Transcript Solar is Nuclear

Swinburne Online Education Exploring the Solar System
Module 20: Inside
Module
the :Sun
Activity 1:
Solar is Nuclear
© Swinburne University of Technology
Summary:
In this Activity, we will investigate
(a) the method by which the Sun produces energy...
If it’s not chemical or gravitational, what is it?
How reliable is our understanding of it?
(b) the temperature and pressure inside the Sun, and
how these maintain the Sun’s energy production.
Early ideas
It was pretty obvious to humans in prehistoric times that
the Sun was highly unusual and obviously not a normal
“Earth-like” object.
The Sun seemed to glide slowly and regularly through the
sky while other daytime objects either fell to Earth, or
changed from minute to minute (like clouds) or from day to
day (like the Moon).
Gravitational collapse?
As we saw in the Activity The Sun: Ruler of the Solar
System, much later on, scientists wondered whether
gravitational collapse has caused the material in the Sun
to heat up.
If that was the case, the Sun would not burn but instead
glow.
Sun shrinks,
and in doing so
radiates a lot
of heat and light
Time problems
However if the Sun glowed due to gravitational
contraction alone, then it would never have kept shining
for the last 5 billion years, nor could it be expected to
last much longer.
Another possibility
DELETE or overwrite
We now know that the Sun not only emits light but also
emits other forms of electromagnetic radiation and
streams of particles.
protons, ions
visible light
electrons
other
electromagnetic
radiation
neutrinos
“solar wind”
This happens in “nuclear reactions” too.
Nuclear interactions and
its associated energy first
came to people’s attention
about a century ago. It
became obvious that the
Sun must be energised by
these interactions. From
then on the Sun was often
referred to as a “nuclear
furnace”.
But ...
Proceed with Caution!
There are several parts of the
term “nuclear furnace” that are
misleading.
We have to treat the term with
extreme caution … or better
still, avoid using it altogether.
“Furnace”
First, the word “furnace” (which comes from an
old Latin term, fornax, meaning “warm”) is
usually associated with burning, which is a
chemical reaction.
“Burning” usually means the combination of a substance
(such as carbon, paper, wood) with oxygen.
You start off with carbon (for instance, in the form of coal)
and oxygen (from the air) and end up with carbon dioxide
(and ash) … and a good deal of heat, as well.
Chemical reactions occur in and between molecules
(which are collections of atoms). Chemistry is the study of
molecules and chemical reactions.
In a chemical reaction, atoms join, leave or move between
molecules to make new molecules. For instance …
CH4
+
(methane)
2O2
(oxygen)
react to form
2 H2O
(water)
+ CO2
(carbon dioxide)
“What does
CH4 mean?”
Strong bonds
Chemical reactions rely on strong bonds existing between
the atoms inside molecules: these bonds occur when those
atoms share electrons.
In the methane molecule, a
carbon atom shares electrons
with four hydrogen atoms.
After the chemical reaction,
it shares electrons with two
oxygen atoms instead.
No molecules, thanks
However it is far too hot for molecules to exist in the Sun.
The molecules simply fall apart in the incredibly high
temperature, so in the Sun there can be no chemistry.
too hot for
molecules
molecule
made of
atoms
molecule is
dissociated
So no chemistry, thanks
In fact, it is even too hot for atoms as we know them to
exist. The heat causes atoms to fall apart too, and what you
have instead is a kind of “melt” called plasma.
too hot even for atoms
molecule is
dissociated
plasma
So we must avoid using the term “furnace” with respect to the Sun.
No fission either, thanks!
Second, most people think that “nuclear”
reactions in the Sun are fission
reactions.
However fission mostly occurs when
elements are pretty heavy (for example,
uranium and plutonium)...
hydrogen
nucleus
helium
nucleus
heavier
nucleus
FisFis
Fission
sion
sion
Fission is the process whereby the nucleus of an atom
splits into several different pieces. These pieces usually
include several particles and some sort of
electromagnetic radiation.
Such radiation may be used for good purposes (e.g.
radiotherapy), although the results can also be
unwelcome (e.g. mutation, radiation poisoning, cancer).
particles
electromagnetic
radiation
electromagnetic
radiation
particles
Although fission can and does occur in some much
lighter elements, the lighter the element the less
likely it will be that fission can occur.
The large, complex collections of particles
in a big nucleus are far more likely
to break up.
