Answer to Question 1
Population I stars are metal rich, containing 1 to 3 percent metals, whereas Population II stars are metal poor, typically containing less than 0.1 percent metals. Population I stars are relatively young, and Population II stars are old. Population I stars belong to the disk component of the galaxy and are sometimes called disk population stars. They have nearly circular orbits in the plane of the galaxy and formed within the last few billion years. The Sun is a Population I star, as are the type I Cepheid variables. Population II stars belong to the spherical component of the galaxy and are sometimes called the halo population stars. These stars have randomly tipped orbits ranging from circular to highly elliptical. They are old stars that formed when the galaxy was young.
Answer to Question 2
main sequence
The mark-recapture technique is often used to estimate population size.
Population growth and growth rate, 1950 - 2100
As the world’s population continues to grow, demands on limited resources for food production
Breakdown of the Population by Schooling and Wages
Relationship Between Income per Capita and Population Growth
Difference Between a Sidereal Month and a Synodic Month