For a classification of all stars, see The H-R Diagram.
Technically, a subgiant star is a star which had exhausted all hydrogen in its core and is fusing hydrogen in a shell surrounding the core. The star is not fusing helium yet.
Lifetime[]
The subgiant phase is not a long-lasting point in the life of a star. It is a transition point.
- The most massive M - type stars might reach this point, but because they are too small, helium fusion will not start. Once hydrogen is exhausted, they will decay into helium White dwarfs.
- Stars like our Sun significantly increase their luminosity while passing through the subgiant phase, before becoming Red giants. The transition will last up to a few million years.
- Stars weighting over 20 times our Sun will pass fast through this phase, almost unnoticed. They will become Supergiant stars.
Hosted planet[]
The subgiant phase does not last long compared to the life of a star, but still long enough compared to a human lifetime. This makes terraforming of a planet feasible.
For the Solar System, during this phase, Mars will be in the habitable zone.