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The Stars and the Sun II. Basic Properties of the Sun

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Title: PDP for NSS Phys. Curriculum: The Stars and the Sun II Subject: Basic properties of the sun Author: Chu Ming-chung Last modified by: PHY Created Date – PowerPoint PPT presentation

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Title: The Stars and the Sun II. Basic Properties of the Sun


1
The Stars and the Sun II.Basic Properties of the
Sun
Chu Ming-chung ??? Department of Physics The
Chinese University of Hong Kong mcchu_at_phy.cuhk.edu
.hk
http//www.phy.cuhk.edu.hk/gee/mctalks/mcpdp.html
2
Summary
  • Nuclear reactions pp chain, CNO cycle, quantum
    tunneling, solar neutrino problem
  • pressure-gravity balance main-sequence star
  • Pressure-temperature thermostat
  • The sun features and activities
  • The sun and Earths climate

3
Basic Properties of the Sun
  • 2.1 Stellar Energy
  • 2.2 Solar Neutrino Problem
  • 2.3 Stability of Stars
  • 2.4 The sun

4
2.1 Stellar energy
  • A star is a super nuclear plant

5
The stars and the sun
  • The sun is a typical main sequence star.
  • If we understand the sun, we understand most
    stars.

6
The sun
7
Stellar Energy
  • Solar output 4x1026 W, stable for 109 years
  • Gravitational energy? Shrinking ?sun hotter
    ?radiates energy. How much would the sun need to
    shrink per year to sustain the output?

This does not contradict with the observation.
So whats wrong?
Ans. Can only sustain this contraction for 107
years! (Ro 7x108 m)
http//www.aoc.nrao.edu/smyers/courses/astro12/L1
5.html
8
Stellar Energy
  • Chemical energy? H ions ?H atoms release binding
    energy. How much can be released? How long can
    this sustain the sun?

Need a long term and stable source of energy!
What is it?
Fundamental forces Gravitation, EM, Weak, Strong
9
Energy source thermonuclear fusion
  • p-p chain 4 protons ? 1 helium nucleus
    positrons (???) neutrinos (???) energy

10
The p-p chain
p-p fusion rate in the sun 5x1013 s-1m-3 For
each p, one fusion in 9x109 years, lifetime of
Sun.
Most d in the universe are primordial!
Slow, but large energy release per reaction ?
sustain solar output for a long time!
11
  • Some mass is converted to energy per reaction E
    mc2.

Estimate the maximum thermal energy produced per
reaction 4p?4He 2 e 2 neutrinos (assumed
massless). Given mass of p 938.27 MeV, mass
of 4He nucleus 3727.36 MeV, 1 MeV 1.6x10-13
J. .
Ans. 4x 938.27 - 3727.36 26 MeV 4x10-12 J.
Note that the positrons also are annihilated soon
to give out energy. KE of particles are similar
and small.
How long can the sun be sustained this way?
Ans. Each reactions consume 4 protons. There
are 1.2 x 1057 protons in the sun. Total energy
output can be 4x10-12x3x1056 1.2x1045 J. Can
sustain the sun for 1.2x1045/4x1026 s 9.5x1010
years!
12
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13
How can we test this theory of solar energy
production?
Ans. By measuring the neutrinos produced in the
p-p fusion! Neutrinos penetrate through the sun
and carry information of the conditions at the
solar core.
How many neutrinos are produced by the sun each
second? How many come to Earth? Given 1 MeV
1.6x10-13 J
14
But Is the temperature high enough to ignite p-p
fusion?
Given nuclear force is short-range 10-15 m
15
  • Quantum mechanics Nuclear reaction can occur
    even if KE is not high enough
  • Tunneling effects (????)
  • very small amount of protons may undergo
    reactions!

16
  • Quantum mechanics particles are also waves
    ?uncertainty principle tunneling if
    , the particle has some probability of
    having a large p

17
Alternative fusion process only 10 of solar
energy
  • Carbon-Nitrogen-Oxygen cycle (CNO cycle)
  • Much more efficient than p-p chain, rate ?T 20,
    T?1.5?107 K
  • Dominant reaction in massive star (gt1.1M?) ?
    massive stars burn much faster ? massive stars
    have much shorter lives

18
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19
How is the nuclear energy transported to the
surface?
  • Radiation (??) Energy carried by EM radiation
  • Convection (??) Hot gas rises and cooler gas
    sinks, carrying energy outward

20
2.2 Solar neutrino problem
  • Neutrinos hardly interact with matter
  • they penetrate the interior layers of the sun and
    reach the earth directly

21
  • detection of which is an indirect evidence of
    nuclear reactions in the solar core
  • The sun should produce 2x1038 neutrinos/s
  • Earth should receive 4x1010 neutrinos/cm2/s
  • Background about 300/cm2/s are from Big Bang
    each person emits about 3x108/day (radioactive
    materials) and receives about 1010/day from
    nuclear power plants
  • but we detect only 1/3 as theoretically expected
  • something is wrong with our theory?

