What does FGB mean in UNCLASSIFIED

FGB stands for "first giant branch". It is an astronomical term used to describe a particular phase of the star's life. During this phase, the star increases in brightness, size, and temperature as it moves through its evolution.

The first giant branch is actually a part of stellar evolution, starting after hydrogen shell burning has stopped. When there is no more hydrogen to fuel the fusion process, the core contracts and causes the outer layers of material near the surface to expand and cool, which allows gravity to take over and start pulling matter inward. This pulls much more helium into the star's core than what was already there before the contraction happened.

The length of time that a stars spends on FGB depends on how massive it is; however generally speaking stars spend roughly 10-50 million years on this phase after they have reached their main sequence stage.

Any type of star that has enough mass to become a red giant can enter into first giant branch (FGB). This includes stars like our own Sun or any other type of stars on the main sequence such as O-type stars or M-type stars.

After leaving FGB, stars continue to evolve until becoming either white dwarfs or neutron stars depending on its mass before entering into first giant branch. In addition some further development may occur including pulsation instabilities that can produce variable red giants known as Mira variables which have periodic flux changes due to stellar pulsations or supernova explosions which occur usually when massive stars die off from exhaustion at end stages when all fusion reactions in their core ceased working many millions years later after their birth from interstellar clouds etc.