The Fate of the Sun – What Will Happen?

When First Formed

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

Within the SunIn 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.

A Dying Fire Produces Ashes

After About 10 Billion YearsAt 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.

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

He

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!)

“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.

So What Happens Then? the central heat diffuses outward, and is

radiated away; so the sustaining pressure is reduced;

and the sun’s gravity will start to win!

That is, the central parts of the sun will contract.

Furthermore… this contraction leads to the release

of gravitational potential energy.

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.)

So: a Simple ExpectationYou expect the Sun to shrink and get

hotter.

Indeed, it does so – but only in the inner parts.

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.

Like So:

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!

This Doesn’t Go On Forever

The core is shrinking and getting hotter.

Eventually, the central parts reach a temperature of ~ 108 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.

Stability Restored!In the new reactions

He + He heavier elements,(mainly carbon)

This releases energy, keeping the core hot. This stops any further contraction.

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 1010 yrs.

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.

But each one of these ‘completed reactions’ 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.

Binding Energy Again

An Unceasing FightThe 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)

and each reaction produces less energy than H He

Analogy:Keeping Warm!

Good Fuel – and Bad

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.

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

get still hotter in the middle, and ignite some new fusion reaction that creates even heavier nuclei

Desperate Measures(Each successivefuel that kicks in is of progressively lower quality)

How Long Can This Last?

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??