Also explain why the observations you describe point to a black hole rather than a neutron star or a white dwarf. Young main sequence stars are referred to a 'metal-rich' or as having roughly the same metal content as the Sun. Discuss one piece of observational evidence in support of the idea that stars synthesize metals. Suppose you had access to a radio telescope with high angular resolving power. Describe how you would determine the speed and rotation of a nearby and well-resolved spiral galaxy.
Spiral galaxies contain large amounts of dark matter.
Suppose you observed a spiral galaxy that had no dark matter and another that, like our Galaxy, had substantial amounts of dark matter.