* Significantly more luminous: Supergiants are far brighter, often thousands of times brighter than red main sequence stars.

* Slightly cooler: Supergiants generally have slightly lower surface temperatures than red main sequence stars, although they are still considered "cool" stars compared to blue stars.

Here's a breakdown:

* Red Main Sequence Stars: These are relatively small, cool, and faint stars. They fuse hydrogen into helium in their cores at a relatively slow rate.

* Supergiants: These are enormous stars, much larger than red main sequence stars. They have exhausted the hydrogen fuel in their cores and have begun fusing heavier elements, which releases more energy and causes them to swell to immense sizes. This also causes them to be much brighter.

Why the cooler temperature?

Although supergiants are much brighter, their surface temperature is slightly lower. This is because the huge size of supergiants spreads their energy over a larger surface area, making their surface cooler than smaller stars like red main sequence stars.

In summary:

Supergiants are much larger, brighter, and slightly cooler than red main sequence stars. They are stars in a later stage of evolution, having exhausted the hydrogen fuel in their cores and begun fusing heavier elements.