequence of
elliptical nebulae. The values of C_{e} were read from the smooth curve
in Fig. 6.
======+=========+================+==========+===========
e | C_{e} | 5 log (1 – e) | C_{0} | Res.
------+---------+----------------+----------+-----------
0 | 10.30 | 0.0 | 10.30 | –0.14
1 | 10.65 | – .23 | 10.42 | – .02
2 | 11.00 | .48 | 10.52 | + .10
3 | 11.35 | 0.78 | 10.57 | + .13
4 | 11.70 | 1.11 | 10.59 | + .15
5 | 12.05 | 1.50 | 10.55 | + .11
6 | 12.40 | 1.99 | 10.41 | – .03
7 | 12.75 | –2.62 | 10.13 | –0.31
+---------+----------------+----------+-----------
Mean | | | 10.44 | 0.12
------+---------+----------------+----------+-----------
[19] _Mt. Wilson Contr._, No. 191; _Astrophysical Journal_, ❰52❱, 162,
1920.
[20] Pease, _Mt. Wilson Comm._, No. 51; _Proceedings of the National
Academy of Sciences_, ❰4❱, 21, 1918.
[21] Pease, _Mt. Wilson Comm._, No. 32: _ibid._, ❰2❱, 517, 1916.
[22] _Astrophysical Journal_, ❰55❱, 406, 1922.
[23] _Monthly Notices_, ❰82❱, 133, 1922.
[24] _Astronomical Journal_, ❰28❱, 75, 1914.
[25] _Mt. Wilson Contr._, No. 297; _Astrophysical Journal_, ❰62❱, 168,
1925.
[26] The latest and most reliable results bearing on the distribution
of faint (hence apparently distant) nebulae are found in Seares’s
revision and discussion of the counts made by Fath on plates of the
Selected Areas with the 60-inch reflector. When the influence of the
cluster in Virgo is eliminated the density appears to be roughly
uniform for all latitudes greater than about 25°.
[27] _Monthly Notices_, ❰78❱, 3, 1917.
[28] Haas, _Introduction to Theoretical Physics_, ❰2❱, 373, 1925.