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Stellar Luminosity

What is a Standard Candle? Alexander L. Rudolph Professeur Invité, UPMC Professor of Physics and Astronomy California State Polytechnic University, Pomona. Stellar Luminosity. Apparent brightness is a measure of how bright a star appears on Earth

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Stellar Luminosity

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  1. What is a Standard Candle?Alexander L. RudolphProfesseur Invité, UPMCProfessor of Physics and AstronomyCalifornia State Polytechnic University, Pomona EUHOU Teacher Training - April 2012

  2. 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

  3. 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 EUHOU Teacher Training - April 2012

  4. 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 EUHOU Teacher Training - April 2012

  5. 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 EUHOU Teacher Training - April 2012

  6. 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 EUHOU Teacher Training - April 2012

  7. 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 EUHOU Teacher Training - April 2012

  8. 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 object’s luminosity, and measure its apparentbrightness, we can find its distance EUHOU Teacher Training - April 2012

  9. 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 star’s 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 EUHOU Teacher Training - April 2012

  10. 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.

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