The final evolutionary stage of most stars before they die is the Asymptotic Giant Branch (AGB). (For some background on how stars get to this point, see the page on the life of stars). Stars on the AGB have a complicated core structure with a core of inert carbon surrounded by a helium burning shell, which is in turn surrounded by an inert helium shell. Above this layer the star is still burning hydrogen in another shell. This complicated core is surrounded by a hydrogen-rich envelope that reflects the star's original composition.
The cores of AGB stars contract and heat up, driving the outer envelope to tremendous expansion. The star brightens and cools, evolving in a manner very similar to red giants, only with higher luminosity. The double-shell structure is inherently unstable to thermal pulses, which occur every 10,000 to 100,000 years (depending on the mass of the star). The thermal pulses temporarily reconfigure the interior of the star, allowing material rich in carbon to be dredged from the core to the surface of the star. After a few thermal pulses, the star will become a carbon star, with more carbon than oxygen at its surface.
The multi-shell structure of stars on the AGB makes them unstable to pulsations in their outer layers. Nearly all stars on the AGB will reach a point when they undergo a series of expansions and contractions, which makes them vary in brightness and color. We call these stars Long-Period Variables (LPVs).
These variables pulsate with typical periods of anywhere between 40 days and two or three years. Their amplitudes can be barely noticeable, or they can vary in brightness by a factor of 200 or more (i.e. several magnitudes). Long-period variables are classified based on the amplitude of the pulsations and the regularity of the period. Mira variables show the strongest amplitudes and most regular periods; semi-regulars have more poorly defined periods. In irregular variables, the lack of repeatability makes it impossible to determine a period.
Table 1. Classes of long-period variables
| Class | Visual amplitude (magnitude) |
Period (days) | Variations | Evolutionary state |
|---|---|---|---|---|
| Mira | > 2.5 | 80-1000 | fairly regular | AGB |
| SRa | < 2.5 | 80-400 | fairly regular | AGB |
| SRb | < 2.5 | 40-800 | semi-regular | AGB |
| SRc | < 2.5 | 40-2000 | semi-regular | Supergiant |
| Lb | < 1.5 | indeterminate | irregular | AGB |
| Lc | < 1.5 | indeterminate | irregular | Supergiant |
The stars that produce SRb and Lb light curves are usually on the AGB, while stars that produce SRc and Lc light curves are supergiants, which are much more massive and much more luminous.
All of these long-period variables undergo mass-loss, where the outer envelope is slowly ejected into space. As material moves away from the central star, it cools. Eventually, its temperature will drop enough to allow dust grains to condense out, much like water vapor condenses to form clouds in the Earth's atmosphere.
Stars on the AGB have expanded to extreme sizes. If the Sun were on the AGB, its outer layers would approach Earth's orbit! Consequently, the escape velocity from the star is much lower than when the star is on the main sequence. The pulsations in the envelope of an AGB star are a key part of the mass-loss process, because they can accelerate the star almost to the escape velocity. But they don't quite reach it, so another push is needed. That push comes from radiation pressure on the dust. Starlight accelerates the freshly condensed dust grains, and the dust grains collide with the gas and drag it outward with it. Literally, the star blows itself a way! Together, the pulsations and radiation pressure on the dust drive the mass-loss process, although it is not clear if one of the mechanisms dominates and if so, which one.
The chemistry of the dust will depend on the chemistry of the ejected gas. In all of these stars, hydrogen dominates the mix, followed by helium (stars start with roughly 73% H and 25% He). For most stars, the next-most abundant element is oxygen. But in many evolved stars, carbon outnumbers oxygen because nuclear processed material from the interior has been dredged to the surface. Carbon monoxide (CO) forms commonly in cool stellar atmospheres but does not condense into dust. CO will form until it exhausts all of the available oxygen or carbon, leaving the more plentiful element to dominate the dust mixture. Stars where the carbon and oxygen abundances are nearly equal are called S stars, and one might expect these to have unusual dust chemistries. They certainly form some unusual molecules in their atmospheres, like zirconium oxide (which is how they're identified).
Most stars on the AGB have only moderate mass-loss rates (~10-7 solar masses / year), but as they evolve, the mass-loss rate will increase. The general belief is that the most highly evolved stars on the AGB are long-period Mira variables with mass-loss rates ~10-5 solar masses / year or more. At these rates, only a few thousand years are required to completely embed the star deep within its own circumstellar dust shell. Such a star will disappear completely from the optical skies, but it will re-emerge as a very bright infrared source.
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Last modified 29 December, 2014. © Gregory C. Sloan.