Life of a star - the Sun
Throughout its life star must battle the force responsible for its creation - GRAVITY
Gravity draws together raw materials - gas and dust
- pulls inward, shrinking star
nuclear fusion - joining of light nuclei to form heavier provides counter force as unleashes vast amounts of energy that streams outward
Stars start when diffuse, cold gas cloud starts collapsing (probably cold because hot cloud would not collapse).
- Sun formed 4.7 billion years ago from cloud gas and dust triggered by supernova explosion -
We conclude this because the sun is not "primitive". It contains other elements in addition to H, He. These elements include
As matter falls inward gravitational is energy converted to heat. Also the explosion made many unstable elements - radioactive decay of these contributed to raise T within collapsing dust cloud.
As it collapses, it rotates faster and faster to conserve angular momentum. The rapid rotation forces some of original material to form disc around central body. When reaches about 800,000 oK proton-proton chain starts. Contraction continues, T increases, luminosity increases, much gas dissipated.
Main sequence in star life = when rate nuclear energy production in core counteracts gravitational collapse. This is most of its life.
- relatively quiescent fusing H to He in core. - sun started He 27%, now 62% will be 100% in 6 billion years
Gradually brightens. During next 1.1 billion years sun will get 10% brighter (earth oceans will boil away)
6.5 billion years from now sun will have more than doubled its present luminosity, exhausted H in core - H fusion moves outward, outer layers swell - and cool
- red giant (process takes about 1.3 billion years) Becomes 166 times present size (it will extend to between Mercury and Venus), luminosity 2300 times as great as now.
- as grows also sheds mass so less gravity - so planets may move further out
core contracts (170,000 km to 3,500 km) grows hotter, mostly He. As density increases, T rises to 100 million oK till He ignites. 2 He nuclei fuse to Be (short life 10-16sec) or 3 He nuclei fuse to 1 Carbon. 4 He nuclei fuse to one Oxygen.
luminosity of core may reach 10 million times that of surface. Surface will become less bright, radius will shrink because expanding He core cools surrounding H shell so less heat to surface.
many other nuclear reactions - some produce neutrons. These may be absorbed by nucleus of an atom. If more than one or two neutrons absorbed into nucleus it becomes unstable, emits electron and get next element up (neutrons changes to proton) This is process to make all elements and isotopes heavier than Boron up to and including Bismuth-209 (none heavier as are unstable)
- 100 million years quiet period till burns all He, then He in a shell of gas just outside core ignites. Still have shell of burning H
- core contracts as no fuel, series explosions, triggers 20 million years expansion and brightening, and shed additional mass.
- grows to about earth's present orbit but earth will have moved away
Finally sun ejects outer layers, see collapsed core - white dwarf. Size, about that present earth. Stays in this state for many billion years then gradually fades to dark dwarf.
Earth may still be there, not engulfed because attraction of sun less as smaller so earth has moved away
If star more massive than earth continue cyclical process. Carbon core contracts, T to a billion oK can then fuse Oxygen. Get a number of nuclear reactions that produce a flood of alpha particles and neutrons. Finally get Fe -which is at peak of binding energy curve. Further fusion consumes energy rather than producing.
Stars with mass 25X sun exhausts H in core in a few million years, fuse He for .5 million years, fuse C for 600 years, O for 6 months, Si for 1 day. Ends up with layered structure, Fe core surrounded by concentric shells progressively lighter elements. No more fusion in Fe core so catastrophic collapse takes less than 1 second. Release huge among of power - star blows up - supernova explosion. 90% original mass flies away in space.
More information on the Sun