Answer to Question 1
There is oxygen in Earth's atmosphere because of plant life. When Earth was young, its atmosphere had no free oxygen, that is, oxygen not combined with other elements. Oxygen is very reactive and quickly forms oxides in the soil or combines with iron and other substances dissolved in water. Only the action of plant life keeps a steady supply of oxygen in Earth's atmosphere via photosynthesis, which makes energy for plants by absorbing carbon dioxide and releasing oxygen. Beginning about 2 billion to 2.5 billion years ago, photosynthetic plants in the oceans had multiplied to the point where they made oxygen at a rate faster than chemical reactions could remove it from the atmosphere. After that time, atmospheric oxygen increased rapidly
Answer to Question 2
Models of stars show that there is a main sequence because stars support their weight by generating energy in their interiors. Gravity pulls all of the atoms in a star inward, but their weight is balanced by the outward pressure of the hot gas inside the star. The gas is heated by nuclear reactions, and the outward flow of that energy keeps each layer hot enough to support the weight pressing down on it. The main sequence is the location in the H-R diagram for stars in equilibrium, fusing hydrogen into helium.
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