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| Galaxies |
1. Spiral tracers must have short enough lifetimes that they can't move further from their place of birth in their lifetime than the width of a spiral arm. The Sun, and certainly any cooler main sequence stars, have lifetimes far too long to trace the spiral structure. For example, the Sun has had time since its birth to revolve some 20 times around the galaxy, crossing through spiral arms many times. Thus, it has long since left the spiral arm where it presumably was born. Even if red dwarfs did not live much longer than the Sun, they would also not be good tracers because they are faint and too difficult to see. But given their long lifetimes, that is not a significant point.
2. Spirals contain many kinds of stars, but the most luminous are the bright, hot, young, O and B stars, which are blue. These dominate the light of the galaxy.
3. The key is the word "observed". The bright blue spirals are easier to see than the faint small ellipticals. But careful counts for more nearby regions where faint ellipticals are easier to see suggest that there are more of them out there (by a factor of about three) than spirals; we just can't see them at large distances.
4. We see more spirals, but that is partially because spirals are usually much brighter than irregular galaxies. Careful counts for nearby galaxies where faint irregulars are easier to see suggest that the number of spirals and of irregulars is about the same (each is about 20 percent of the total).
5. We are not certain. Perhaps there is no clear distinction between dwarf ellipticals and large globular clusters.
6. The Hubble Law applies on the large scale. Superposed on the Hubble expansion is local or "peculiar motion", which can offset the Hubble flow for nearby galaxies. Andromeda is close enough that the gravitational interactions in the Local Group overcome the Hubble expansion and it is moving toward us.
7. We are not certain. They appear to be almost devoid of visible matter, but may well contain substantial dark matter.
8. From the Hubble law, a recessional velocity of 3100 km/s implies a distance of about 155 million light years. From the Small-Angle Formula Calculator, an angular size of 4 arc minutes at a distance of 155 million light years implies a physical diameter of about 180,000 light years.