Question

Describe the life history of a 1 solar mass star from the moments after the helium flash when the star moves into the horizontal branch through to the white dwarf stage. Include as many details as possible and describe its evolutionary track on the H-R diagram as it progresses.

Answer 1

After exhausting its core hydrogen, a 1 solar mass star leaves the main sequence and begin fusion in a hydrogen shell around the helium core and become a giant on the red giant branch. In 1 solar mass stars, the helium core becomes a region of degenerate matter that does not contribute to the generation of energy. It continues to grow and increase in temperature as the hydrogen fusion in the shell contributes more helium. The core eventually reaches the temperature necessary for the fusion of helium into carbon through the triple-alpha process. The initiation of helium fusion begins across the core region, which will cause an immediate temperature rise and a rapid increase in the rate of fusion. Within a few seconds the core becomes non-degenerate and quickly expands, producing an event called helium flash. The output of this event is absorbed by the layers of plasma above, so the effects are not seen from the exterior of the star. The star now changes to a new equilibrium state, and its evolutionary path switches from the red giant branch (RGB) into the horizontal branch of the H-R diagram.

Asymptotic giant branch phase-

After the star has consumed the helium at the core, hydrogen and helium fusion continues in shells around a hot core of carbon and oxygen. The star follows the asymptotic giant branch on the H-R diagram, tracing the original red giant evolution, but with even faster energy generation. AGB stars are typically long-period variables, and suffer mass loss in the form of a stellar wind. A star may lose 50 to 70% of its mass during the AGB phase.

Circumstellar envelope-

The extensive mass loss in AGB stage results in a circumstellar envelope surrounding the star.It is a part of the star that has a roughly spherical shape and is not gravitationally bound to the star core. Usually circumstellar envelopes are formed as a result of the dense stellar wind.

Post AGB stage-

The star ultimately reaches the tip of the asymptotic-giant-branch and run out of fuel for shell burning. This star is not sufficiently massive to start full-scale carbon fusion, so it contracts again, going through a period of post-asymptotic-giant-branch superwind to produce a planetary nebula with an extremely hot central star. The central star then cools to form a white dwarf.