Since the Sun is 98%
hydrogen and helium
(the lightest elements),
there can be hardly any
fission in the Sun.
He
26%
other
2%
H
72%
Another type of “nuclear”
There are actually a number of different kinds of
“nuclear” reaction, involving different forces, particles
and energies.
While fission occurs when nuclei split up into smaller
particles, there is a type of nuclear interaction where the
reverse happens.
This type of nuclear
interaction is called
fusion.
Fusion
It is very difficult under Earth conditions to make fusion
occur: the particles being fused often have the same
electrostatic charge (positive, in the case of nuclei) and
therefore repel each other very strongly.
So a cloud of gas has to be very
compressed (or collapse a great
deal under its own weight) before
the high pressure and
temperature can overcome this
repulsion, and fusion can begin.
Electrostatic repulsion
stops impact
… but high pressure
and temperature
encourage impact
When fusion does occur, it not only involves the
formation of a new atom from several old ones, but
there is also the release of some energy in the form of
electromagnetic radiation (heat, light, x-rays and so on)
and perhaps particles such as neutrinos, electrons etc.
Change figure
to match movie
particle
new
nucleus
electromagnetic
radiation
electromagnetic
radiation
particle
To see an animation click here
Ninety percent of the time, fusion in the Sun involves
hydrogen nuclei being fused to make helium:
Several Reactions
Start with 4 protons
under enormous
pressure and
temperature
End up with a
“normal” helium nucleus,
two gamma rays,
two positrons and
two neutrinos
Here is that process broken into its three steps:
1. Two protons fuse
to make deuterium,
releasing a positron
and a neutrino
2. The deuterium fuses
with another proton to make
a light helium nucleus
and a gamma ray
3. Two light helium nuclei
fuse to make “normal”
helium, plus two protons
proton
neutron
positron
neutrino
gamma ray
hydrogen nucleus
one positive charge
like a proton
but with no charge
“positive electron”
one positive charge
no charge
and no mass
a very energetic
photon
Here are the symbols and equations used
by physicists to show how the various
particles and so on “add up” for this
reaction:
Two
helium-3
nuclei
A
hydrogen
nucleus
Two
hydrogen
nuclei
combines
a “heavy”
combinewith
make
a
combine
totomake
one
hydrogen
nucleus
to
helium-4
nucleus.
“heavy”
hydrogen
nucleus
produce helium-3.
(also
deuterium).
Twocalled
hydrogen
nuclei
gamma ray
AA positron
andisa emitted.
neutrino
are emitted.
are emitted.
This reaction starts with protons (bare hydrogen nuclei)
and so is called the proton-proton chain.
If you combine all of the equations for the entire chain,
you find that six protons end up producing a helium
nucleus, two positrons, two neutrinos and two gamma
rays, with two left-over protons released as well.
61H+  4He++ + 21H+ + 2e+ + 2n + 2
[By the way, the positrons don’t just sit there.
They fly off and combine with electrons, but that’s another story.]
Here it is in one diagram:
Click here to see animation
Energy production
Now, just for a moment remember what started all this talk
about fusion and fission and nuclear reactions:
it was to work out how the Sun produces so much energy.
Although there is an exchange of
energy in most of the steps, it is the step
where a gamma ray is emitted that is of
most interest.
It turns out that if you compare the mass that you start with
and the mass you end up with there is a difference …
… and that difference is exactly accounted for by one of
the most widely-known and least-understood equations in
physics:
E=
2
mc
According to this equation, energy (E) and mass (m) may
be interchangeable: for example, in fission reactions and
in fusion reactions like the proton-proton chain.
c is the speed of light in a vacuum: 3 x 108 ms-1.
Here is that equation at work with respect to the protonproton chain:
BEFORE:
four protons
AFTER:
helium nucleus
plus two positrons
plus two neutrinos
… and two gamma rays
Initial total mass = 6.693 x 10-27 kg
Final total mass = 6.645 x 10-27 kg
Difference = 0.048 x 10-27 kg
… and according to E = mc2
this is equivalent to ...
Energy = 0.43 x 10-11 joules
… which is just the energy observed
in the two gamma rays
E = mc2
Energy =
0.43 x 10-11
joules
Big deal! 0.43 x 10-11 joules sounds tiny
even with the help of c2 (I hear you say).
However to produce the Suns luminosity
a huge 6 x 1011 kg of hydrogen must be
converted into helium each second.
