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Galaxies

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Galaxies and Cosmology ... The study of galaxies is thus intimately connected with cosmology the study of ... effects of expansion in virtually all galaxies ... – PowerPoint PPT presentation

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Title: Galaxies


1
Galaxies
  • Dr Bryce
  • 2950

2
Class notices
3
Edwin Hubble
4
The Great Debate
  • Before Hubble, some scientists argued that
    spiral nebulae were entire galaxies like our
    Milky Way, while others maintained they were
    smaller collections of stars within the Milky Way
  • The debate remained unsettled until someone
    finally measured their distances

5
Hubble settled the debate by measuring the
distance to the Andromeda Galaxy using Cepheid
variables as standard candles
6
Distribution of Galaxies
  • We find few galaxies in the plane of the Milky
    Way
  • The Milky Way obscures our view

7
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 cosmology the study of the
    structure and evolution of the universe

8
Hubble Ultra Deep Field
Spiral Galaxy
9
Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
10
Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Irregular Galaxies
Spiral Galaxy
11
halo
disk
bulge
Spiral Galaxy
12
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
13
Hubble Classification
  • Tuning fork diagram
  • Not an evolutionary flow (unlike HR diagrams)

14
Hubbles galaxy classes
Spheroid Dominates
Disk Dominates
15
Elliptical Galaxies
  • Have a large range of sizes from 1 million solar
    masses to 1014 solar masses
  • Stellar motions are randomly orientated, stars
    are orbiting the galactic centre but not in an
    organised fashion

16
Line of Sight
  • Ellipiticals are classified according to their
    appearance, but this actually depends on the
    angle through which they are viewed

17
Elliptical Galaxy All spheroidal component,
virtually no disk component Few young stars Not
much interstellar medium
18
Barred Spirals
  • Called Sb
  • Young stars in disk
  • Significant amounts of interstellar gas and dust

19
Barred Spiral Galaxy Has a bar of stars across
the bulge
20
Spiral Galaxies
  • Called S
  • Young stars in disk
  • Significant amounts of interstellar gas and dust

21
Lenticular Galaxy Has a disk like a spiral
galaxy but much less dusty gas (intermediate
between spiral and elliptical)
22
Irregular Galaxy
23
Spiral galaxies are often found in groups of
galaxies (up to a few dozen galaxies)
24
Elliptical galaxies are much more common in huge
clusters of galaxies (hundreds to thousands of
galaxies)
25
Considering distance
26
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27
Step 1 Determine size of solar system using radar
28
Step 2 Determine distances of stars out to a few
hundred light-years using parallax
29
Luminosity passing through each sphere is the
same Area of sphere 4p
(radius)2 Divide luminosity by area to get
brightness
30
The relationship between apparent brightness
and luminosity depends on distance We can
determine a stars distance if we know its
luminosity and can measure its apparent
brightness A
standard candle is an object whose luminosity we
can determine without measuring its distance
31
Step 3 Apparent brightness of star clusters
main sequence tells us its distance
32
Step 4 Because the period of a Cepheid variable
star tells us its luminosity, we can use these
stars as standard candles
33

Cepheid variable stars are very luminous
34
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35
Step 5 Apparent brightness of white-dwarf
supernova tells us the distance to its
galaxy (up to 10 billion light-years)
36
Step 6 Tully-Fisher Relation Entire galaxies
can also be used as standard candles because
galaxy luminosity is related to rotation speed
37
Hubbles law
38
The spectral features of virtually all galaxies
are redshifted ? Theyre all moving away from us
39
Hubbles Law
  • Hubbles Law velocity H0 x distance

40
Redshift of a galaxy tells us its distance
through Hubbles Law distance
velocity H0
41
Distances of farthest galaxies are measured from
redshifts
42
Thought Question
  • Your friend leaves your house. She later calls
    you on her cell phone, saying that shes been
    driving at 60 miles an hour directly away from
    you the whole time and is now 60 miles away. How
    long has she been gone?
  • A. 1 minute
  • B. 30 minutes
  • C. 60 minutes
  • D. 120 minutes

43
Thought Question
  • Your observe a galaxy moving away from you at 0.1
    light-years per year, and it is now 1.4 billion
    light-years away from you. How long has it taken
    to get there?
  • A. 1 million years
  • B. 14 million years
  • C. 10 billion years
  • D. 14 billion years

44
The expansion of the Universe
45
The expansion rate appears to be the same
everywhere in space The universe has no center
and no edge (as far as we can tell)
46
One example of something that expands but has no
center or edge is the surface of a balloon
47
The cosmic horizon
  • The universe does not seem to have an edge
  • But there is a horizon i.e. a place beyond which
    we cant see
  • The cosmic horizon is a boundary in time NOT
    space
  • We cannot see anything with a lookback time
    greater than 14 billion years

48
Important points
  • We observe the effects of expansion in virtually
    all galaxies
  • Measurements of the rate of expansion tell us
    that it started 14 billion years ago
  • Astronomers use many techniques to verify this
    rate, do not depend on just one set type of
    observation
  • The Universe (Spacetime) is expanding and the
    galaxies are being carried along

49
Cosmological Principle
  • The universe looks about the same no matter
    where you are within it
  • Matter is evenly distributed on very large scales
    in the universe
  • No center no edges
  • Not proved but consistent with all observations
    to date

50
Hubbles constant tells us age of universe
because it relates velocities and distances of
all galaxies Age 1 / H0
Distance Velocity
51
Deep observations show us very distant galaxies
as they were much earlier in time (Old light
from young galaxies)
52
Distances between faraway galaxies change while
light travels Astronomers think in terms of
lookback time rather than distance
distance?
53
Expansion stretches photon wavelengths causing a
cosmological redshift directly related to
lookback time
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