Download What is a standard candle?

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
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)
Luminosity
Apparent brightness =
4p ´ (distance)2
•
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
L = 4p (1400 W/m2)(1.5 ´ 1011 m)2
= 4 ´ 1026 W
Brightness 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
EUHOU Teacher Training - April 2012
Brightness Quiz II
A 50 W and a 100 W bulb are placed the same distance
from 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
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?
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
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?
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
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?
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
Standard Candles
•
We can use this equation to
find the distances to objects
in the universe using the
concept of a standard candle
Apparent brightness =
•
Luminosity
4p ´ (distance)2
Thus, if we know an object’s
luminosity, and measure its
apparent brightness, we can
find its distance
EUHOU Teacher Training - April 2012
Standard Candles - Examples
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Cepheid variable stars (Inter. Fig. II)
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•
•
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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
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?
a)
b)
c)
d)
Galaxy A
Galaxy B
They are located at the
same distance.
There is insufficient
information to tell.