Lecture 18: Energy Generation in Stars
Astronomy 101/103
Terry Herter, Cornell University
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Lecture
Topics
  • Conclusion of star formation
  • What makes stars shine
    • Possible energy sources
    • Fusion
  • Energy Transport in stars

T Tauri
Stars
  • Named after the first one found.
  • Newly formed stars which are just clearing away the surrounding material.
  • Surface eruptions, rapid variations of light output and mass loss via "wind".
  • Many are surrounded by disks!!
    • Could planets be forming there?
  • Sun was probably once a T Tauri star.

What makes
the Sun
Shine
The Sun emits 4x1026 Watts
  • People power on the earth is about 600 billion Watts
  • US Power consumption is about 1013 Watts
  • Equivalent to > 100 billion nuclear bomb/sec

We know from dating of rocks on the Earth and Moon that the Sun is at least 4.5 billion years old.

  • => Need about 6x1043 J of energy

Energy
Sources

Possible Energy Sources for Main Sequence Stars

  • Chemical Reactions
    • Such as a burning fire
    • ==> Sun's lifetime ~ 1000 years!!
  • Gravitational Compression
    • Shrinking - use gravity, like a water fall
    • Must collapse ~50 feet per year!
    • ==> Sun's lifetime ~ 15x106 years.
  • Nuclear reactions
    • Convert mass to energy
    • Much more energy per unit mass than chemical reactions


Mass
to
Energy
  • Albert Einstein - 1905
    • Equivalence between mass and energy
      E = mc2
  • Main-sequence stars release energy by converting Hydrogen into Helium
    • 4 H1 ==> He4 + energy
    • Superscript is number of protons + neutrons.

E = mc2

4 H1 ==> He4 + energy

  • mass of 4 H1 = 6.693x10-27 kg
  • mass of 1 He4 = 6.645x10-27 kg
  • The mass difference is released as energy
    • ~0.048 x10-27 kg per reaction
  • E = mc2 = 0.048x10-27 kg x (3x108 m/sec)2
    • E ~ 4.3 x 10-12 Joules
    • Energy is created out of mass!
  • Converting the hydrogen into helium for the Sun would produce about 1044 J of energy
    • Enough to keep the Sun burning for about 10 billion years

Fusion
and Fission
  • Fusion is the process of creating heavier elements from lighter ones.

    e.g. 4 H1 => He4 + energy

  • Fission is the breaking up of (typically) heavy nuclei to make lighter ones.

    e.g. U235 + n => Ba141 + Kr92 + 3n + energy

  • Stars are fueled by fusion
    • Not enough heavy elements

Atomic
Nuclei


Stellar
Cores

  • Tcore~15x106 K
  • Particles move very fast.
  • Collision results in:
    • Deuterium + Positron + Neutrino + Energy

Reactions
in Stars
  • Proton-Proton Chain
    • Most efficient in lower mass stars
    • T > 10,000,000 K
  • CNO Cycle
    • Most efficient in higher mass stars
    • T > 16,000,000 K
    • Hans Bethe (Cornell) 1939

P-P chain:

The P-P Chain: Reactions

The P-P Chain: Illustration


CNO cycle:

The CNO Cycle: Reactions

  • Carbon is the catalyst for the reaction. It is returned to be used again!

 


CNO cycle:

The CNO Cycle: Illustration

 

 


The
Neutrino
  • A particle produced in stellar nuclear reactions is the neutrino, designed by the the Greek symbol n
  • The neutrino has no charge & small mass (close to zero)
    • - very little interaction with matter
  • The Sun is transparent to neutrinos.
  • "Neutrino telescopes" can look at the interior of the Sun.
  • Tank of chlorine (cleaning fluid) in S. D. mine. n's occasionally interact to convert Cl37 to Ar37
  • There appear to be too few n's!

Energy
Transport
in Stars
  • How does the energy get out?
  • Energy can be transported by
    • Conduction
    • Convection
    • Radiation
  • Stars use the latter two methods
  • The trade between convection and radiation depends on the star and region within a star

Model
of the
Sun

 

 

Region
Temp. (106 K)
Density
(g/cm3)
Energy
Transport
Core
15~
100
Convective
Radiative zone
3~
1
Radiative
Convective zone
1~
0.1
Convective

 

Interiors of
Stars

 


Energy
Transport
Summary
  • Massive stars (> 2 Msun) have small convective cores and large radiative envelopes.
  • Low mass stars (< 1 Msun) have small radiative cores and large convective envelopes.

 


Balance
of
Life
  • Hydrostatic Equilibrium is the balance of gravity and pressure in each layer of a star.
  • It keeps a star from collapsing (or expanding).
  • This balance is maintained as a star ages, so that its size might shrink or grow to maintain it.

 

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