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What's the difference between the RGB and the AGB? I can't seem to find an clear distinction anywhere.


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up vote 6 down vote accepted

Red giants and asymptotic giants have some close similarities, and one actually evolves into the other. Both have an extended envelope of relatively cool, non-burning material (mostly $\rm{H}$, $\rm{He}$). They also each have a core of dense, non-burning material; in the case of the red giant this is mostly $\rm{He}$, while for the asymptotic giant it's $\rm{C}$ and $\rm{O}$.

The burning shell in the red giant is $\rm{H}$. For stars of the right mass, the conditions (density, temperature) in the core will periodically be sufficient to ignite the $\rm{He}$ causing a "core flash".

Red giant structure:

Red Giant Structure

After the red giant branch of stellar evolution there is a brief period where the $\rm{He}$ core burns called the horizontal branch. Once the He core is exhausted (it's been converted to $\rm{C}$ and $\rm{O}$), the star starts on the asymptotic giant branch. This branch has two parts, the early asymptotic giant branch (E-AGB) and the thermal-pulse asymptotic giant branch (TP-AGB).

E-AGB structure:

Asymptotic Giant Structure

Stars on the E-AGB are like red giants, but in addition to a $\rm{H}$ burning shell there is a $\rm{He}$ burning shell (the energy output is dominated by the He burning shell). In the TP-AGB, the $\rm{H}$ shell picks up again and dominates the energy output, but periodically as the $\rm{He}$ produced by $\rm{H}$ burning is accreted onto the $\rm{He}$ shell, "helium shell flashes" occur, analogously to helium core flashes in red giants.

Source/Reference: Carroll & Ostlie "An Introduction to Modern Astrophysics: 2nd Edition" (Pearson)

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Great and thorough explanation. Thank you! – Matt Aug 20 '13 at 14:57
Happy to help :) Out of curiosity, where did you find info on RGB and AGB stars that didn't explain the difference? – Kyle Oman Aug 20 '13 at 14:59
@Matt If you want to see stellar evolution tracks that place the giant branches in perspective, figures 13.4 and 13.5 of Carroll & Ostlie (2nd ed.) do an excellent job. – Chris White Aug 21 '13 at 0:12

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