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The Sun

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The Sun Internal structure of the Sun Nuclear fusion Protons, neutrons, electrons, neutrinos Fusion reactions Lifetime of the Sun Transport of energy in the sun – PowerPoint PPT presentation

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Date added: 8 July 2019
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Title: The Sun


1
The Sun
  • Internal structure of the Sun
  • Nuclear fusion
  • Protons, neutrons, electrons, neutrinos
  • Fusion reactions
  • Lifetime of the Sun
  • Transport of energy in the sun
  • Radiation versus convection
  • The atmosphere of the Sun
  • Demos 8B10.35, 4B20.10

2
The Sun
3
The Sun
4
The Sun
5
Properties of the Sun
  • Mass 2?1030 kg
  • Radius 696,000 km
  • Luminosity 3.8?1026 W
  • Age 4.6 billion years
  • Surface temperature 5,800 K
  • Composition 70 H, 28 He, 2 other

6
Properties of the Sun
7
Could chemical reactions power the Sun?
But the Sun is about 4.6 billion years old.
8
Could gravitational contraction power the Sun?
Better, but still off by a factor of 100.
9
Nuclear burning
10
Nuclear burning
  • Elementary particles
  • Protons (orange) found in nuclei, positive
    charge
  • Neutrons (blue) found in nuclei, no charge
  • Electrons (e-) orbit nuclei, negative charge
  • Photons (g) particles of light (gamma-rays)
  • Positrons (b) anti-matter electrons, positive
    charge (e in book)
  • Neutrinos (n) ghost particles, no charge, can
    easily pass through normal matter

11
Convert proton to neutron
  • To convert a proton to a neutron
  • A positron (b) and a neutrino (n) must be
    produced and released

12
Make nuclei out of protons and neutrons
  • 1H normal hydrogen nucleus proton
  • 2H deuterium hydrogen nucleus (unstable)
  • proton plus neutron (in heavy water)
  • 3He light helium nucleus (unstable(
  • two protons plus one neutron
  • 4He normal helium nucleus
  • two protons plus two neutrons

13
Nuclear burning
14
Nuclear burning
4(1H) ? He4 energy 2 neutrinos
MH 1.673?10-27 kg MHe 6.645?10-27 kg
Rate of fusion reactions is
Our Sun converts 4 ?109 kg of matter into energy
each second.
15
Nuclear burning
Total energy available is
Lifetime is
16
Internal Structure of the Sun
We want to make a model of the Sun, what do we
need to know?
17
Gas in the Sun is in hydrostatic equilibrium
18
Fish in water are in hydrostatic equilibrium
19
Transport of energy through the radiative zone
Photons produced via fusion scatter many times in
the Suns dense interior - a random walk.
20
Random Walk
In one dimension, a random walk can be thought of
as tossing a coin if heads then go left, if
tails then go right.
Coin tosses follow the binomial distribution.
For large numbers of tosses, the distribution
becomes the normal distribution.
The average total distance traveled for n steps
of length l is
21
Random Walk
The same formula holds in 2 and 3 dimensions. For
the Sun, the average distance between collisions
is about l 1 mm. Photons travel at the speed
of light, so the time between collisions is t
l/c 10-3 m /(3?108 m/s) 3 ?10-12 s The
radius of the Sun is L 7?108 m. The average
number of collisions before a photon escapes is
n (L/l)2 (7?108 m/ 10-3 m)2 5 ? 1023 The
average photon stays in the Sun for a time T
tn (3 ?10-12 s)(5 ? 1023) 1.5 ?1012 s
50,000 years A more accurate estimate gives
120,000 years
22
Q A drunken pirate takes a 0.2 m randomly
oriented step each second. He starts at the
center of a small island of radius 20 m. On
average, how long will it take before he steps
into the ocean?
  1. 20 sec
  2. 100 sec
  3. 1000 sec
  4. 10,000 sec
  5. 100,000 sec

23
Internal Structure of the Sun
24
Convective zone
25
Do the following transport energy by convection
or radiation?
  1. A gas oven
  2. A microwave
  3. A heat lamp
  4. An electric radiator

