Chapter 15 A Universe of Galaxies

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Chapter 15A Universe of Galaxies
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15.1 Islands of Stars

• Our Goals for Learning

What are the three major types of galaxies?
How are galaxies grouped together?
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What are the three major types of galaxies?
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Hubble Ultra Deep Field
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Hubble Ultra Deep Field
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Disk Component stars of all ages, many gas clouds
Spheroidal Component bulge halo, old
stars, few gas clouds
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Disk Component stars of all ages, many gas clouds
Blue-white color indicates ongoing star formation
Spheroidal Component bulge halo, old
stars, few gas clouds
Red-yellow color indicates older star population
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Disk Component stars of all ages, many gas clouds
Blue-white color indicates ongoing star formation
Spheroidal Component bulge halo, old
stars, few gas clouds
Red-yellow color indicates older star population
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Beyond the three main galaxy types

• Dwarf galaxies (dwarf spirals, dwarf ellipticals,
  dwarf irregulars) smaller and less massive
  other properties very similar
• Low surface brightness galaxies stars are spread
  out over a larger-than-normal area can be dwarf
  or giant, spiral or elliptical
• Merging/interacting galaxies (discussed later)

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How are galaxies grouped together?
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Spiral galaxies are often found in groups of
galaxies (up to a few dozen galaxies)
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Elliptical galaxies are much more common in huge
clusters of galaxies (hundreds to thousands of
galaxies)
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What have we learned?

• What are the three major types of galaxies?
• (1) Spiral galaxies have prominent disks and
  spiral arms.
• (2) Elliptical galaxies are rounder and redder
  than spiral galaxies and contain less cool gas
  and dust.
• (3) Irregular galaxies are neither disklike nor
  rounded in appearance.

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What have we learned?

• How are galaxies grouped together?
• Spiral galaxies tend to collect in groups of
  galaxies, which contain up to several dozen
  galaxies.
• Elliptical galaxies are more common in clusters
  of galaxies, which contain hundreds to thousands
  of galaxies, all bound together by gravity.

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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?

1. High-luminosity stars
2. Medium-luminosity stars
3. 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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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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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 36, page 127-130

• Page 128 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
  127.
• Then do the same for Part II.

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Part II, page 129 (instructions on page 127)

• 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 (pages 129-130).