Astronomy Cast Ep. 177: Mysteries of the Milky Way, Part 2

How many spiral arms does the Milky Way have?:
There are man different types of spiral galaxies, from normal and barred, to even smaller classifications in each broader group. The arms are caused by density waves moving through the Galaxy. When you look at the galaxy in different wavelengths, you get different data on how many arms there are. Therefore, blue light was selected as a constant of observation. In blue light, the Milky Way appears to have 2 spiral arms.
Sibling Stars: Did other stars form in our solar nebula? What happened to them?
Everything starts out bound together in a star forming region. Over time, the star-forming region stretches out and the sibling stars start to separate. The best way to match with siblings is to match the Sun's composition with other stars'. Unfortunately, non-sibling stars can match as well.
Mass-extinctions:
Seem to trend every 60 million years. Are we due?
When the solar system goes through the Galaxy's disk, it goes through areas of high and low density, we become susceptible to cosmic rays and higher asteroid rates in the high density areas.
G-dwarfs: dwarf stars in the G part of the H-R diagram. They should be more abundant than large stars, but aren't. Why?
There should be a group of stars formed right after the Big Bang that are almost purely hydrogen and helium, but we haven't found a single one yet. They are called Population 3 stars. One explanation is that without the presence of metals, only giant stars could form out of pure hydrogen and helium. The other solution is that the first stars were so giant that they lived and died before any small stars could form. This prevents any G dwarves from forming, and therefore nothing that formed before the first generation of supernovae is capable of still being alive today because the supernova polluted space with metals.
Where are the intermediate mass black holes? We've found stellar mass and supermassive black holes, but only a couple of intermediate size black holes in the whole galaxy. We don't have enough evidence currently to have any real plausible conclusions.