Why do Don and Ra talk about a lenticular galaxy when our galaxy is spiral?

[zenmaster]

Don probably just made a technical mistake. Generally speaking, the Milky Way is 'lenticular' in shape, but that is not the correct astronomical term to use among those available.

Here's a description of galactic evolution from Larson - which differs from conventional understanding in that lenticular and globular galaxies are still thought to be older structures (mainly due to an incorrect theory of stellar evolution).

Closely connected with the velocity is the shape of the rotating structure. The correlation in this case is so obvious that in actual practice the velocity is generally inferred from the shape rather than measured directly, although measurements have been made in some cases where conditions are favorable. Increased rotational velocity in the elliptical galaxies results' in greater eccentricity. Beginning with the globular clusters, which are rotating very slowly and are spherical or nearly spherical, the elliptical units pass through all stages of eccentricity down to strongly lenticular shapes. At this point the spiral disk develops. The structure of the young spiral can be described as loose: the arms are thick and widely separated and the nucleus is rather inconspicuous. As the galaxy grows older and larger the nucleus becomes more prominent and the increased rotational velocity causes the arms to thin out and wind up more tightly. In the limiting condition the galaxy is practically all nucleus and the spiral arms are wound around this central mass so tightly that in effect they become part of it. These changes in appearance in the final stage account for some of the apparent deviations from the normal relation between size and age. There are a number of very large galaxies which are classified as elliptical, although they are greatly in excess of the size which normally results in the development of the spiral structure. The logical explanation is that these are not actually elliptical galaxies; they are the tightly wound, rapidly rotating, giant spirals which have reached the end of the road as galaxies and are ready to take the next step in the evolutionary cycle. Some particularly interesting inferences along this line can be drawn from the characteristics of the giant galaxy Messier 87, one of the well-known examples of this class, and this subject will receive further attention later.