Spectrographic rotation 2.0×10^9 ☉
Öpik’s method 2.6×10^8
Here again the resulting masses are of the same order. They can be made
to agree as well as those for M 31 by the not unreasonable assumption
that the absolute luminosity of the nebula is 2 mag. or so brighter
than normal.
Öpik’s method leads to values that are reasonable and fairly consistent
with those obtained by the independent spectrographic method.
Therefore, in the absence of other resources, its use for deriving the
mass of the normal nebula appears to be permissible. The result, 2.6 ×
10^8 ☉, corresponding to an absolute magnitude of –15.2, is probably
of the right order. The two test cases suggest that this value may be
slightly low, but the data are not sufficient to warrant any empirical
corrections.