Here's the breakdown:

* Main Sequence Stars: These are the most common type of stars, like our Sun. They fuse hydrogen into helium in their cores, generating energy and creating outward pressure that balances gravity.

* Hydrogen Depletion: As a star fuses hydrogen, it eventually runs out in the core. This causes the core to contract and heat up.

* Shell Burning: The increased temperature and pressure allow hydrogen fusion to begin in a shell surrounding the core, causing the star to expand dramatically.

* Red Giant Stage: The expanded star becomes much cooler and redder, hence the name "red giant."

What kinds of main sequence stars form red giants?

* Low-mass stars (like our Sun): These stars will evolve into red giants and eventually become white dwarfs.

* Intermediate-mass stars: These stars will also become red giants, but they will eventually undergo further evolution, potentially leading to supernova explosions and neutron stars or black holes.

It's important to note: Red giant formation is a natural stage in the evolution of most stars, not a specific type of star on its own.