Stars Tom Burbine tburbinemtholyoke'edu

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StarsTom Burbinetburbine_at_mtholyoke.edu

• http//www.mtholyoke.edu/courses/tburbine/emmanuel
  .ppt

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• What is the difference between a star and a
  planet?

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• A star is a celestial body that radiates energy
  derived from thermonuclear fusion in its interior
• A planet is a celestial body that is not massive
  enough to cause thermonuclear fusion

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• What is the closest star to the Earth?

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• The Sun
• The second closest is Proxima Centauri (4.2 light
  years away)

http//www.astronomy.com/asy/objects/images/sun_fu
ll_disk_soho_09_14_1997.jpg
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Today

• We are going to go over
• Composition of Stars
• Thermonuclear Fusion
• Stellar Classification

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Stars are made up of Atoms

• Atoms are made up of 3 types of particles
• Protons positive charge (1)
• Electrons negative charge (-1)
• Neutrons neutral charge (no charge)
• Protons and Neutrons are found in the nucleus

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Lithium Atom
http//www.biocrawler.com/encyclopedia/ImageAtom_
diagram.png
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Periodic Table
http//athene.as.arizona.edu/lclose/teaching/a202
/images/periodic.gif
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Elements

• Different elements have different numbers of
  protons
• Hydrogen (H) is the simplest (one proton)
• Helium (He) is the second-simplest (two protons)

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Composition of Suns photosphere
http//filer.case.edu/sjr16/advanced/sun_astar.htm
l
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Energy levels where an electron can reside To go
to a higher energy level, an electron needs to
gain energy To go to a lower energy level, an
electron needs to lose energy
Hydrogen
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cant happen
Hydrogen
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Energy levels Vary for Different Atoms
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Since Energy Levels Vary for Different Atoms

• Different atoms emit and absorb energy at
  different energies

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c frequencywavelength c is speed of light
Higher the frequency, Higher the energy of the
photon Higher the wavelength, Lower the energy of
the photon
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Intensity
Longer wavelength Shorter frequency, shorter
energy
http//www.physics.utoledo.edu/lsa/_p1750/SPlab.j
pg
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Structure of the Sun
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Sun

• Core 15 million Kelvin where fusion occurs
• High temperature due to the weight of the outer
  layers of the sun, which causes fusion to occur
  which provides the energy and maintains the high
  temperature
• Photosphere 5,800 Kelvin surface of Sun

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Nuclear Fusion

• Nuclear fusion is the process by which atoms join
  together to form a heavier nucleus.
• It is accompanied by the release of energy for
  atoms lighter than iron

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Energy Source for Sun

• Fusing hydrogen into helium
• Hydrogen nucleus 1 proton
• Helium nucleus 2 protons, 2 neutrons
• Need high temperatures for this to occur

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http//www.astronomynotes.com/starsun/s3.htm
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http//www.astronomynotes.com/starsun/s3.htm
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How does Fusion Convert Mass to Energy

• What is the most famous formula in the world?

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E mc2

• m is mass in kilograms
• c is speed of light in meters/s
• E (energy) is in joules
• very small amounts of mass may be converted into
  a very large amount of energy

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Who came up with it?
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http//msnbcmedia3.msn.com/j/msnbc/Components/Phot
os/z_Projects_in_progress/050418_Einstein/050405_e
instein_tongue.widec.jpg
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http//observe.arc.nasa.gov/nasa/exhibits/stars/st
ar_6.html
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Process is actually more complicated
Steps 1 and 2 must occur twice
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Reaction in Sun

• Takes an average of 109 years to complete
• It if occurred faster, Sun would run out of fuel

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Neutrinos

• Neutrinos almost massless particles
• No charge
• First postulated in 1930 by Wolfgang Pauli to
  explain conservation of energy in the decay of a
  neutron into a proton and an electron.
• It takes a neutrino about 2 seconds to exit the
  Sun

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The solar neutrino problem

• Less neutrinos appear to be produced from the Sun
  than expected from models

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How was the Homestake Gold Mine used to detect
neutrinos?

• A 400,000 liter vat of chlorine-containing
  cleaning fluid was placed in the Homestake gold
  mine
• Every so often Chlorine would capture a neutrino
  and turn into radioactive argon
• Modelers predict 1 reaction per day
• Experiments found 1 reaction every 3 days
• Later discovered that this experiment was only
  sensitive to one of three types of neutrinos

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• How much longer will it take the Sun to use up
  all its fuel?
• When the Sun uses up its fuel it will start
  expanding, which will be bad for people living on
  Earth

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Things you need to know

• Energy source for sun is four hydrogen atoms
  combining to produce one helium atom
• About 0.7 of the original mass is turned into
  energy during this process
• 10 of the Suns mass is hot (8 million Kelvin
  or above) enough to undergo fusion
• Mass of the Sun 2 x 1030 kg

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And

• Total lifetime (energy available)
• (rate energy/time at which sun emits
  energy)
• rate energy/time at which the Sun emits energy
  is equal to 3.8 x 1026 Watts (Joules/second)

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And

• Time left Lifetime current age
• Current age 5 billion years

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Calculation

• Mass of the Sun that is turned into energy
• m 2 x 1030 kg 10 0.7
• m 1.4 x 1027 kg of Sun can be turned into
  energy
• E mc2
• E 1.4 x 1027 kg times 9 x 1016 m2/s2
• E 1.26 x 1044 Joules

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Calculation

• Lifetime 1.26 x 1044 Joules/3.8 x 1026
  Joules/second
• Lifetime 3.3 x 1017 seconds
• Lifetime 1.05 x 1010 years
• Time left 10.5 billion years 5 billion years
• Time left 5.5 billion years

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Fusion

• The rate of nuclear fusion is a function of
  temperature
• Hotter temperature higher fusion rate
• Lower temperature lower fusion rate
• If the Sun gets hotter or colder, it may not be
  good for life on Earth

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What is happening to the amount of Helium in the
Sun?

• A) Its increasing
• B) its decreasing
• C) Its staying the same

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What is happening to the amount of Helium in the
Sun?

• A) Its increasing
• B) its decreasing
• C) Its staying the same

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So how does the Sun stay relatively constant in
Luminosity (power output)
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Classification of Stars

• Stars are classified according to luminosity and
  surface temperature
• Luminosity is the amount of power (energy/second)
  it radiates into space
• Surface temperature is the temperature of the
  surface

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Hertzsprung-Russell Diagram

• If you plot temperature versus luminosity of
  stars, the stars plot in specific regions

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OBAFGKM

• Oh Be A Fine Girl/Gal Kiss Me
• Play song
• http//www.mtholyoke.edu/courses/tburbine/ASTR223/
  OBAFGKM.mp3

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Remember

• Temperature on x-axis (vertical) does from higher
  to lower temperature
• O hottest
• M - coldest

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Hertzsprung-Russell Diagram

• Most stars fall along the main sequence
• Stars at the top above the main sequence are
  called Supergiants
• Stars between the Supergiants and main sequence
  are called Giants
• Stars below the Main Sequence are called White
  Dwarfs

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Radius

• Smallest stars on the main sequence fall on the
  bottom right
• Largest stars on main sequence fall on the top
  left
• At the same size, hotter stars are more luminous
  than cooler ones
• At the same temperature, larger stars are more
  luminous than smaller ones

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Main Sequence Stars

• Fuse Hydrogen into Helium for energy
• On main sequence, mass tends to decrease with
  decreasing temperature

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Any Questions?