Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional)...

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15.2 Distances of Galaxies Our Goals for Learning • How do we measure the distances to galaxies? • What is Hubble’s Law? • How do distance measurements tell us the age of the universe?

Transcript of Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional)...

Page 1: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

15.2 Distances of Galaxies

• Our Goals for Learning• How do we measure the distances to galaxies?

• What is Hubble’s Law?

• How do distance measurements tell us the age of the universe?

Page 2: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

How do we measure the distances to galaxies?

Page 3: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

Step 1

Determine size of solar system using radar

Page 4: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

Step 2

Determine distances of stars out to a few hundred light­years using parallax

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Brightness alone does not provide enough information to measure distance

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   The relationship between apparent brightness and luminosity depends on distance:

                                  Luminosity     Brightness =                                 4 pi  x   (distance)2

  We can determine a star’s distance if we know its luminosity and can measure its apparent brightness:

                                  Luminosity     Distance  =                               4 pi  x   Brightness

  A standard candle is an object whose luminosity we can determine without measuring its distance.We can use standard candles to measure distances.

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Step 3

Apparent brightness of star cluster’s main sequence tells us its distance

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Which kind of stars are best for measuring large distances?

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8%

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1. High­luminosity stars

2. Medium­luminosity stars

3. Low­luminosity stars

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Cepheid  variable stars are very luminous

    

Page 10: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

Cepheid variable stars with longer periods have greater luminosities

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Step 4

The period of a Cepheid variable star tells us its luminosity.  Its luminosity plus its apparent brightness tells us its distance. 

So we can use Cepheid variable stars as standard candles to measure distances.

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Edwin Hubble, using Cepheids as standard candles, was the first to measure distances to other galaxies

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Measuring distances using Cepheids has been a key mission of the Hubble Space Telescope

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Hubble’s extra­sharp vision allows us to observe individual Cepheid variable stars in galaxies up to 100 million light­years away

Galaxy  M100

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Clicker Question: You’ve found a Cepheid variable star in Galaxy A with a period of 100 days.  You've also found a Cepheid variable star in Galaxy B with a period of 10 days.  Both Cepheids have the same brightness as seen from Earth.  Which galaxy is farther away from us?

• GET YOUR ANSWER READY:

• A) Galaxy A• B) Galaxy B• C) Both are the 

same distance

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Which galaxy is farther away?

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1. Galaxy A

2. Galaxy B

3. Both are the same distance

Page 17: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

White­dwarf supernovae can also be used as standard candles

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Step 5

Apparent brightness of white­dwarf supernova (another kind of standard candle) tells us the distance to its galaxy

(up to 10 billion light­years)

Supernova

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In summary, we measure galaxy distances using a chain of techniques known as the distance ladder

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What is Hubble’s Law?

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By measuring velocities and distances of galaxies, Hubble found that a galaxy’s velocity and distance are related in a special way

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The spectral features of virtually all galaxies are redshifted  ⇒  They’re all moving away from us

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Hubble’s Law:    velocity = H0 x distance

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Redshift of a galaxy tells us its distance through Hubble’s Law:

 distance = 

velocity    H0

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Distances of farthest galaxies are measured from redshifts

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What have we learned?• How do we measure the distances to galaxies?• Our measurements of galaxy distances depend on a 

chain of methods. The chain begins with radar ranging in our own solar system and parallax measurements of distances 

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What have we learned?

• What is Hubble’s law?• Hubble’s law tells us 

that more distant galaxies are moving away faster. It allows us to determine a galaxy’s distance from the speed at which it is moving away from us, which we can measure from its Doppler shift. 

Page 28: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

Activity #49, page 185­188

• Page 186 has 3 photos of a flat (2­dimensional) universe of galaxies, taken at 3 different times.

• Use it to answer the questions in Part I on page 185.• We’ll do those as clicker questions, then do the 

same for Part II.

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2. If you were in one of the other galaxies, would the MW (Milky Way) galaxy appear 

to be moving away from you?

 Yes  N

o

 It de

pends

88%

3%9%

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1. Yes2. No3. It depends

Page 30: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

4. Are any of the galaxies moving closer to each other?

 Yes  N

o

 May

be

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4%

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1. Yes2. No3. Maybe

Page 31: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

5. If we measured radial velocities of other galaxies, what would we see?

 All g

alaxie

s red

shift

ed

 All g

alaxie

s blues

hifted

 Some g

alaxie

s red

shi...

88%

10%2%

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1. All galaxies redshifted

2. All galaxies blueshifted

3. Some galaxies redshifted, some galaxies blueshifted

Page 32: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

6. If someone in another galaxy measured radial velocities of galaxies besides their 

own, what would they see?

 All o

ther

 galax

ies re

d...

 All o

ther

 galax

ies blu...

 Some g

alaxie

s red

shi...

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1. All other galaxies redshifted

2. All other galaxies blueshifted

3. Some galaxies redshifted, some galaxies blueshifted

Page 33: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

Part II, page 187 (instructions on page 186)

• Using Figure 2, fill in the blank entries in the lower right of Table 1: you’re measuring how far galaxies A and B moved between images II and III, divided by the time elapsed between images II and III.

• Then, using the bottom row of Table 1 (which you’ve just filled in), in Figure 3 plot the velocities of galaxies A and B as a function of their distance.

• Then answer questions 8D, 9, 10.

Page 34: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

8D. Figure 3 shows that the farther away a galaxy is, the ___ it is moving away from 

the reference galaxy.

 Faster

 Slow

er

 Neit

her –

 spee

d is i

n...

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1. Faster2. Slower3. Neither – speed is 

independent of distance

Page 35: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

9. How long has galaxy B been travelling if it started at the origin in Fig. 2, moved at constant speed, and ended up at its position in Image III?

6%

39%

55%

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1. 2 billion years

2. 4 billion years

3. 8 billion years

Page 36: Our Goals for Learning - York University · • Page 186 has 3 photos of a flat (2dimensional) universe of galaxies, taken at 3 different times. • Use it to answer the questions

10.  Who do you think is correct?

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10%

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1. Sam

2. Samantha