The Sun as a Star

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The Sun as a Star

• Susan Cartwright
• University of Sheffield

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What do we know about the Sun?

• Its size and its distance from us
• Its luminosity (energy generated per second)
• Its (surface) chemical composition and
  temperature
• How it generates energy
• Its (approximate) age
• from radioactive dating of solar system material
• Its history and future
• from our understanding of stars in general

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The Suns composition and temperature

• The spectrum of the Sun tells us both its
  composition and its temperature

4
How the Sun generates energy

• Fusion of hydrogen to helium
• How does this work?
• He atom weighs slightly less than 4 H atoms
  (0.7)
• E mc2
• How do we know?
• Only mechanismthat lasts long enough!
• We detect neutrinos

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The Sun as a Star
Joe Morris, http//joemorris.mystarband.net/
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How typical is the Sun?

• Compare Sun with nearby stars
• It is on the main sequence
• It is brighter and more massive than average
• but fainter and less massive than most bright
  naked-eye stars
• It is not a binary
• this is not rare, but probably more than half of
  all stars are binaries
• It has planets
• this is probably very common

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The Suns life and death Youth

• The Sun was born from a giant cloud of cool gas
• this contracted under gravity
• as it contracted it heated up
• eventually the core reached 10 million degrees
  and hydrogen fusion began

Most of following images taken from
http//outreach.atnf.csiro.au/education/senior/ast
rophysics
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The Suns life and death Youth
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The Suns life and death Prime

• The Suns main sequence lifetime is 10 billion
  years
• Less massive stars live longer
• More massive stars live less long
• The Sun gets slightly brighter as it evolves on
  the main sequence
• faint young Sun problem
• Why didnt early Earth freeze solid??

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The Suns life and death Old Age

• When the Sun has used up its core hydrogen, it
  will become a Red Giant
• Red giants still use hydrogen as fuel, but
  outside the (pure helium) core
• The helium core will get larger and hotter
• Eventually it will reach 100 million degrees and
  helium fusion will begin
• this is much less efficient and will not last as
  long

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The Suns life and death Old Age
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The Suns life and death Death

• As helium fusion continues, the Sun will shed its
  outer envelope
• the hot inner core will be revealed as a White
  Dwarf
• about 0.6 times the Suns current mass, but only
  the size of Neptune

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The Suns life and death Death
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The lives of other stars

• Stars up to about 8 times the Suns mass live and
  die like the Sun
• only more massive ones do it faster
• Stars more than 8 times the Suns mass can fuse
  heavier elements
• they are responsible for making most of the
  Periodic Table!
• But they die young in a spectacular explosion
  called a Supernova

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The Life and Death of Stars
Simulations from http//rainman.astro.uiuc.edu/ddr
/stellar/beginner.html
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Planetary Systems
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Other Stars with Planets

• 276 planets detected to date!

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Conclusions

• The Sun is a star
• a little brighter than most
• a bit more massive than most
• not a binary
• Studying the Sun can tell us about stars
• it is far closer than any other star and can be
  studied in much more detail
• Studying other stars can tell us about the Sun
• range of ages, masses and chemical composition
  available for study
• planets are common range of very different
  planetary systems can be studied

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Gliese 581
Probably like Venus
Possibly habitable!