The Evolution of the Sun (from astro... 
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The Evolution of the Sun
http://astrosun2.astro.cornell.edu/academics/courses//astro201/evol_sun.htm
- A cloud of gas and dust begins to contract under the force of gravity. In regions of star birth, we find gaseous nebulae and molecular clouds. These sites of pre-birth are dark patches called globules.
- The protosun collapsed. As it did, its temperature rose to about 150,000 degrees and the sun appeared very red. Its radius was about 50 present solar radii.
- When the central temperature reaches 10 million degrees, nuclear burning of hydrogen into helium commences.
- The star settles into a stable existence on the Main Sequence, generating energy via hydrogen burning. This is the longest single stage in the evolutionary history of a star, typically lasting 90% of its lifetime. Thermonuclear fusion within the Sun is a stable process, controlled by its internal structure.
- The hydrogen in the core is completed burned into helium nuclei. Initially, the temperature in the core is not hot enough to ignite helium burning. With no additional fuel in the core, fusion dies out. The core cannot support itself and contracts; as it shrinks, it heats up. The rising temperature in the core heats up a thin shell around the core until the temperature reaches the point where hydrogen burning ignites in this shell around the core. With the additional energy generation in the H-burning shell, the outer layers of the star expand but their temperature decreases as they get further away from the center of energy generation. This large but cool star is now a red giant, with a surface temperature of 3500 degrees and a radius of about 100 solar radii.
- The helium core contracts until its temperature reaches about 100
million degrees. At this point,
helium burning ignites, as helium is converted into carbon (C) and
oxygen (O). However, the core cannot expand as much as required to
compensate for the increased energy generation caused by the helium
burning. Because the expanion does not compensate, the temperature stays
very high, and the helium burning proceeds furiously. With no safety
valve, the helium fusion is uncontrolled and a large amount of energy is
suddenly produced. This helium flash occurs within a few hours
after helium fusion begins.
The core explodes, the core temperature falls and the core contracts again, thereby heating up. When the helium burns now, however, the reactions are more controlled because the explosion has lowered the density enough. Helium nuclei fuse to form carbon, oxygen, etc..
- The star wanders around the red giant region, developing its distinct layers, eventually forming a carbon-oxygen core.
- When the helium in the core is entirely converted into C, O, etc., the core again contracts, and thus heats up again. In a star like the Sun, its temperature never reaches the 600 million degrees required for carbon burning. Instead, the outer layers of the star eventually become so cool that nuclei capture electrons to form neutral atoms (rather than nuclei and free electrons). When atoms are forming by capturing photons in this way, they cause photons to be emitted; these photons then are readily available for absorption by neighboring atoms and eventually this causes the outer layers of the star to heat up. When they heat up, the outer layers expand further and cool, forming more atoms, and releasing more photons, leading to more expansion. In other words, this process feeds itself.
- The outer envelope of the star blows off into space, exposing the hot, compressed remnant core. This is a planetary nebula .
- The core contacts but carbon burning never ignites in a one solar mass star. Contraction is halted when the electrons become degenerate, that is when they can no longer be compressed further. The core remnant as a surface temperature of a hot 10,000 degrees and is now a white dwarf .
- With neither nuclear fusion nor further gravitational collapse possible, energy generation ceases. The star steadily radiates is energy, cools and eventually fades from view, becoming a black dwarf.
With this understanding of how the Sun will evolve, we can follow its evolution on the HR diagram.
Suggested readings:
- "Giants in the Sky: The Fate of the Sun", Kaler, James, B., 1993, Mercury, Mar-Apr, pp. 34-41.
http://astrosun2.astro.cornell.edu/academics/courses//astro201/evol_sun.htm
- If the Sun became a red giant will t... LCD
18 y
3,772
This is a reply to # 274,957
If the Sun became a red giant will the Earth still be able to support life here?
http://curious.astro.cornell.edu/question.php?number=48
Jagadheep: No, the Earth will not be able to support life if the Sun becomes a giant star. Giant stars have large radii as their name implies. When the Sun becomes a giant star, it may become so large as to engulf Earth, in which case the planet will be destroyed. Even if this does not happen, the sun will expand so far out that the temperatures on Earth will become extremely high so that all oceans will evaporate away, and there will be no water left on Earth. So, no life which depends on water will be able to survive.
When the sun starts expanding in about 5 billion years, what will be the first signs of this process?
Karen: The Sun is a relatively low mass star and as such its death will be relatively mundane (at least by Astronomical standards). The Sun's luminosity and radius have been increasing since it started life and will continue to gradually increase in this manner for another 4.5 billion years or so. When the hydrogen in the core is all used up energy generation will stop there, however it will continue in a thin shell around the core. It is this which makes the Sun expand since it heats up the outer layers more. Funnilly enough this makes the very outer layer cooler so that sun will actually redden as well as becoming brighter and expanding. I suspect that this reddening might be the first signs the the Sun has left the Main Sequence.
How long will it take from the process starts til the earth is engulfed, or at least uninhabitable?
Timescales are difficult in evolutionary models of stars. It's not clear quite what'll happen to the Earth either. It could be engulfed by the Sun, or it might get pushed out into a larger orbit and freeze as the Sun expands. The Sun will be a Red Giant for a few million years. By then I think it's safe to say that the Earth will be uninhabitable.
Will the earth catch fire while humans still live here or will the planet simply dry out?
I think that the temperature would kill life before anything caught on fire. It would only need to be 100F or so all the time for humans to be wiped out (we don't survive long in the desert, right).
Is it probable that life on earth will survive that long, or will asteroids wipe us out before then?
Probability wise, it's likely that the Human Race will have been killed off by the time the Sun leaves the Main Sequence. I don't think that any species in history has dominated the Earth for that long. Of course we could be the first....
Related questions:
- Is the Sun expanding? Will it ever explode?
- Can supernovae hinder the formation of life in galaxies?
- Will the sun go supernova in six years and destroy Earth (as seen on Yahoo)?
More questions about The Earth: Previous | Next
More questions about The Sun: Previous | Next
http://curious.astro.cornell.edu/question.php?number=48
