The Sun

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The Sun

• 2950
• Dr Bryce

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Class Notices
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Radius 6.9 x 108 m (109 times Earth) Mass
2 x 1030 kg (300,000 Earths) Luminosity
3.8 x 1026 watts
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Is it on FIRE?
Chemical Energy Content
10,000 years
Luminosity
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Is it CONTRACTING?
Gravitational Potential Energy
25 million years
Luminosity
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E mc2 - Einstein, 1905
It can be powered by NUCLEAR ENERGY!
Nuclear Potential Energy (core)
10 billion years
Luminosity
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Weight of upper layers compresses lower layers
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Gravitational or Hydrostatic equilibrium Energy
provided by fusion maintains the pressure
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Gravitational contraction Provided energy that
heated core as Sun was forming Contraction
stopped when fusion began
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Interior of the Sun
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Fusion Small nuclei stick together to make a
bigger one (Sun, stars)
Fission Big nucleus splits into smaller
pieces (Nuclear power plants)
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High temperature enables nuclear fusion to happen
in the core
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Sun releases energy by fusing four hydrogen
nuclei into one helium nucleus
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Fig.17.02
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IN 4 protons OUT 4He nucleus 2 gamma rays 2
positrons 2 neutrinos Total mass is 0.7 lower
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Solar Thermostat
Rise in core temperature causes fusion rate to
rise, so core expands and cools down
Decline in core temperature causes fusion rate to
drop, so core contracts and heats up
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Energy output
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Energy gradually leaks out of radiation zone in
form of randomly bouncing photons
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Convection (rising hot gas) takes energy to
surface
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Bright blobs on photosphere are where hot gas is
reaching surface
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Patterns of vibration on surface tell us about
what Sun is like inside
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Neutrinos created during fusion fly directly
through the Sun Observations of these solar
neutrinos can tell us whats happening in core
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Solar neutrino problem Early searches for solar
neutrinos failed to find the predicted number
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Solar neutrino problem Early searches for solar
neutrinos failed to find the predicted
number More recent observations find the right
number of neutrinos, but some have changed form
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Limb darkening
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Corona

• Visible during total solar eclipses
• Very hot 1 million K
• But very tenuous

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Temperature in the Solar Atmosphere
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Solar activity is like weather

• Sunspots
• Solar Flares
• Solar Prominences
• All are related to magnetic fields

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Sunspots Are cooler than other parts of the
Suns surface (4000 K) Are regions with strong
magnetic fields
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Butterfly diagram
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Zeeman Effect We can measure magnetic fields in
sunspots by observing the splitting of spectral
lines
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Charged particles spiral along magnetic field
lines
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Loops of bright gas often connect sunspot pairs
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The solar cycle
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The Solar Cycle
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Charged particles streaming from Sun can disrupt
electrical power grids and can disable
communications satellites
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Star-Forming Clouds

• Stars form in dark clouds of dusty gas in
  interstellar space
• The gas between the stars is called the
  interstellar medium

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Composition of Clouds

• We can determine the composition of interstellar
  gas from its absorption lines in the spectra of
  stars
• 70 H, 28 He, 2 heavier elements in our region
  of Milky Way

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Molecular Clouds

• Most of the matter in star-forming clouds is in
  the form of molecules (H2, CO,)
• These molecular clouds have a temperature of
  10-30 K and a density of about 300 molecules per
  cubic cm

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Molecular Clouds

• Most of what we know about molecular clouds comes
  from observing the emission lines of carbon
  monoxide (CO)

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Interstellar Dust

• Tiny solid particles of interstellar dust block
  our view of stars on the other side of a cloud
• Particles are lt 1 micrometer in size and made of
  elements like C, O, Si, and Fe

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Interstellar Reddening

• Stars viewed through the edges of the cloud look
  redder because dust blocks (shorter-wavelength)
  blue light more effectively than
  (longer-wavelength) red light

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Interstellar Reddening

• Long-wavelength infrared light passes through a
  cloud more easily than visible light
• Observations of infrared light reveal stars on
  the other side of the cloud

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Observing Newborn Stars

• Visible light from a newborn star is often
  trapped within the dark, dusty gas clouds where
  the star formed

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Observing Newborn Stars

• Observing the infrared light from a cloud can
  reveal the newborn star embedded inside it

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Glowing Dust Grains

• Dust grains that absorb visible light heat up and
  emit infrared light of even longer wavelength

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Glowing Dust Grains

• Long-wavelength infrared light is brightest from
  regions where many stars are currently forming

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Gravity versus Pressure

• Gravity can create stars only if it can overcome
  the force of thermal pressure in a cloud
• Emission lines from molecules in a cloud can
  prevent a pressure buildup by converting thermal
  energy into infrared and radio photons

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Mass of a Star-Forming Cloud

• A typical molecular cloud (T 30 K, n 300
  particles/cm3) must contain at least a few
  hundred solar masses for gravity to overcome
  pressure
• Emission lines from molecules in a cloud can
  prevent a pressure buildup by converting thermal
  energy into infrared and radio photons that
  escape the cloud

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Resistance to Gravity

• A cloud must have even more mass to begin
  contracting if there are additional forces
  opposing gravity
• Both magnetic fields and turbulent gas motions
  increase resistance to gravity

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Fragmentation of a Cloud

• Gravity within a contracting gas cloud becomes
  stronger as the gas becomes denser
• Gravity can therefore overcome pressure in
  smaller pieces of the cloud, causing it to break
  apart into multiple fragments, each of which may
  go on to form a star

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Fragmentation of a Cloud

• This simulation begins with a turbulent cloud
  containing 50 solar masses of gas

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Fragmentation of a Cloud

• The random motions of different sections of the
  cloud cause it to become lumpy

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Fragmentation of a Cloud

• Each lump of the cloud in which gravity can
  overcome pressure can go on to become a star
• A large cloud can make a whole cluster of stars

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Isolated Star Formation

• Gravity can overcome pressure in a relatively
  small cloud if the cloud is unusually dense
• Such a cloud may make only a single star

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Stellar winds