Evolution of stars of different masses are represented in the Hertzsprung-Russell diagram. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD The asymptotic giant branch is marked AGB in the case of a star of 2 solar masses.

The Asymptotic Giant Branch is the name given to a region of the Hertzsprung-Russell Diagram populated by evolving low to medium-mass stars. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth This is a period of stellar evolution undertaken by all low to intermediate mass stars (0. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime 6-10 solar masses) late in their life.

Stellar evolution

When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus The star's luminosity increases greatly, and it becomes a red giant, following a track leading into the upper-right hand corner of the HR diagram. A red giant is a luminous Giant star of low or intermediate mass (roughly 0

Eventually, once the temperature in the core has reached approximately 3x10⁸K, helium burning begins. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves back towards the left hand side of the HR diagram. This is the horizontal branch (for population II stars) or red clump (for population I stars). The horizontal branch ( HB) is a stage of Stellar evolution which immediately follows the Red giant branch in stars whose masses are similar to the Sun's In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen The red clump is a feature in the Hertzsprung-Russell diagram of Stars The red clump is considered the metal -rich counterpart to the Horizontal branch In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen After the completion of helium burning in the core, the star again moves to the right and upwards on the diagram. Its path is almost aligned with its previous red giant track, hence the name of Asymptotic Giant Branch. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 In pure and Applied mathematics, particularly the Analysis of algorithms, real analysis and engineering asymptotic analysis is a method of describing Stars at this stage of stellar evolution are known as AGB stars.

The AGB stage

The AGB phase is divided into two parts, the early AGB (E-AGB) and the thermally pulsing AGB (TP-AGB). During the E-AGB phase the main source of energy is helium fusion in a shell around a core consisting mostly of carbon and oxygen. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the During this phase the star swells up to giant proportions to become a red giant again. The star may become as large as one astronomical unit. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the After the helium shell runs out of fuel, the TP-AGB starts. Now the star derives its energy from fusion of hydrogen in a thin shell, inside of which lies the now inactive helium shell. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical However, on periods of 10,000 to 100,000 years the helium shell switches on again, and the hydrogen shells switches off, a process known as a helium shell flash. A helium flash is the sudden beginning of Helium fusion in the core of intermediate mass Stars of less than about 2 Due to these flashes, which only last a few thousand years, material from the core region is mixed into the outer layers, changing its composition, a process referred to as dredge-up. Because of this dredge-up AGB stars may show S-process elements in their spectra. The S-process or slow-neutron -capture-process is a Nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in Subsequent dredge-ups can lead to the formation of Carbon stars. A carbon star is a late type giant star similar to the Red giants (or occasionally Red dwarf) Star whose atmosphere contains more Carbon than

AGB stars are typically long period variables, and suffer large mass loss in the form of a stellar wind. A long period variable is a type of Variable star in which variations in brightness occur over long timescales of months or years A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. A star may lose 50 to 70% of its mass during the AGB phase.

Circumstellar envelopes of AGB stars

The extensive mass loss of AGB stars means that they are surrounded by an extended circumstellar envelope (CSE). Given a mean AGB lifetime of one Myr and an outer velocity of 10 km/s, its maximum radius can be estimated as 10¹⁹ cm. This article is about the unit of time For the Magic the Gathering creatures see Creatures of Magic The Gathering. This is a maximum value since the wind material will start to mix with the interstellar medium at very large radii, and it also assumes that there is no velocity difference between the star and the interstellar gas. Dynamically most of the interesting action is quite close to the star, where the wind is launched and the mass loss rate is determined. However, the outer layers of the CSE show chemically interesting processes, and are due to size and lower optical depth easier to observe. Optical depth is a measure of transparency, and is defined as the negative Logarithm of the fraction of Radiation (or Light) that is scattered

The temperature of the CSE is set by heating and cooling processes for the gas and the dust, but is dropping with radial distance from the photosphere of the star's of some 2200 K. A chemical picture of an AGB CSE outwards was suggested by Marwick (2000) something like this:

1. Photosphere: LTE chemistry;
2. Pulsating stellar envelope: Shock chemistry;
3. Dust formation zone;
4. Chemically quiet;
5. Interstellar UV radiation: Photodissociation of molecules - complex chemistry

Here the dichotomy between oxygen-rich and carbon-rich stars will have an inital say. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties Photodissociation, photolysis, or photodecomposition is a Chemical reaction in which a Chemical compound is broken down by Photons In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 In the dust formation zone the so-called refractory elements (Fe, SI, Mg, . . . ) are removed from the gas phase and end up in dust grains. The newly formed dust will immediately assist in surficide reactions. Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including Solid - Liquid The stellar winds from AGB stars are sites of cosmic dust formation, and are believed to be the main production sites of dust in the universe.

The stellar winds of AGB stars are also often the site of maser emission. An astrophysical maser is a naturally occurring source of stimulated Spectral line emission typically in the Microwave portion of the Electromagnetic The masering molecules are SiO, H₂O, and OH.

After these stars have lost nearly all of their envelopes, and only the core regions remain, they evolve further into short lived preplanetary nebulae. A protoplanetary nebula or preplanetary nebula (PPN is an Astronomical object which is at the short-lived episode during a Star 's rapid Stellar The final fate of the AGB envelopes are represented by planetary nebulae (PNe). A planetary nebula is an Emission nebula consisting of a glowing shell of Gas and plasma formed by certain types of Stars when they die

References

• H. J. Habing, Hans Olofsson; Asymptotic Giant Branch Stars, Springer (2004). ISBN 0387008802.

See also

• Carbon stars
• Mira
• Planetary nebulae
• Red giant