DIMENSIONS OF EXTRA-GALACTIC NEBULAE
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 1.4×109 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 | |
|---|---|---|
| Obs | Cal. | |
| E0 | 360 | 340 |
| E1 | 430 | 380 |
| E2 | 500 | 430 |
| E3 | 590 | 490 |
| E4 | 700 | 570 |
| E5 | 810 | 680 |
| E6 | 960 | 850 |
| E7 | 1130 | 1130 |
| Type | Diameter in Parsecs |
|---|---|
| Sa | 1450 |
| Sb | 1900 |
| Sc | 2500 |
| SBa | 1280 |
| SBb | 1320 |
| SBc | 2250 |
| Irr | 1500 |