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Galaxies and the Foundation of Modern Cosmology

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Galaxies and the Foundation of Modern Cosmology what are the three major types of galaxies? How are galaxies grouped together? Hubble Deep Field Galaxies and ... – PowerPoint PPT presentation

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Title: Galaxies and the Foundation of Modern Cosmology


1
Galaxies and the Foundation of Modern Cosmology
2
  • what are the three major types of galaxies?
  • How are galaxies grouped together?

3
Hubble Deep Field
  • Our deepest images of the universe show a great
    variety of galaxies, some of them billions of
    light-years away.

4
Galaxies and Cosmology
  • A galaxys age, its distance, and the age of the
    universe are all closely related.
  • The study of galaxies is thus intimately
    connected with cosmologythe study of the
    structure and evolution of the universe.

5
What are the three major types of galaxies?
6
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9
Spiral galaxy
10
Disk Component stars of all ages, many gas clouds
Spheroidal Component bulge and halo, old
stars, few gas clouds
11
Blue-white color indicates ongoing star formation.
Disk Component stars of all ages, many gas clouds
Spheroidal Component bulge and halo, old
stars, few gas clouds
Red-yellow color indicates older star population.
12
Thought Question
  • Why does ongoing star formation lead to a
    blue-white appearance?
  • A. There arent any red or yellow stars.
  • B. Short-lived blue stars outshine others.
  • C. Gas in the disk scatters blue light.

13
Barred Spiral Galaxy Has a bar of stars across
the bulge. We few barred galaxies in the
past. Bars are a sign of maturity or stability
14
Lenticular Galaxy Has a disk like a spiral
galaxy but much less dusty gas (intermediate
between spiral and elliptical).
15
Elliptical Galaxy All spheroidal component,
virtually no disk component
Red-yellow color indicates older star population.
16
Irregular Galaxy Neither spiral nor elliptical.
Blue-white color indicates ongoing star formation.
17
Hubbles galaxy classes
Spheroid dominates
Disk dominates
18
How are galaxies grouped together?
19
Spiral galaxies are often found in as groups of
galaxies (up to a few dozen galaxies per group).
20
Elliptical galaxies are much more common in huge
clusters of galaxies (hundreds to thousands of
galaxies).
21
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?

22
How do we measure the distances to galaxies?
23
Brightness alone does not provide enough
information to measure distance.
Are Bright Stars Nearby or Luminous?
24
Step 1 Determine size of solar system using
radar.
Radar Pulses
25
Step 2 Determine distances of stars out to a few
hundred light-years using parallax.
26
Luminosity passing through each sphere is the
same. Area of sphere 4p
(radius)2 Divide luminosity by area to get
brightness.
27
The relationship between apparent brightness
and luminosity depends on distance ONLY.
Luminosity
Brightness 4p
(distance)2 We can determine a stars distance
if we know its luminosity and can measure its
apparent brightness.
Luminosity Distance
4p ? Brightness A standard
candle is an object whose luminosity we can
determine without measuring its distance.
28
Step 3 Distance from cluster Main Sequence
Fitting.
Apparent brightness of star clusters main
sequence tells us its distance. Pleiades is a
bit less than 3 times further away than Hyades
can you see why? Hyades is close enough to
measure its distance will stellar parallax, so we
can use it to measure the distance to all
clusters in our galaxy.
29
Thought Question
  • Which kind of stars are best for measuring large
    distances?
  • A. High-luminosity stars
  • B. Low-luminosity stars

30
Thought Question
  • Which kind of stars are best for measuring large
    distances?
  • A. High-luminosity stars
  • B. Low-luminosity stars

31
Cepheid variable stars are very luminous.
32
Cepheid Variable Stars
  • The light curve of this Cepheid variable star
    shows that its brightness alternately rises and
    falls over a 50-day period.

33
Cepheid variable stars with longer periods have
greater luminosities.
34
Step 4 Using Cepheid Variables as Standard
Candles
Because the period of a Cepheid variable star
tells us its luminosity, we can use these stars
as standard candles. We can calibrate the
period-luminosity relationship of Cepheid stars
by measuring these in star clusters in our galaxy
(recall we can measure the distance to clusters
using Hyades as a baseline). Once calibrated the
Cepheid period-luminosity relation can be used to
determine the distances to galaxies within a few
tens of mega parsecs.
35
White dwarf supernovae can also be used as
standard candles.
36
Step 5 Apparent brightness of a white dwarf
supernova tells us the distance to its galaxy (up
to 10 billion light-years).
37
What is Hubbles law?
38
Hubbles Law
  • We can use Hubbles Law to determine the distance
    to extremely distance galaxies.
  • If we measure the Doppler shift we can tell from
    the redshift how fast a galaxy is moving away.
  • From the previous plot give the recession
    velocity we can estimate the distance.

39
  • Now work on the Galaxy Classification section of
    the lecture tutorial book.
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