After the Sun's core hydrogen is depleted by nuclear fusion the core will consist primarily of 1) carbon. 2) deuterium. 3) helium. 4) oxygen. The material that makes up the Sun was once part of 1) the Big Bang. 2) another star. 3) a molecular cloud. 4) all of the above. Most of the brightest stars in the sky are 1) relatively hot small stars. 2) relatively cool giant stars. 3) relatively cool small stars. 4) relatively hot giant stars. Ejnar Hertzsprung Henry Norris Russell Hertzsprung– Russell diagram http://www.universetoday.com/wp-content/uploads/2009/02/hrdiagram.jpg Hertzsprung– Russell diagram http://upload.wikimedia.org/wikipedia/commons/6/6b/HRDiagram.png A main sequence star is stable because of self-regulation; 1) the nuclear reaction rate does not depend on temperature. 2) a slight contraction leads to higher gas pressure. 3) a slight contraction decreases the nuclear reaction rate. 4) a slight contraction leads to lower internal pressure.