Red giant

Red giants are big stars that have left main sequence. They change fast and will, at some point, go supernova or will decay into White dwarfs. However, because human lifetime is so short compared to a star, they can be a good destination for settlers. They have a certain Habitable Zone.

The star
First of all, there is not one single type of red giants. There are sub-giants, giants, supergiants and super-supergiants. Second, they don't all have the same color. This is the fate that our sun, Sol, will face one day.

As a star finishes its hydrogen supply, it starts to fuse helium into heavier elements. Then, when helium is exhausted, it will start using whatever it finds. Depending on star mass, there is an upper limit. Anyway, since no human has lived enough to witness the whole transformation and we only have seen stars in different moments, we cannot know exactly which star transforms into what. The transformation is not very fast. B - type stars, for example, follow the next pattern: First, when the star starts fusing helium, it turns into a larger but colder star, blowing more heat (white, but not red, similar to what the star was first). Then, when helium is exhausted, it starts to visibly increase in size and to become orange, then red. The process is very slow first, but much faster later.

Red giants have terrible solar winds. They lose matter in a very fast way. Such a strong wind is like a scourge to the poor nearby planets, so strong that it can blow a significant part of a gas giant's atmosphere away. Well, in the first stages, while the star is a yellow or orange giant, still a planet cannot be endangered that much.

Not all red giants have the same luminosity corresponding to the same size (this is why there are subgiants, giants, supergiants and super-supergiants). So, each one is different and there is no general pattern.