Question

How are AGB stars formed?

Answer 1

AGB stars are stars approaching the end of their life.

Explanation:

Asymptotic Giant Branch stars form a track on the Hertzprung-Russell diagram. They are not newly formed stars, rather they are the evolution of old stars.

Stars between about 0.6 and 10 solar masses use Hydrogen fusion to create energy. When the supply of Hydrogen is exhausted, the star's core starts to collapse under gravity. This causes the temperature to rise and the outer layers expand into a red giant. When the temperature gets to over `3*10^8`K Helium fusion starts and the star's luminosity increases.

When the supply of Helium is exhausted the star becomes a red giant again. This is the start of the AGB phase.