What is a Standard Candle? Alexander L. Rudolph Professeur Invit

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What is a Standard Candle?Alexander L.
RudolphProfesseur Invité, UPMCProfessor of
Physics and AstronomyCalifornia State
Polytechnic University, Pomona
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Stellar Luminosity

• Apparent brightness is a measure of how bright a
  star appears on Earth
• Luminosity is a measure of how much energy per
  second (W) a star emits
• The apparent brightness of an object declines
  with distance (inverse square)
• If we measure apparent brightness (energy/sec/m2)
  and we know distance, we can get the luminosity
  of the star
• For Sun, apparent brightness 1400 W/m2 and
  d 150 million km 1.5 1011 m

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Brightness Quiz I

• Two identical 50 W bulbs are placed at different
  distances
• from you. Which one appears brighter?
• The closer one
• The further one
• They appear the same brightness
• There is not enough information to tell

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Brightness Quiz II

• A 50 W and a 100 W bulb are placed the same
  distance
• from you. Which one appears brighter?
• The 50 W bulb
• The 100 W bulb
• They appear the same brightness
• There is not enough information to tell

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Brightness Quiz III

• A 50 W and a 100 W bulb are placed at different
  distances
• from you. If the 100 W bulb is closer, which one
  appears
• brighter?
• The 50 W bulb
• The 100 W bulb
• They appear the same brightness
• There is not enough information to tell

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Brightness Quiz IV

• A 50 W and a 100 W bulb are placed at different
  distances
• from you. If the 50 W bulb is closer, which one
  appears
• brighter?
• The 50 W bulb
• The 100 W bulb
• They appear the same brightness
• There is not enough information to tell

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Brightness Quiz V

• Two identical stars, one 5 light years from
  Earth, and a
• second 50 light years from Earth are discovered.
  How much
• fainter does the farther star appear to be?
• square root of 10
• 10
• 100
• 1,000
• the farther star does not appear fainter, since
  it is identical

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The Distance Ladder

• Radar ranging (few AU)
• Parallax (AU few ? 1000 light years)
• Main sequence fitting (100s ? 10s of thousands of
  light years)
• Cepheid variable stars (100s of thousands ? 100s
  of millions of light years)
• Distant standards (Tully-Fisher, White Dwarf
  supernovae 10s of millions to 10 billion light
  years)

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Distance and Parallax

• As the Earth orbits the Sun, relatively nearby
  stars appear to move relative to more distant
  stars
• Because even the nearest stars are so distant,
  there is a simple relationship between distance
  and apparent angle a star moves
• 1 parsec 3.26 light years

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Standard Candles

• We can use this equation to find the distances to
  objects in the universe using the concept of a
  standard candle
• Thus, if we know an objects luminosity, and
  measure its apparent brightness, we can find its
  distance

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Standard Candles - Examples

• Cepheid variable stars (Inter. Fig. II)
• Very bright stars which vary in luminosity in a
  regular way
• A relation exists between variation period and
  the stars luminosity
• Calibrated using nearby Cepheids of known
  distance
• White Dwarf (Type I) supernovae
• All WD Sne cross the Chandrasekhar limit and
  explode in the same way hence all have similar
  luminosities
• Calibrated in nearby galaxies of known distance

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Galactic Distances Quiz

• Cepheid variable stars are located in two
  different galaxies, A and B.
• Both stars have the same average apparent
  brightness. The star in
• galaxy A has a bright-dim-bright period of 10
  days, while the one in
• galaxy B has a bright-dim-bright period of 30
  days. Which of the two
• galaxies is at a greater distance from us?
• Galaxy A
• Galaxy B
• They are located at the
• same distance.
• There is insufficient
• information to tell.