How does a main sequence star become a red giant?

Answer 1

After a star has used all the availably hydrogen in its core, it contracts until the temperature and pressure in a region around the now-dead helium core become high enough to start hydrogen fusion once again. This "shell fusion" actually produces more energy than core fusion did, causing the star to expand. The expansion gives the star a much larger surface area, so although it is radiating more energy, its surface is now cooler (redder) than it was before.

When the Sun turns into a red giant (in five billion years or so), it will expand to probably somewhere beyond the orbit of Venus.

Answer 2

Moderate sized main sequence stars like the sun will become red giants towards the end of their life. This happens when they exhaust their internal hydrogen supplies and the outer layers expand greatly. The outer layers become cooler , and so redder, but they are so large that the total luminosity increases greatly. Larger hotter main sequence stars become blue giants when they expand. The outer layers are still quite hot and the giant star is extremely luminous. These will eventually cool and become red giants. So the difference between red giants and blue giants is temperature. Blue giants are hotter than red giants.

Answer 3

A main sequence star becomes a red giant after it has exhausted the fuel it has. You can read more in wikipedia:

Answer 4

when it spins so fast that it just burns away the layers of the stars