Answer to Question 1
Shapley began his research on the star system by noticing that, although open star clusters are scattered all along the Milky Way, more than half of all globular clusters lie in or near the constellation Sagittarius. The globular clusters seem to be distributed in a great cloud with a center located in the direction of Sagittarius. Shapley assumed that the orbital motion of these clusters is controlled by the gravitation of the entire star system, and for that reason he concluded that the center of the star system must lie somewhere toward Sagittarius.
Answer to Question 2
Zero Age Main Sequence
Structure and function of the thick and thin filaments.
The threespine stickleback (Gasterosteus aculeatus) has been used as a model organism for the study
Pocket gophers and ecosystem structure (data from Huntly and Inouye 1988).
The a helix, a common secondary structure in proteins
Levels of protein structure: Primary structure
Basic structure of the autonomic nervous system.