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Future Solar Evolution .... The Fate of Our Sun !


Stars the size of our sun won't die in a spectacular explosive supernova,
but their deaths leave a temporary beauty that is as unique as each star.

The sun is currently about half way through it's life as a main sequence star.
It is becoming gradually more luminous all the time.
It's fuel is the hydrogen in it's core.

What will happen to the sun after it's time on the main sequence?

The Sun will eventually burn up all of it's hydrogen core, leaving a helium ash core surrounded by hydrogen.

When the hydrogen shell burns it will expand the sun into a Red Giant while continually increasing the helium ash core.

The colour of a star helps identify it's temperature - a hot star is white while a cool star is red.

Our sun will expand until it is almost as large as the current orbit of the Earth, definately a giant star.

The gradual increase in the core temperature that has been happening due to the increase in pressure suddenly causes the helium core to burn, helium fusion now begins to create a carbon ash core.

This renewed core fuel burning causes the red giant sun to contract and get hotter.

The helium core is eventually exhausted leaving a carbon core with a helium shell.

Once again, increasing core temperatures result in shell fuel ignition and the helium shell begins fusion depositing more carbon ash into the core.

Once again the sun will have expanded, this time it will grow massive, as much as 1000 times it current size.

Within the core of the massive giant, instabilities cause the sun to pulsate, contracting and expanding a number of times, allowing the outer layers of atmosphere to be cast outward from sun.

Now without a fuel to burn, the core compacts, shrinking away from the expanding outer layers of gas.

These layers of gas are excited by the particles and radiation given off the hot central exposed core and glow as a planetary nebula.



After about another 75,000 years the core becomes what is called a white dwarf star, composed mostly of carbon.
The white dwarf will eventually cool to a black dwarf, a dark remnant of what was once our brilliant Sun.

Maybe one day, a nearby supernovae will cause it to be recycled back into the interstellar medium
to eventually help form a new star.

Solar Evolution on the H-R Diagram

Click on the diagram to determine the surface temperature,
and luminosity of the Sun along its evolutionary path (in red).
The Main Sequence is in blue.
The temperature and luminosity values are given in terms of the Sun's values.

Java Code by Dr S Morgan - University of Northern Iowa
The Java Runtime Environment is required to run this applet - visit www.java.com for the free software.

1. Main Sequence Sun - burning a hydrogen core depositing helium ash with a continual gradual expansion. - (Main Sequence)
2. Expansion and cooling - burning a hydrogen shell, depositing more helium. - (Sub Giant Branch)
3. Rapid expansion into a Red Giant. - (Red Giant Branch)
4. Helium Flash causes sun to contract and become warmer - burning helium core. - (Early Asymptotic Giant Branch)
5. Huge increase in size causes lower temperatures - burning a helium shell - (Thermal-Pulse Asymptotic Giant Branch)
6. Instability causes pulsations which create a planetary nebula. - (Post Asymptotic Giant Branch)
7. Core contracts and cools to become a White Dwarf. - (Pre-White Dwarf)


* the term "burning", when used to refer to fuels in stars, refers to the fusion process involving those fuels.
* the term "ash" is used to refer to the unburning result of a fusion process.



Some references & resources:-
Free NASA Resources for Educators
The Once & Future Sun
Wikipedia Solar Evolution page
NASA & Hubble Space Telescope images
Columbia Uniiversity - Solar Evolution
Dr S Morgan
Solar evolution and the distant future of Earth - pdf
Astronomy Today
UCSD Astronomy lecture





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02/05/2007