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Red Giants

When no hydrogen is left in the core, the hydrogen burning stops. The helium core of the star starts to collapse. This gravitational contraction will heat up the hydrogen envelope surrounding the core. Fusion therefore begins in the envelope and the star expands.
Red Giant 1 Red Giant 2 The star becomes very big and the surface temperature decreases, although the core temperature is still very high and the total luminosity is also high. Since the average luminosity is low, the star appears red. This big red star is a red giant. Figure 08-08a is a schematic diagram depicting this initial phase of a red giant with a (main-sequence) mass of 1 Msun.

Figure 08-08a Red Giant 1
[view large image]

Figure 08-08b Red Giant 2 [view large image]

All red giants are variable stars. The core keeps on contracting and heating up until it is hot enough for the triple-alpha process (also known as helium flash) to take place. In this reaction, three helium nuclei will fuse together to form a carbon nucleus. Since the hydrogen-burning shell and helium-burning core do not produce energy in a stable and steady manner, the star will pulsate and generate strong stellar wind. Eventually, the entire outer shell will be ejected. The gas ejected will form a thin shell around the star. This is the planetary nebula. For the core, it does not heat up sufficiently for carbon-burning. When there's no more nuclear burning, it shrinks. But the electron degenerate pressure prevents it from complete collapse. It grows fainter as well as hotter and becomes a white dwarf. Figure 08-08b is a schematic diagram depicting the final phase of a red giant with a (main-sequence) mass of 1 Msun.

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