Evolution of stars

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categorie: Engleza

nota: 9.82

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The actual process of star formation remains shrouded in mystery because stars form in dense, cold molecular clouds whose dust obscures newly formed stars from our view.

For reasons which are not fully understood, but which may have to do with collisions of molecular clouds, or shockwaves passing through molecular clouds as the clouds pass through spiral structure in galaxies, or m[...]
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The actual process of star formation remains shrouded in mystery because stars form in dense, cold molecular clouds whose dust obscures newly formed stars from our view.

For reasons which are not fully understood, but which may have to do with collisions of molecular clouds, or shockwaves passing through molecular clouds as the clouds pass through spiral structure in galaxies, or magnetic-gravitational instabilities (or, perhaps all of the above) the dense core of a molecular cloud begins to condense under its self-gravity, fragmenting into stellar mass clouds which continue to condense forming protostars.

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6. Red Giant - Helium Flash
As the Helium core of the star contracts, nuclear reactions continue in a shell surrounding the core. Initially the temperature in the core is too low for fusion of helium, but the core-contraction liberates gravitational energy causing the helium core and surrounding hydrogen-burning shell to increase in temperature, which, in turn, causes an increase in the rate of nuclear reactions in the shell.

In this instance, the nuclear reactions are producing more than enough energy to satisfy the luminous energy output. This extra energy output pushes the stellar envelope outward, against the pull of gravity, causing the outer atmosphere to grow by as much as a factor of 200. The star is now cool, but very luminous - a Red Giant.

(You do the arithmetic: 200 x 700,000km = ?; where will the outer radius of the sun be?)

• Age: 100 million yrs from Point 5
• R ~ 200Rsun
• Tcore = 200,000,000K
• Tsurface = 3500K
• Energy Source: P-P Chain in shell around core;

Ignition of Triple-Alpha Process.

The contraction of the core causes the temperature and density to increase such that, by the time the temperature is high enough for Helium nuclei to overcome the repulsive electrical barrier and fuse to form Carbon, the core of the star has reached a state of electron degeneracy. Degeneracy comes about due to the Pauli Exclusion Principle, which prohibits electrons from occupying identical energy states.

The core of the Red Giant is so dense that all available lower energy states are filled up. Because only high-energy states are available, the core resists further compression -- there is a pressure due to the electron degeneracy. This pressure has a significant difference from pressure produced by the Ideal Gas Law -- it is independent of temperature. This removes a key element in the feedback-stability mechanism that regulates hydrogen burning on the main sequence.

H-R Diagram from Helium Burning to White Dwarf.

7. Helium Burning Main Sequence

Once again the core of the star readjusts to allow for a new source of energy, in this case fusion of Helium to form Carbon via the Triple-Alpha Process. The Triple alpha process releases only about 20% as much energy as hydrogen burning, so the lifetime on the Helium Burning Main Sequence is only about 2 billion years.

• Age: About 10,000 yrs from point 6.
• Tsurface = 9000K
• Tcore = 200,000,000K
• Energy Source: Triple-alpha process in core;

P-P Chain in shell
During this phase some Carbon and Helium will fuse
12C + 4He --> 16O
resulting in the formation of a Carbon-Oxygen core. When the Helium is exhausted in the core of a star like the sun, no further reactions are possible. Helium burning may occur in a shell surrounding thecore for a brief period, but the lifetime of the star is essentially over.

8. Planetary Nebula

When the helium is exhausted in the core of a star like the sun, the C-O core will begin to contract again. Central temperatures will never reach high enough values for Carbon or Oxygen burning, but the Helium and Hydrogen burning shells will conyinue burning for a while. Throughout the star's lifetime it is losing mass via a stellar wind, like the solar wind.

This mass loss increases when the star swells up to the size and low gravity of a Red Giant. During Helium Burning, thermal pulses, caused by the extreme temperature sensitivity of the 3-alpha Process, can cause large increases in luminosity with accompanying mass ejection. During Helium Shell Burning, a final thermal pulse produces a giant "hiccough" causing the star to eject as much of 10% of its mass, the entire outer envelope, revealing the hot inner regions with temperatures in excess 100,000K, shown in this animation of the Helix, below.

The resulting Planetary Nebuala is the interaction of the newly ejected shell of gas with the more slowly moving ejecta from previous events and the ultraviolet light from the hot stellar remnant, which heats the gas and causes it to fluoresce. The Ring Nebula in Lyra (Messier Database, Web Nebulae) shown below is HST images of Planetary Nebulae: The Ring Nebula and the young Planetary Nebula known as MyCn18, the Hourglass Nebula.

• More about Planetary Nebulare from George Jacoby's (b& w) Planetary Nebula Gallery.
• Planetary Nebulae at the SED's Messier Gallery.
• The Planetary Nebula Observer's HomePage includes more links to Planetary Nebula Resources.
• Univ. of Calgary Planetray Nebula Homepage with theoretical models of PN emission structure.
• Bruce Balick's HST Images of Planetary Nebulae.

9. White Dwarf

As the nebula disperses, the shell nuclear reactions die out leaving the stellar remnant, supported by electron degeneracy, to fade away as it cools down. The white dwarf is small, about the size of the earth, with a density of order 1 million g/cm3, about equivalent to crushing a volkswagen down to a cubic centimeter or a "ton per teaspoonful."

• R ~ Rearth (a few thousand km)
• Tsurface = 30000K - 5000K
• Energy Source: "Cooling Off".
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