The Solar Engine in Detail

The Solar Engine in Detail
Our Objective
To test whether we are right about the
nuclear reactions happening in the sun!
Only then can we confidently predict its
future (and the eventual demise of the
Two Complications
Astrophysicists have to understand the
details of all the steps in the reaction
chain; and
We need to know if there are any other
reactions that contribute significantly.
Remember the Chain of Events
- but not all steps are equally likely:
some happen more readily than others
An Analogy
Getting Through the Airport
Once on the ground, your airplane taxis to the gate.
The walkway is attached, the doors are opened.
Passport control  possible diversion to health and immigration.
Baggage collection (long delay?!)
Customs clearance (perhaps even longer?)
Finding the gate for your connecting flight, check-in, security,...
The Slowest Link
Determines Your Progress
A super-fast luggage carousel is irrelevant if you
have to spend two hours at customs inspection!
Similarly In the Sun
Not all steps are equally speedy.
An average proton enjoys a long life in the
sun, whizzing around for billions of years.
(Only a very tiny fraction get consumed at
any given time.)
…but then
Once a deuterium forms, it is almost
instantly consumed.
(Analogy: you spend hours waiting for
immigration clearance, but then find your
luggage in a jiffy!)
So, In the P-P Cycle
H + H  D + positron + neutrino
(after 14 billion years for an average proton)
D + H  He3 + gamma ray
(after 6 seconds)
He3 + He3  He4 + 2 protons + gamma ray
(after 1 million years)
It is a question of
cross-sections for
different interactions.
Different Probability
of Interacting
A typical hydrogen nucleus (a freemoving proton) has a long life in the
core of the sun!
There will be very little deuterium, since
it gets consumed so quickly once it is
Next: Be Sure to Consider All Contributions
(How Do You Heat Your House?)
Other Reactions Can Happen
(producing small amounts of Be, Li, and B)
Even More Reactions
A star made of pure Hydrogen would undergo the
pp cycle (it would have no choice!).
But in almost all stars, some heavier elements are
present – including a bit of Carbon, Nitrogen,
and Oxygen.
In such stars, an independent reaction cycle can
occur: the CNO cycle.
The CNO Cycle
(No need to know these details!)
Net effect: 4H  He + energy!
CNO play a role, but it’s really H  He
So the CNO Cycle Simply
Facilitates H  He
It is analogous to a catalytic process.
The sequence of reactions leaves the C,N,O
essentially unchanged.
(Look back at the equations.)
But CNO Differs from PP
In PP, you have to slam two protons
together from time to time.
In CNO, you have to slam a proton into a
nucleus of Carbon or Nitrogen.
This is much harder to do! Why?
The Stronger Repulsion!
Consider the Charges:
Carbon has 6 protons; Nitrogen has 7;
Oxygen has 8.
These nuclei repel any approaching protons
quite strongly.
The proton will not hit the C,N, or O nucleus
unless it is moving very fast indeed.
Strong Temperature Dependence
The reactions in the CNO cycle are
significant contributors only when the
temperature is considerably higher than in
the core of the sun.
So it is only really important in the hotter
(more massive) stars.
Too Much Detail for You?
Don’t worry about all the equations
and the details of the reactions.
The important lesson is that astrophysicists
need to incorporate all these details if we
are going to understand the sun, and its
fate, correctly!