Gravitational Equilibrium How does the Sun remain ... - Duke Physics

Duke Physics 55 Spring 2007
Lecture #15: The Sun

ADMINISTRATIVE STUFF
­ Quiz 3
­ Homework #6 due
­ Homework #7 available

OUTLINE
BDSV Chapter 14.1, 14.2
­ Some properties of the Sun
­ What's inside the Sun?
­ How does the Sun remain stable?
­ How does the Sun shine?
­ How do we know this?

LUMINOSITY: power (energy/time) output in
electromagnetic radiation
Solar luminosity
is 3.8 x 10 26 Watts!
1 second's worth of energy would supply all human
needs for half a million years!

What's inside the Sun?

Anatomy of the Sun: moving outwards

Deepest Layers
Core:
where energy
is generated
Radiation zone:
radiation carries
energy outward
Convection zone:
churning gas
carries energy
outward
p

Intermediate Layers
Chromosphere:
10,000 K,
emits UV
Photosphere:
6,000 K, what appears
as the 'surface' from p Earth

Outer Layers
Solar wind:
particles
expelled
Corona:
1 million K,
emits x­rays
p

How does the Sun remain stable?
Gravitational Equilibrium
Weight of overlying material is supported by
underlying pressure⇒ higher pressure in deeper layers

How does the Sun shine?
Many ancients thought it
was literally on fire
1800's: chemical burning?
gravitational contraction?
(As the Sun shrinks, gravitational
potential energy is converted into
radiative energy)
But, neither of these provide enough energy...
Sun would last only a short time

The answer: mass is converted
to radiative energy!
E=mc 2

How does it actually happen?
Nuclear fusion
­ nuclei fuse to make final nucleus with
mass less than sum of initial masses
­ mass difference released in form of radiation
The Sun loses 4 billion kg/second!

This can only happen at very high temperature
Nuclei repel each other unless they get very close together
At very short
distances,
the nuclear
strong force
kicks in,
and sticks
protons together
In order to get very close, they have to be going very fast;
it's hot enough only in the core of the Sun

Hydrogen fuses into helium via the
proton­proton chain, via several steps
Overall: 4 protons (H) in, 1 He + 2 gamma rays
+ 2 positrons + 2 neutrinos out
(more steps later, but this is the main one)

The Sun is a Mass of Incandescent Gas
(and plasma...)

Solar Thermostat: rate of fusion is highly sensitive
to temperature, thanks to gravitational equilibrium

PRS Question
If the electrical charge on each proton were
suddenly magically to become greater, then:
a. The core of the Sun would become larger and hotter.
b. The core of the Sun would become larger and cooler.
c. The core of the Sun would become smaller and
hotter.
d. The core of the Sun would become smaller and
cooler.
e. Not enough information to answer.

If the electrical charge on each proton were
suddenly magically to become greater, then:
a. The core of the Sun would become larger and hotter.
b. The core of the Sun would become larger and cooler.
c. The core of the Sun would become smaller and
hotter.
d. The core of the Sun would become smaller and
cooler.
e. Not enough information to answer.
More charge requires higher temperature for fusion.
Fusion at current temperature would slow down or stop;
the core would contract and heat up gravitationally
... whether fusion would restart depends on how
much higher temperature is necessary!

The gamma rays slowly work their
way out from the core to the surface
exchanging energy with the solar matter
(losing energy per photon ⇒ going from gamma
to mostly visible wavelengths)
RADIATIVE DIFFUSION: takes about 1 million
years for energy to reach the surface!

What evidence do we have for this
mechanism of energy generation?
Mathematical models incorporating known
physics (gravity, pressure, temperature):
these make predictions that
we can test against observations

Helioseismology (sun quakes)
Measure
movement
of the
Sun's
surface
using
Doppler
shift
Pattern depends on temperature, density, etc:
test solar models!

Another way of testing the picture:
NEUTRINOS: 'ghostly' particles that have
very tiny mass and hardly interact with
anything (only via the weak interaction)
2 neutrinos are produced for each fusion reaction
(a million from the Sun through your hand each second!)

Neutrinos can be detected in giant underground detectors
Giant: because neutrino interactions are so rare
Underground: to shield out cosmic rays
My experiment in Japan: Super­Kamiokande
50 ktons of ultrapure water 1 km underground

Inside Super­K, filling with water

Image of the Sun in neutrinos from Super­K

Several experiments now see solar neutrinos; number
is now consistent with expected from nuclear fusion
Homestake mine,
HUSA
(cleaning fluid)
Super­K, Japan
(water)
SNO, Canada
(heavy water)

Cosmic Gall, by John Updike
Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids through a drafty hall
Or photons through a sheet of glass.
They snub the most exquisite gas,
Ignore the most substantial wall,
Cold-shoulder steel and sounding brass,
Insult the stallion in his stall,
And scorning barriers of class,
Infiltrate you and me! Like tall
And painless guillotines, they fall
Down through our heads into the grass.
At night, they enter at Nepal
And pierce the lover and his lass
From underneath the bed - you call
It wonderful; I call it crass.

WUN2K
Sun is stable by
gravitational
equilibrium

WUN2K
The Sun produces energy by nuclear fusion,
which requires very hot temperatures in the core
We know this from
mathematical models,
tested by helioseismology,
and neutrino detection

WUN2K
'Solar thermostat' maintains equilibrium

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