When the distances are known, it is possible to derive actual
dimensions and hence to calibrate the curve in Figure 6, which exhibits
the apparent diameters as a function of type, or stage in the nebular
sequence, for nebulae of a given apparent magnitude. The mean maximum
diameters in parsecs corresponding to the different mean types are
given in Table XVII. For the elliptical nebulae, values are given both
for the statistical mean observed diameters and for the diameter as
calculated for the pure types.
Spirals at the last stage in the observed sequence have diameters of
the order of 3000 parsecs. Assuming 1:10 as the ratio of the two
axes, the corresponding volume is of the order of 1.4×10^9 cubic
parsecs, and the mean luminosity density is of the order of 7.7
absolute magnitudes per cubic parsec as compared with 8.15 for the
galactic system in the vicinity of the sun. These results agree with
those of Seares who, from a study of surface brightness, concluded
that the galactic system must be placed at the end of, if not actually
outside, the series of known spirals when arranged according to
density.[19]
TABLE XVII
========+=======================+=======+=====================
Type | Diameter in Parsecs | Type | Diameter in Parsecs
+-----------+-----------+ |
| Obs. | Cal. | |
--------+-----------+-----------+-------+---------------------
E0 | 360 | 340 | Sa | 1450
E1 | 430 | 380 | Sb | 1900
E2 | 500 | 430 | Sc | 2500
E3 | 590 | 490 | |
E4 | 700 | 570 | SBa | 1280
E5 | 810 | 680 | SBb | 1320
E6 | 960 | 850 | SBc | 2250
E7 | 1130 | 1130 | Irr | 1500
--------+-----------+-----------+-------+---------------------