This turns out not to be a problem. The
Suns mass is 2 x 1030 kg. Based on this,
it has adequate fuel to have been
undergoing nuclear fusion for the last
4.6 billion years (the age of the Solar
System), and to continue for another
5 billion years!
Other important fusion reactions
Although in our Sun it is the proton-proton chain which
dominates (91%), in other stars other reactions are very
important. Here are the main ones:
CNO cycle
A complex series of reactions in which the transformation
carbon - nitrogen - carbon - nitrogen - oxygen - nitrogen - carbon
facilitates the conversion of four protons to one helium nucleus
(plus energy)
Helium “burning”
Three helium nuclei fuse to create one carbon nucleus
(plus energy).
This is also called the “triple-alpha reaction”.
Carbon “burning”
Carbon is fused to form heavy elements (plus energy):
in particular, iron is the final product of much
carbon burning.
Why all the p-p in our Sun?
Q:
Why in our Sun is there mostly proton-proton
reaction and hardly any of the other types of
reaction?
A:
It is because of the difficulty in making these
reactions occur, and that depends on three
things ...
Pressure, temperature and chemical
composition
All nuclei contain protons and so are positively-charged,
and things with the same charge will repel each other.
So unless the nuclei are forced to come so close to each
other that this electrostatic repulsion is overcome, fusion
will not occur.
Higher pressure within a gas makes the particles in the
gas be, on average, closer to each other.
Higher temperature within a gas makes the particles in
the gas move a great deal faster and so they may
randomly come closer to each other.
Finally, if the elements aren’t there, they
can’t react!
A closer look
Let’s watch two positively-charged nuclei approaching
each other and see what happens.
no
Oh,
No!
allLet’s
rightfuse!
No!
Oh,
Okay!
all
right
No!
No!
no
…
thethey
nuclei
areeach
very other
close more
to each
Asbut
thesuddenly,
nuclei getwhen
closer,
repel
and
other
nuclear
force takes
and fusion
can occur.
morethe
strongly
because
of theover
electrostatic
repulsion
…
Censors Note: Nuclei used in this slide were older than the age of thermonuclear consent
More P and T required
The more positively-charged a nucleus is, the harder it
will be to get it close enough to another nucleus that
fusion can occur. More pressure (P) and temperature (T)
will be needed.
Reaction...
Proton-proton reaction
CNO cycle
triple-alpha reaction
becomes significant at
8 million degrees Kelvin (K)
20 million K
100 million K
carbon “burning”
600 million K
A proton has only one positive charge; carbon has 6,
nitrogen 7 and oxygen 8; and an alpha particle has two.
The heart of the Sun
Mathematical models can be applied to estimate what the
temperature and pressure will be in various zones inside
the Sun (and other stars).
Fusion can only occur in the
core of our Sun. The heat
produced in the core is
transported to the surface
through the other layers, and
we’ll have a look at that in the
next Activity.
160
16
140
14
120
12
100
10
80
8
60
6
Fusion in the Sun occurs where the
temperature exceeds 8 million K
40
4
20
Temperature (million K)
Density (g/m3)
Temperature and density in the Sun
density
temperature
2
0
0
… that is, only in the
the0.8
Sun0.9
0.1 0.2 0.3innermost
0.4 0.5 25%
0.6 of
0.7
Radius (fraction of whole)
0
1
One quarter of
the radius is just
one 64th of the
volume!
The journey ahead
In this Activity, we looked at some ways that energy is
produced in the natural world, and explored why fusion is
the only option to explain the energy production of the
Sun.
Because of the extreme temperature and density required,
this can only occur in the interior region which we call the
core of the Sun.
In the next Activity we will look at how energy is transported
to the surface of the Sun once it is produced.
Image Credits
Temperature variations in the corona August 1998 (colour enhanced)
http://antwrp.gsfc.nasa.gov/apod/image/9808/activesun_trace.jpg
Now return to the Module home page, and read
more about the Sun in the Textbook Readings.
Hit the Esc key (escape)
to return to the Module 20 Home Page
Chemical symbols
When astronomers, chemists, biologists and other people working in the
sciences want to write about particular elements such as hydrogen, helium,
oxygen and so on, they use a letter or letters which relate to the Latin name
for the substance.
A molecule is a combination of atoms. The “formula” for the molecule is written
using a subscript if there is more than one atom of an element.
C = carbon
O = oxygen
CO
carbon monoxide
CO2
carbon dioxide
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