Transcript Galaxies - List of Physics Courses

Galaxies
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What is a galaxy?
How many stars are there in an average
galaxy?
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About how many galaxies are there in the
universe?
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What is the name of our galaxy?
Galaxy
• Gravitationally bound stars, gas, dust, neutron stars, black holes, etc.
• Billions of stars in a galaxy and billions of galaxies in the universe
• Milky Way: 1,000 LY thick and 100,000 LY across!
Active Galactic Nuclei (AGN)
•Normal Galaxies – one
million to one trillion
solar luminosities
•AGN - several hundreds
of trillions of solar
luminosities!
–Spectrum is not black body!
What does this mean?
AGN
What is the power source that generates such
enormous luminosities in AGNs?
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AGN Energy Source
•Rapidly varying intensity
=> small size
•High luminosity and small
size => supermassive
black hole
•Billions of solar masses!
•Consumes entire stars!
Cosmic Distance Scale
•Closest galaxy:
Andromeda – 2.5 million
light years away!
•“Standard Candles”
•Extremely bright events
with well-known
luminosities
•Can you think of an example
of such an event?
Cosmic Distance Scale
• “Standard Candles”
•Extremely bright events
with well-known
luminosities
•Ex. Type Ia (carbondetonation) supernovae
•Works out to ~ 3 billion light
years!
Structures of Galaxies
On intermediate scales, galaxies form identifiable structures.
Groups
A few to a few dozen galaxies bound together by their combined gravity.
Clusters
Largest (known) gravitationally bound structures
- typically containing thousands of galaxies.
The Virgo Cluster of about
2500 galaxies (central part shown). The center of the Hercules Cluster
Superclusters
Larger structures made up of clusters and groups. 10,000's of galaxies.
The Local Supercluster consists of the Virgo Cluster,
the Local Group and several other groups.
Cosmological Structure?
We have seen that galaxies are organized into
clusters and superclusters on intermediate
scales in the universe.
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How is matter distributed in the universe on the
very largest scales?
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No Structure on Largest Scales
(Galaxies distributed fairly uniformly)
•Surprising given observations
of Groups, clusters, and
superclusters on smaller scales
•Cosmological Principle
•Universe is homogeneous and
isotropic
•Homogeneity => ?
•Isotropy => ?
No Structure on Largest Scales
(Galaxies distributed fairly uniformly)
•Surprising given structure
on smaller scales
•Cosmological Principle
•Universe is homogeneous and
isotropic
•Homogeneity => universe has
no edge
•Isotropy => universe has no
center
Olbers` Paradox
If the universe is
homogeneous, isotropic,
infinite, and unchanging,
then the night sky should be
as bright as the sun.
Why isn't it?
The universe is expanding!
=> Assumption that universe is unchanging is
incorrect.
=> Doesn’t matter whether universe is finite or
infinite, we only see a finite part.
=> Light from objects greater than 14 billion light
years away has not had time to reach us!
Cosmological Scales
How do we know that the universe is
expanding?
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Almost all galaxies exhibit show redshifted spectral lines
=> they are receding from us!
Size of universe is not static, nor shrinking due to influence of gravity.
Universe is expanding.
Redshift or recessional velocity is proportional to distance:
=> the farther away a galaxy is, the faster it is receding.
V = H0 x D
velocity (km / sec)
(Hubble's Law)
Distance (Mpc)
1 pc ~ 3 Light Years
Hubble's Constant (km / sec / Mpc)
Or graphically. . .
Current estimate:
H0 = 60 - 75
km/sec/Mpc
Hubble Expansion Analogy
1 Mpc
6 Mpc
12 Mpc
1 pc ~ 3 Light Years
2 Mpc
12 Mpc
24 Mpc
Accuracy increases with distance.
● The farther out , the faster the expansion.
● Works for any chosen point of reference.
● As the ruler cools, all points converge.
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The Fate of the Universe
What property determines the ultimate fate of
the universe?
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Affect of Density on the Universe
Density Determines the Eventual
Fate of the Universe
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How is the expansion rate changing with time?
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Will the universe expand forever?
Question
What is it that is causing the expansion of the
universe to accelerate?
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We aren't sure, but we call it dark energy .
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Repulsive nature suggests that universe may expand forever.
• Largest contributor to cosmic density, more than normal matter or
even dark matter!
Recent Progress!
• Cosmological constant – One possible source of dark energy
• Effect of nonzero vacuum energy (pair creation)
• Knew it was repulsive, but couldn't calculate strength
• Recent work yields value consistent w/ observations
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Review Question
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The spectrum of an AGN is _____.
–A)
mainly due to starlight
–B)
very dim
–C)
non-blackbody
–D)
highly stable
Review Question
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Hubble's law tells us that the ______.
–A)
closer a galaxy is, the faster it moves away
–B)
farther a galaxy is, the faster it moves away
–C)
farther a galaxy is, the slower it moves away
–D)
farther a galaxy is, the larger it is
Review Question
Isotropy implies that the universe has no
_____.
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–A)
center
–B)
curvature
–C)
horizon
–D)
edge