Notes

MS Star
• How long the stage lasts – in general terms
While the sun will spend about 10 billion years on the main sequence, a star 10 times as massive will stick around for only 20 million years.
• General Size
Diverse sizes, very small to very large
• Elemental composition at that stage
Stars begin their main sequence phases with cores filled with 74-percent hydrogen, 25-percent helium and 1-percent other elements.
• Relationship between pressure and gravity at that stage
All main sequence stars (including the Sun) are in hydrostatic equilibrium. That is, the inward force of gravity, which tends to compress the star, is balanced by the outward force due to the pressure.
• What is happening at the stage
Main sequence stars are stars in the phase where they are fusing hydrogen atoms to form helium. Once a star runs out of hydrogen and starts fusing helium to form heavier elements, a star enters its dying phase.

Red Giant
• How long the stage lasts – in general terms
Stars spend approximately a few thousand to 1 billion years as a red giant.
• General Size
Red giant stars reach sizes of 100 million to 1 billion kilometers in diameter (62 million to 621 million miles), 100 to 1,000 times the size of the sun
• Elemental composition at that stage
helium is able to fuse into carbon. Depending on the mass of the star, the helium burning might be gradual or might begin with an explosive flash. The energy produced by the helium fusion causes the star to expand outward to many times its original size.
• Relationship between pressure and gravity at that stage
Gravity takes the lead and compresses the star smaller and tighter. Temperatures increase with the contraction, eventually reaching levels where helium is able to fuse into carbon.
• How that stage transitions into the next stage
Eventually, the helium in the core runs out and fusion stops. The star shrinks again until a new helium shell reaches the core. When the helium ignites, the outer layers of the star are blown off in huge clouds of gas and dust known as planetary nebulae. The core continues to collapse in on itself. Smaller stars such as the sun end their lives as compact white dwarfs. The material of larger, more massive stars fall inward until the star eventually becomes a supernova


SOURCES FOR MS STAR:
http://www.space.com/22437-main-sequence-stars.html
https://www.reference.com/science/main-sequence-stars-abbebc39f698451d#
http://www.astronomy.ohio-state.edu/~ryden/ast162_4/notes14.html

SOURCES FOR RED GIANT:
http://www.space.com/22471-red-giant-stars.html