1. EUHOU Teacher Training - April 2012
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

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. EUHOU Teacher Training - April 2012
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. 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?
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. 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?
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. 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?
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. 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?
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. 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)

9. EUHOU Teacher Training - April 2012
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
EUHOU Teacher Training - April 2012

10. 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
Thus, if we know an object’s luminosity, and measure its apparent brightness, we can find its distance
EUHOU Teacher Training - April 2012

11. 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

12. 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.