next up previous
Next: The core helium burning Up: Structural evolution of LIM Previous: The main sequence (MS)

The red giant branch (RGB) phase

The transit of a star from its main sequence position on the H-R diagram to the red giant region is so rapid that stars are rarely observed between these two regions. The evolution to the red giant phase is indicated on the H-R diagram (Fig. 2.2) as point C. At this stage, the stellar core is exhausted of hydrogen and the star has to rearrange its structure. The helium core contracts rapidly and heats up, while hydrogen in a shell surrounding the core continues to burn. For low-mass stars, the core becomes so dense that it becomes degenerate*. In this state, the core has a degenerate gas pressure which depends only density, not temperature. This enables the core to attain a pressure sufficient to support the weight of the upper layers of gas, even though no fusion reactions are taking place in the core. For intermediate-mass stars, the helium core does not become degenerate, but burns the helium to create a carbon-oxygen core (which does become degenerate). As the core contracts, the outer layers expand and cool and the star swells to become a red giant. Low mass stars (Mstar < ~ 2Mtex2html_wrap_inline518 ) may spend up to ~20% of their lives as red giants, whereas intermediate mass stars ( ~2Mtex2html_wrap_inline518 < Mstar < ~ 8Mtex2html_wrap_inline518 ) have much shorter red giant phases (<7% of their MS lifetime). During the RGB phase there is a deep convective outer envelope, which penetrates the deep layers containing the remnants of hydrogen burning. Thus, newly synthesized nuclei are mixed and transported to the surface. This process is called the ``first dredge-up''. As this name implies, there are further ``dredge-up'' scenarios, and thus the red giant branch is only the first time the star travels up the giant branch of the H-R diagram. This has lead to the red giant branch also being called the ``first ascent of the giant branch''.

* According to the Pauli Exclusion Principle only two electrons with opposite spins can have a given energy in a given volume at one time. Consequently in a very dense gas, the atoms are so tightly packed that they are only held apart by the fact that the electrons cannot exist in the same space. This creates the degenerate gas pressure that prevents further contraction.

Next: The core helium burning Up: Structural evolution of LIM Previous: The main sequence (MS)

Wicked Witch's Webpage
Angela's Home Page
email me
Angela's Research page
Angela's Research Page