26
Internal Structure of the Sun
  • Equation of hydrostatic equilibrium
  • Equation of mass continuity
  • Equation of state
  • Equations of energy production and transport

27
Internal Structure of the Sun
28
How do we know?
Core temperature 15,600,000 K, density 150?
water Surface temperature 5800 K, average density
1.4? water
29
How can we check if fusion really powers the Sun
30
Test fusion hypothesis by looking for neutrinos
Neutrinos are only produced in nuclear
reactions. Ray Davis shared the 2002 Nobel prize
in Physics for originally detecting neutrinos
from the Sun.
31
The Solar Neutrino Problem
Neutrinos are detected, but only at 1/3 the rate
expected. Solution - Neutrinos change flavor
as they transit from sun to Earth, from electron
neutrinos, to tau (?) and muon (µ) neutrinos
32
We see oscillations on the surface of the Sun
33
Helioseismology is a way to probe the Suns
interior using the Suns own vibrations.
  • The surface of the Sun vibrates up and down in
    oscillations which can go deep through the Sun.
  • We can observe these oscillations from Earth by
    looking at the Doppler shifts of different pieces
    of the Sun.

34
Waves inside the Sun
The pattern of waves on the surface is determined
by the conditions deep inside the Sun.
35
What direct observational evidence supports the
model of thermonuclear reactions in the Suns
core?
  1. Neutrinos
  2. Gamma rays
  3. Sun spot counts
  4. WMD inspections

36
The Suns Atmosphere
  • Photosphere - the 5800 K layer we see.
  • Chromosphere a thin layer, a few 1000 km thick,
    at a temperature of about 10,000 K. Can be seen
    during solar eclipse.
  • Corona Outermost layer, 1,000,000 km thick, at
    a temperature of about 1,000,000 K.

37
Outer layers of sun
1 photosphere, 2 chromosphere, 3 corona Why
the outer layers of the Suns atmosphere are
hotter is a puzzle.
38
Photosphere
39
Chromosphere
40
Corona
41
Corona
42
Limb darkening
43
Limb darkening
44
Sunspots are low temperature regions in the
photosphere
Sun spots are about 4000 K (2000 K cooler than
solar surface) and have magnetic fields up 1000?
the normal solar magnetic field. They can be as
large as 50,000 km and last for many months.
45
Particles spiral around magnetic field lines
Magnetic field
Motion of charged particle (electron, proton,
nucleus)
46
Sunspots are low temperature regions in the
photosphere
The large magnetic fields in sunspots decrease
the flow of heat via convection causing the
sunspot to become cool.
47
Sunspot cycle
48
Sunspots can be used to measure the rotation of
the Sun
Near the equator the Sun rotates once in 25
days. The poles rotate more slowly, about once
every 36 days.
49
Sunspot cycle
Each 11 years, the Suns magnetic field changes
direction. Overall cycle is 22 years.
50
Granulation
51
Which statement is not correct?
  1. The solar coronal temperature is about 106 K.
  2. Sunspots are very cool and dark, with
    temperatures of about 300 K.
  3. The Suns core has a temperature about 107 K.
  4. The chromosphere is hotter than the photosphere.

52
Solar magnetic fields also create other phenomena
  • Prominences
  • Flares
  • Solar wind
  • Coronal mass ejections

53
Particles spiral around magnetic field lines
Magnetic field
Motion of charged particle (electron, proton,
nucleus)
Particles, that we see, get trapped along
magnetic field lines, that we dont see,
stretching out from the Sun.
54
Prominences -Cooler than photosphere.
55
Solar flares -Hotter, up to 40,000,000 KMore
energetic
56
Coronal mass ejections -eruption of gas, can
reach Earth and affect aurora, satellites
Movie
57
Coronal mass ejection
Movie
58
Aurora
59
Review Questions
  1. How long could the Sun continue to burn H at its
    current luminosity?
  2. What is produced in the fusion of H to He?
  3. If the radius of the Sun were doubled, how much
    longer would it take photons produced in the
    Suns core to escape?
  4. What is the connection between sunspots and the
    Suns magnetic field?
  5. What is the sunspot cycle?
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