by neufer » Thu Mar 05, 2009 11:20 pm
http://en.wikipedia.org/wiki/Asymptotic_Giant_Branch wrote:
<<When a [low to intermediate mass] 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. 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. The star may become as large as one astronomical unit. After the helium shell runs out of fuel, the [T]hermally [P]ulsing [A]symptotic [G]iant
ranch (TP-AGB) phase starts. Now the star derives its energy from fusion of hydrogen in a thin shell, inside of which lies the now inactive helium shell. However, over periods of 10,000 to 100,000 years, the helium shell switches on again, and the hydrogen shell switches off, a process known as a helium shell flash or thermal pulse. Due to these pulses, 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. Subsequent dredge-ups can lead to the formation of Carbon stars.
[A]symptotic [G]iant ranch stars are typically long period variables, and suffer large mass loss in the form of a stellar wind. A star may lose 50 to 70% of its mass during the AGB phase.
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 to be roughly 30 light years. This is a maximum value since the wind material will start to mix with the interstellar medium at very large radii. The outer layers of the CSE show chemically interesting processes, and due to size and lower optical depth are easier to observe.
After these stars have lost nearly all of their envelopes, and only the core regions remain, they evolve further into short lived preplanetary nebulae. The final fate of the AGB envelopes are represented by planetary nebulae (PNe).>>
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http://en.wikipedia.org/wiki/Mira wrote:
<<Mira A is currently an Asymptotic Giant Branch (AGB) star, in the thermally pulsing AGB phase. Each pulse lasts a decade or more, and an amount of time on the order of 10,000 years passes between each pulse. With every pulse cycle Mira increases in luminosity and the pulses grow stronger. This is also causing dynamic instability in Mira, resulting in dramatic changes in luminosity and size over shorter, irregular time periods.
The overall shape of Mira A has been observed to change, exhibiting pronounced departures from symmetry. These appear to be caused by bright spots on the surface that evolve their shape on time scales of 3–14 months. Observations of Mira A in the ultraviolet band by the Hubble Space Telescope have shown a plume-like feature pointing toward the companion star.
Mira A is a well-known example of a category of variable stars known as Mira variables. It—and the other ca 6000-7000 known stars of this class—are all red giants whose surfaces oscillate in such a way as to increase and decrease in brightness over periods ranging from about 80 to more than 1000 days.
In the particular case of Mira, its increases in brightness take it up to about magnitude 3.5 on average, placing it among the brighter stars in the Cetus constellation. Individual cycles vary too; well-attested maxima go as high as magnitude 2.0 in brightness and as low as 4.9, a range almost 15 times in brightness, and there are historical suggestions that the real spread may be three times this or more. Minima range much less, and have historically been between 8.6 and 10.1, a factor of four times in luminosity. The total swing in brightness from absolute maximum to absolute minimum (two events which did not occur on the same cycle) is 1700 times. Interestingly, since Mira emits the vast majority of its radiation in the infrared, its variability in that band is only about two magnitudes. The shape of its light curve is of an increase over about 100 days, and a return twice as long.
..................................................
- . The Tempest – Act 1, Scene 2
.
MIRA-nda: The sky, it seems, would pour down stinking pitch,
. But that the sea, mounting to the welkin's cheek,
. Dashes the fire out.
..................................................
In 1638 Johann Holwarda determined a period of the star's reappearances, eleven months; he is often credited with the discovery of Mira's variability. Johannes Hevelius was observing it at the same time and named it "Mira" (meaning "wonderful, astonishing") in 1662's Historiola Mirae Stellae, for it acted like no other known star. Ismail Bouillaud then estimated its period at 333 days, less than one day off the modern value of 332 days (and perfectly forgivable, as Mira is known to vary slightly in period, and may even be slowly changing over time).>>
[quote=" http://en.wikipedia.org/wiki/Asymptotic_Giant_Branch "]
<<When a [low to intermediate mass] 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. 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. The star may become as large as one astronomical unit. After the helium shell runs out of fuel, the [T]hermally [P]ulsing [A]symptotic [G]iant [B]ranch (TP-AGB) phase starts. Now the star derives its energy from fusion of hydrogen in a thin shell, inside of which lies the now inactive helium shell. However, over periods of 10,000 to 100,000 years, the helium shell switches on again, and the hydrogen shell switches off, a process known as a helium shell flash or thermal pulse. Due to these pulses, 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. Subsequent dredge-ups can lead to the formation of Carbon stars.
[A]symptotic [G]iant [B]ranch stars are typically long period variables, and suffer large mass loss in the form of a stellar wind. A star may lose 50 to 70% of its mass during the AGB phase.
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 to be roughly 30 light years. This is a maximum value since the wind material will start to mix with the interstellar medium at very large radii. The outer layers of the CSE show chemically interesting processes, and due to size and lower optical depth are easier to observe.
After these stars have lost nearly all of their envelopes, and only the core regions remain, they evolve further into short lived preplanetary nebulae. The final fate of the AGB envelopes are represented by planetary nebulae (PNe).>>[/quote]--------------------------------------------
[quote=" http://en.wikipedia.org/wiki/Mira "]
<<Mira A is currently an Asymptotic Giant Branch (AGB) star, in the thermally pulsing AGB phase. Each pulse lasts a decade or more, and an amount of time on the order of 10,000 years passes between each pulse. With every pulse cycle Mira increases in luminosity and the pulses grow stronger. This is also causing dynamic instability in Mira, resulting in dramatic changes in luminosity and size over shorter, irregular time periods.
The overall shape of Mira A has been observed to change, exhibiting pronounced departures from symmetry. These appear to be caused by bright spots on the surface that evolve their shape on time scales of 3–14 months. Observations of Mira A in the ultraviolet band by the Hubble Space Telescope have shown a plume-like feature pointing toward the companion star.
Mira A is a well-known example of a category of variable stars known as Mira variables. It—and the other ca 6000-7000 known stars of this class—are all red giants whose surfaces oscillate in such a way as to increase and decrease in brightness over periods ranging from about 80 to more than 1000 days.
In the particular case of Mira, its increases in brightness take it up to about magnitude 3.5 on average, placing it among the brighter stars in the Cetus constellation. Individual cycles vary too; well-attested maxima go as high as magnitude 2.0 in brightness and as low as 4.9, a range almost 15 times in brightness, and there are historical suggestions that the real spread may be three times this or more. Minima range much less, and have historically been between 8.6 and 10.1, a factor of four times in luminosity. The total swing in brightness from absolute maximum to absolute minimum (two events which did not occur on the same cycle) is 1700 times. Interestingly, since Mira emits the vast majority of its radiation in the infrared, its variability in that band is only about two magnitudes. The shape of its light curve is of an increase over about 100 days, and a return twice as long.
..................................................
[list]. The Tempest – Act 1, Scene 2
.
MIRA-nda: The sky, it seems, would pour down stinking pitch,
. But that the sea, mounting to the welkin's cheek,
. Dashes the fire out.[/list]
..................................................
In 1638 Johann Holwarda determined a period of the star's reappearances, eleven months; he is often credited with the discovery of Mira's variability. Johannes Hevelius was observing it at the same time and named it "Mira" (meaning "wonderful, astonishing") in 1662's Historiola Mirae Stellae, for it acted like no other known star. Ismail Bouillaud then estimated its period at 333 days, less than one day off the modern value of 332 days (and perfectly forgivable, as Mira is known to vary slightly in period, and may even be slowly changing over time).>>[/quote]