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Galaxies

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Galaxies What is a galaxy? How many stars are there in an average galaxy? About how many galaxies are there in the universe? What is the name of our galaxy? – PowerPoint PPT presentation

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Title: Galaxies


1
Galaxies
  • What is a galaxy?
  • How many stars are there in an average galaxy?
  • About how many galaxies are there in the
    universe?
  • What is the name of our galaxy?

2
(No Transcript)
3
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?

4
AGN
  • What is the power source that generates such
    enormous luminosities in AGNs?

5
AGN Energy Source
  • Rapidly varying intensity gt small size
  • High luminosity and small size gt supermassive
    black hole
  • Billions of solar masses!
  • Consumes entire stars!

6
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?

7
Cosmic Distance Scale
  • Standard Candles
  • Extremely bright events with well-known
    luminosities
  • Ex. Type Ia (carbon-detonation) supernovae
  • Works out to 3 billion light years!

8
Structures of Galaxies
Groups A few to a few dozen galaxies bound
together by their combined gravity.
9
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
10
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.
11
Cosmological Structure?
  • We have seen that galaxies are organized into
    clusters and superclusters on intermediate scales
    in the universe.
  • How is matter distributed in the universe on the
    very largest scales?

12
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 gt ?
  • Isotropy gt ?

13
No Structure on Largest Scales (Galaxies
distributed fairly uniformly)
  • Surprising given structure on smaller scales
  • Cosmological Principle
  • Universe is homogeneous and isotropic
  • Homogeneity gt universe has no edge
  • Isotropy gt universe has no center

14
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?
15
The universe is expanding!
gt Assumption that universe is unchanging
is incorrect. gt Doesnt matter whether
universe is finite or infinite, we only see a
finite part. gt Light from objects greater than
14 billion light years away has not had time to
reach us!
16
Cosmological Scales
  • How do we know that the universe is expanding?

17
Almost all galaxies exhibit show redshifted
spectral lines gt they are receding from
us! Size of universe is not static, nor
shrinking due to influence of gravity. Universe
is expanding.
18
Redshift or recessional velocity is proportional
to distance gt the farther away a galaxy is,
the faster it is receding.
V H0 x D (Hubble's Law)
velocity (km / sec) Distance
(Mpc) Hubble's Constant (km
/ sec / Mpc)
Or graphically. . .
Current estimate H0 60 - 75 km/sec/Mpc
19
Hubble Expansion Analogy
20
The Fate of the Universe
  • What property determines the ultimate fate of the
    universe?

21
Affect of Density on the Universe
22
Density Determines the Eventual Fate of the
Universe
  • How is the expansion rate changing with time?
  • Will the universe expand forever?

23
Question
  • What is it that is causing the expansion of the
    universe to accelerate?

24
  • We aren't sure, but we call it dark energy .
  • 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

25
Review Question
  • The spectrum of an AGN is _____.
  • A) mainly due to starlight
  • B) very dim
  • C) non-blackbody
  • D) highly stable

26
Review Question
  • 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

27
Review Question
  • Isotropy implies that the universe has no _____.
  • A) center
  • B) curvature
  • C) horizon
  • D) edge
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