Stars and HR Diagrams

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Stars and HR Diagrams
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Nuclear fusion reaction

• In essence, 4 hydrogen nuclei combine (fuse) to
  form a helium nucleus, plus some byproducts
  (actually, a total of 6 nuclei are involved)
• Mass of products is less than the original mass
• The missing mass is emitted in the form of
  energy, according to Einsteins famous formulas
  
• E mc2
• (the speed of light is very large, so there is a
  lot of energy in even a tiny mass)

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Hydrogen fuses to Helium

• Start 4 protons ? End Helium neutrinos
  energy
• Hydrogen fuses to
  Helium

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The Standard Solar Model (SSM)

• Sun is a gas ball of hydrogen helium
• Density and temperature increase towards center
• Very hot dense core produces all the energy by
  hydrogen nuclear fusion
• Energy is released in the form of EM radiation
  and particles (neutrinos)
• Energy transport well understood in physics

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Standard Solar Model
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Hydrostatic Equilibrium

• Two forces compete gravity (inward) and energy
  pressure due to heat generated (outward)
• Stars neither shrink nor expand, they are in
  hydrostatic equilibrium, i.e. the forces are
  equally strong

Heat
Gravity
Gravity
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More Mass means more Energy

• More mass means more gravitational pressure
• More pressure means higher density, temperature
• Higher density, temp. means faster reactions
  more reactions per time
• This means more energy is produced

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How do we know what happens in the Sun?

• We cant look into the Sun
• But come up with theory that explains all the
  features of the Sun and predicts new things
• Do more experiments to test predictions
• This lends plausibility to theory

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Details

• Radiation Zone and Convection Zone
• Chromosphere
• Photosphere
• Corona
• Sunspots
• Solar Cycle
• Flares Prominences

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Understanding Stars

• Understanding in the scientific sense means
  coming up with a model that describes how they
  work
• Collecting data (Identify the stars)
• Analyzing data (Classify the stars)
• Building a theory (Explain the classes and their
  differences)
• Making predictions
• Testing predictions by more observations

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Identifying Stars - Star Names

• Some have names that go back to ancient times
  (e.g. Castor and Pollux, Greek mythology)
• Some were named by Arab astronomers (e.g.
  Aldebaran, Algol, etc.)
• Since the 17th century we use a scheme that lists
  stars by constellation
• in order of their apparent brightness
• labeled alphabetically in Greek alphabet
• Alpha Centauri is the brightest star in
  constellation Centaurus
• Some dim stars have names according to their
  place in a catalogue (e.g. Ross 154)

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Classification by Star Properties

• What properties can we measure?
• distance
• velocity
• temperature
• size
• luminosity
• chemical composition
• mass

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Classification of the Stars Temperature

• Class Temperature Color Examples
• O 30,000 K blue
• B 20,000 K bluish Rigel
• A 10,000 K white Vega, Sirius
• F 8,000 K white Canopus
• G 6,000 K yellow Sun, ? Centauri
• K 4,000 K orange Arcturus
• M 3,000 K red Betelgeuse

Mnemotechnique Oh, Be A Fine Girl/Guy, Kiss Me
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The Key Tool to understanding Stars the
Hertzsprung-Russell diagram

• Hertzsprung-Russell diagram is luminosity vs.
  spectral type (or temperature)
• To obtain a HR diagram
• get the luminosity. This is your y-coordinate.
• Then take the spectral type as your
  x-coordinate, e.g. K5 for Aldebaran. First letter
  is the spectral type K (one of OBAFGKM), the
  arab number (5) is like a second digit to the
  spectral type, so K0 is very close to G, K9 is
  very close to M.

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Constructing a HR-Diagram

• Example Aldebaran, spectral type K5III,
  luminosity 160 times that of the Sun

L
1000
Aldebaran
160
100
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1
Sun (G2V)
O B A F G K M
Type
0123456789 0123456789 012345
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The Hertzprung-Russell Diagram

• A plot of absolute luminosity (vertical scale)
  against spectral type or temperature (horizontal
  scale)
• Most stars (90) lie in a band known as the Main
  Sequence

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Hertzsprung-Russell diagrams

• of the closest stars of the brightest stars