May have been solved by neutrino oscillation
http//wwwlapp.in2p3.fr/neutrinos
22
Neutrino telescopes
  • Telescopes use a large area to collect signals
    from far away
  • Make use of neutrino reactions with protons or
    neutrons, eg.
  • Weak interaction 1 ton ? few events/day
  • Use large amount of materials

23
Kamiokande ???????????
Masatoshi Koshiba
Nobel Prize in Physics, 2002
Drawing from Superkamiokande http//dumand.phys.wa
shington.edu/superk/
24
SuperKamiokande
Photo from Superkamiokande http//dumand.phys.wash
ington.edu/superk
25
SuperKamiokande
Photos from Superkamiokande http//dumand.phys.was
hington.edu/superk
26
Photo from Superkamiokande http//dumand.phys.wash
ington.edu/superk
27
Pure water
Photo from Superkamiokande http//dumand.phys.wash
ington.edu/superk
28
Inside the Kamiokande neutrino telescope
Photo from Superkamiokande http//dumand.phys.wash
ington.edu/superk
29
2.3 Stability of stars Pressure-temperature
thermostat
  • Balance of gravity vs. pressure gradient,
    radiation vs. fusion energy

Why is it stable against perturbations?
30
  • If nuclear reactions slow down
  • temperature decreases, lower thermal pressure
  • gravity contracts the star
  • compression heats the core, increases
  • increases nuclear energy generation

31
  • If nuclear reactions are too fast
  • temperature increases higher thermal pressure
  • stellar core expands
  • expansion cools the core, decreases the density
  • slows down the nuclear reaction rate

32
  • A star is very stable in the main sequence (a
    star of 1 solar mass lasts for ?10 billion years)

Sun on 2003/07/31 Taken in CUHK
33
2.4 The sun
Sunset on 26/12/01
34
Solar activities
  • Flares (??) - energetic outbursts
  • Prominences (??) - ejected hot gas
  • Coronal (??) mass ejection
  • Sunspots (??) - unexplained connection to Earths
    climate
  • Solar cycle min-max-min 11 years

All are associated with magnetic fields!
35
Coronal Mass Ejection speeds as high as 1000
km/s (average 400 km/s)
36
Solar Cycles (????)
37
Aurora ??
38
Solar constant (????)
  • Solar constant total solar energy reaching the
    surface of the earth 1360 J/m2s
  • 1 change in the constant may cause 1-2?C change
    in Earth?s temperature.
  • Variations cause long term changes in the earths
    climate, e.g. the ice ages.
  • There are exceptional periods of time when the
    sun is almost clear of sunspots, e.g., the
    Maunder Minimum (1645-1715) ?????, which
    coincide with cold weather on Earth connection
    between solar activities and Earths climate?
  • The solar constant varies by only 0.2,
    unaffected by solar cycles ? stability of Earths
    climate

39
Observing the sun
  • Dont need a large telescope very bright
  • Need proper filters!
  • Hydrogen-alpha filters select light emitted by
    Hydrogen only see prominences and other surface
    features clearly
  • White-light filters reduce light uniformly

40
Sun on 28/04/03 in H-alpha
41
Solar storms
23/10/2003
42
Mercury Transit ????on 2003/05/07 Taken in CUHK
43
Measuring Earth-Sun distance
Venus Transit 8/6/2004 Taken in CUHK
44
Summary
  • Nuclear reactions pp chain, CNO cycle, quantum
    tunneling, solar neutrino problem
  • pressure-gravity balance main-sequence star
  • Pressure-temperature thermostat
  • The sun features and activities
  • The sun and Earths climate

45
Basic Properties of the Sun
  • 2.1 Stellar Energy
  • 2.2 Solar Neutrino Problem
  • 2.3 Stability of Stars
  • 2.4 The sun

46
The Stars and the Sun II.Basic Properties of the
Sun
Chu Ming-chung ??? Department of Physics The
Chinese University of Hong Kong mcchu_at_phy.cuhk.edu
.hk
http//www.phy.cuhk.edu.hk/gee/mctalks/mcpdp.html
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