1. The Fate of the Sun – What Will Happen?

2. When First Formed The sun is a big ball of ~ 2/3 H, 1/3 He. It is uniform in composition.

3. Within the Sun In the outermost parts, there are no nuclear reactions: it’s not hot enough! At the very center, there are very vigorous reactions: it is extremely hot there. At intermediate zones, reactions occur at moderate rates.

4. A Dying Fire Produces Ashes

5. After About 10 Billion Years At the very center, all the H has turned to He. The core is now essentially pure Helium – the “ashes” of the p-p cycle. (Not literal “ashes” like a wood fire leaves behind, of course.) Nuclear reactions cease! At intermediate zones, only some of the H has turned to He. So reactions continue there, at a moderate pace, because there is some H fuel still left to consume. In the outer parts, the composition stays unchanged. No reactions are happening there at all.

6. Like So [except that there are no ‘sharp’ boundaries] He

7. Why Do The Central Reactions Cease? The He nuclei have two protons each, and repel one another very strongly.   It is not hot enough to force the He nuclei to merge (That requires a temperature of ~100 million degrees!)

8. “Shell Burning” 4 H  He continues in the surrounding shell, where the H has not yet been totally consumed. The central fires go out, however.

9. So What Happens Then? the central heat diffuses outward, and is radiated away; the central heat diffuses outward, and is radiated away; so the sustaining pressure is reduced; so the sustaining pressure is reduced; and and the sun’s gravity will start to win! the sun’s gravity will start to win! That is, the central parts of the sun will contract.

10. Furthermore… this contraction leads to the release of gravitational potential energy. this contraction leads to the release of gravitational potential energy. so, the central parts will get hotter so, the central parts will get hotter (This is the paradoxical behaviour we mentioned earlier: as the Sun radiates away its central energy, it eventually gets hotter rather than cooling off.)

11. So: a Simple Expectation You expect the Sun to shrink and get hotter. Indeed, it does so – but only in the inner parts.

12. Deceptive Behaviour!! Amazingly, the outer parts of the Sun SWELL UP (it gets MUCH BIGGER) and cool off (it becomes redder in colour). It becomes a RED GIANT.

13. Like So:

14. This Expansion Takes A Lot of Energy! Remember that you are ‘lifting’ trillions of tons of material up, through a distance of millions of kilometers, against the pull of the Sun’s gravity! The collapse of the core releases a lot of energy, but only some of it goes into heating the core. A lot of it goes into ‘puffing up’ the outer layers of the Sun!

15. This Doesn’t Go On Forever The core is shrinking and getting hotter. Eventually, the central parts reach a temperature of ~ 10 8 Kelvin – about 10x as hot as the sun’s core is now. This is hot enough for Helium nuclei to fuse together, so nuclear reactions start again.

16. Stability Restored! In the new reactions He + He  heavier elements, (mainly carbon) This releases energy, keeping the core hot. This stops any further contraction.

17. Here’s an Example Calculation [I won’t ask you to repeat this – but do learn from it!] Suppose we started with 12 gazillion protons in the hot central core of the Sun. [Note that “gazillion” is not a real word!] They fuse (in groups of 4) to form 3 gazillion He nuclei. That is, 3 gazillion ‘completed reactions’ have occurred. Each one produces some amount of energy (call it X). The total energy released is 3 gazillion X. This maintains the Sun on the main sequence for 10 10 yrs.

18. Subsequently, As a Red Giant The center contains 3 gazillion He nuclei They can combine (3 at a time) to form 1 gazillion Carbons, through a total of 1 gazillion completed reactions. through a total of 1 gazillion completed reactions. But each one of these ‘completed reactions’ produces quite a bit less than X amount of energy. produces quite a bit less than X amount of energy. (This is because of the Binding Energy curve [next panel]) The total energy released is considerably less than 1 gazillion X.

19. Binding Energy Again

20. An Unceasing Fight The inward pull of gravity is unceasing, and enough energy must be generated to support the star. The new He  C reactions do that! But the He fuel cannot last as long as the H  He phase did because there are fewer reactions (fewer particles) there are fewer reactions (fewer particles) and each reaction produces less energy than H  He and each reaction produces less energy than H  He

21. Analogy: Keeping Warm!

22. Good Fuel – and Bad

23. Consequently: The ‘renewed sun’ will have a considerably shorter life as a red giant than it did on the main sequence. (In fact, it will last only ~10% as long.) We are on the ‘slippery slope’ to stellar death.

24. What Next? By the same reasoning: When He runs out, leaving Carbon ‘ash’ in the core, we expect that the sun will: cease producing energy, lose pressure support, and contract in the core cease producing energy, lose pressure support, and contract in the core get still hotter in the middle, and ignite some new fusion reaction that creates even heavier nuclei get still hotter in the middle, and ignite some new fusion reaction that creates even heavier nuclei

25. Desperate Measures (Each successive fuel that kicks in is of progressively lower quality)

26. How Long Can This Last?

27. Stars Seem to be Doomed! As each fuel source is used up, a less efficient one kicks in, but for a dramatically shorter period of time. Moreover, the ‘peak’ of the binding energy curve means that eventually there will be no further energy supply to be tapped. Stars seem to be doomed. Will gravity win??