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Stellar Luminosity

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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 - PowerPoint PPT Presentation
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What is a Standard Candle? Alexander L. Rudolph Professeur Invité, UPMC Professor of Physics and Astronomy California State Polytechnic University, Pomona EUHOU Teacher Training - April 2012
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Page 1: Stellar Luminosity

What is a Standard Candle?

Alexander L. RudolphProfesseur Invité, UPMC

Professor of Physics and AstronomyCalifornia State Polytechnic University,

Pomona

What is a Standard Candle?

Alexander L. RudolphProfesseur Invité, UPMC

Professor of Physics and AstronomyCalifornia State Polytechnic University,

Pomona

EUHOU Teacher Training - April 2012

Page 2: Stellar Luminosity

Stellar LuminosityStellar 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

• 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

Page 3: Stellar Luminosity

Brightness Quiz IBrightness Quiz I

Two identical 50 W bulbs are placed at different distances

from you. Which one appears brighter?

A. The closer one

B. The further one

C. They appear the same brightness

D. There is not enough information to tell

Two identical 50 W bulbs are placed at different distances

from you. Which one appears brighter?

A. The closer one

B. The further one

C. They appear the same brightness

D. There is not enough information to tell

EUHOU Teacher Training - April 2012

Page 4: Stellar Luminosity

Brightness Quiz IIBrightness Quiz II

A 50 W and a 100 W bulb are placed the same distancefrom you. Which one appears brighter?

A. The 50 W bulb

B. The 100 W bulb

C. They appear the same brightness

D. There is not enough information to tell

A 50 W and a 100 W bulb are placed the same distancefrom you. Which one appears brighter?

A. The 50 W bulb

B. The 100 W bulb

C. They appear the same brightness

D. There is not enough information to tell

EUHOU Teacher Training - April 2012

Page 5: Stellar Luminosity

Brightness Quiz IIIBrightness 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?

A. The 50 W bulb

B. The 100 W bulb

C. They appear the same brightness

D. There is not enough information to tell

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?

A. The 50 W bulb

B. The 100 W bulb

C. They appear the same brightness

D. There is not enough information to tell

EUHOU Teacher Training - April 2012

Page 6: Stellar Luminosity

Brightness Quiz IVBrightness 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?

A. The 50 W bulb

B. The 100 W bulb

C. They appear the same brightness

D. There is not enough information to tell

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?

A. The 50 W bulb

B. The 100 W bulb

C. They appear the same brightness

D. There is not enough information to tell

EUHOU Teacher Training - April 2012

Page 7: Stellar Luminosity

Brightness Quiz VBrightness Quiz VTwo 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?

A. square root of 10

B. 10

C. 100

D. 1,000

E. the farther star does not appear fainter, since it is identical

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?

A. square root of 10

B. 10

C. 100

D. 1,000

E. the farther star does not appear fainter, since it is identical

EUHOU Teacher Training - April 2012

Page 8: Stellar Luminosity

Standard Candles 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 apparent brightness, we can find its distance

• 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 apparent brightness, we can find its distance

EUHOU Teacher Training - April 2012

Page 9: Stellar Luminosity

• 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

• 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

Standard Candles - Examples Standard Candles - Examples

EUHOU Teacher Training - April 2012

Page 10: Stellar Luminosity

Galactic Distances QuizGalactic 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?

a) Galaxy Ab) Galaxy Bc) They are located at the

same distance.d) There is insufficient

information to tell.

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?

a) Galaxy Ab) Galaxy Bc) They are located at the

same distance.d) There is insufficient

information to tell.


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