Answer:
Red giant or super giant → very cool but very luminous
→ found in the upper right of the H-R diagram.
Main sequence →The majority of stars in our galaxy
→ Sun, for example
→ a very hot and very luminous star
White dwarfs → very hot but very dim
→ not much larger in radius than earth
Explanation:
Giant:
When the stars run out of their fuel that is hydrogen for the nuclear fusion reactions then they convert into Giant stars.That's why they are very cool. Giant stars have the larger radius and luminosity then the main sequence stars.
Main Sequence:
Stars are called main sequence stars when their core temperature reaches up to 10 million kelvin and their start the nuclear fusion reactions of hydrogen into helium in the core of the star. That is why they are very hot and luminous. For example sun is known as to be in the stage of main sequence as the nuclear fusion reactions are happening in its core.
White dwarfs:
When the stars run out of their fuel then they shed the outer layer planetary nebula, the remaining core part that left behind is called as white dwarf. It's the most dense part as the most of the mass is concentrated in this part.
A. because everything is balanced.
