15.2 Distances of Galaxies

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15.2 Distances of Galaxies

• Our Goals for Learning

How do we measure the distances to galaxies?
What is Hubbles Law? How do distance
measurements tell us the age of the universe?
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How do we measure the distances to galaxies?
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Step 1 Determine size of solar system using radar
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Step 2 Determine distances of stars out to a few
hundred light-years 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 stars 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 clusters
main sequence tells us its distance
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Which kind of stars are best for measuring large
distances?

• High-luminosity stars
• Medium-luminosity stars
• Low-luminosity stars

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Cepheid variable stars are very luminous
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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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Hubbles extra-sharp vision allows us to observe
individual Cepheid variable stars in galaxies up
to 100 million light-years away
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Clicker Question Youve 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?

• Galaxy A
• Galaxy B
• Both are the same distance

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White-dwarf supernovae can also be used as
standard candles
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Step 5 Apparent brightness of white-dwarf
supernova (another kind of standard candle) tells
us the distance to its galaxy (up to 10 billion
light-years)
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 Hubbles Law?
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By measuring velocities and distances of
galaxies, Hubble found that a galaxys velocity
and distance are related in a special way
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The spectral features of virtually all galaxies
are redshifted ? Theyre all moving away from us
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(No Transcript)
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Redshift of a galaxy tells us its distance
through Hubbles Law distance
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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 Hubbles law?
• Hubbles law tells us that more distant galaxies
  are moving away faster. It allows us to determine
  a galaxys distance from the speed at which it is
  moving away from us, which we can measure from
  its Doppler shift.

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Activity 49, page 185-188

• Page 186 has 3 photos of a flat (2-dimensional)
  universe of galaxies, taken at 3 different times.
• Use it to answer the questions in Part I on page
  185.
• Well 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
• No
• It depends

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

• Yes
• No
• Maybe

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

• All galaxies redshifted
• All galaxies blueshifted
• Some galaxies redshifted, some galaxies
  blueshifted

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6. If someone in another galaxy measured radial
velocities of galaxies besides their own, what
would they see?

• All other galaxies redshifted
• All other galaxies blueshifted
• Some galaxies redshifted, some galaxies
  blueshifted

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Part II, page 187 (instructions on page 186)

• Using Figure 2, fill in the blank entries in the
  lower right of Table 1 youre 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
  youve 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.

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

• Faster
• Slower
• Neither speed is independent of distance

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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?

• 2 billion years
• 4 billion years
• 8 billion years

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10. Who do you think is correct?

• Sam
• Samantha