Chapter 31 Galaxies

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Chapter 31 Galaxies the Universe
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Review Recap
It does this by precisely measuring the speed of
gas and stars around a black hole. This
provides clues for the existence of a black hole.
Astronomers measure the motion of stars
swirling around the black hole to determine the
mass.
The Hubble Space Telescope measures black holes
that cannot be seen.

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A Black Hole

A black hole is an object with a gravitational
pull so intense that not even light can escape.
It pulls in other stars and gas with its
gravitational force.
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31.1 The Milky Way Galaxy

• Objectives
• Determine the size and shape of the Milky Way, as
  well as Earths Location within it.
• Describe how the Milky Way was formed.

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Variable Stars

• By measuring a star's period of pulsation,
  astronomers can determine its luminosity and
  calculate how far away a variable star must be to
  appear as dim or as bright as it does.

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Variable stars comparing and contrastingBoth are
variable stars that have a relationship between
its luminosity and its pulsation period.

• RR Lyrae variables

• Cepheid variables

• have periods of pulsations between 1.5 hours and
  1 day and, on average, have the same luminosity

• have pulsation periods between 1 day and more
  than 100 days.
• the longer the pulsation period the greater the
  luminosity

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The Shape of the Milky Way
Astronomers have determined the shape of the
Milky Way by using radio waves because they
penetrate the interstellar gas and dust without
being scattered or absorbed.
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Discovering Other Galaxies

• Other galaxies were first believed to be nebulae
  or star clusters within the Milky Way.

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Discovering Other Galaxies cont.

• Edwin Hubble's discovery of Cepheid variable
  stars in the Great Nebula in the Andromeda
  constellation proved the existence of galaxies
  outside the Milky Way.

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Mass of the Milky Way
Studies provide evidence that there is a great
amount of unseen matter called dark matter
composed of dim stellar remnants that have mass.
6 x 1011 times (the mass of our Sun)
Estimates vary from 200 billion x , up to 1,000
billion x the Mass of our Sun.
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Dark Matter
The strongest evidence that the universe
contains a larger amount of dark matter than
visible matter is that by analyzing the motion of
member galaxies, the mass of galaxy clusters can
be determined. This mass of whole galaxy clusters
is always much larger than the sum of only the
visible masses of the galaxies.
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Formation and Evolution of the Milky Way
The halo and bulge of the Milky Way are made of
old stars which suggests that the halo and bulge
formed first.
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Quiz 31.1 The Shape of the Milky Way
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31.2 Other Galaxies in the Universe

• Objectives
• Describe how astronomers classify galaxies.
• Identify how galaxies are organized into clusters
  and super clusters.
• Describe the expansion of the universe.

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Superclusters

• Gigantic formation of clusters of galaxies
  hundreds of millions of light-years in size are
  called superclusters.

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Active Galaxies

• Core of a galaxy in which highly energetic
  objects or activities are located. (also known as
  AGNs!)

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Quasars

• The redshift in many quasars is very large, which
  means that those quasars are far away.

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The Expanding Universe Hubbles Law

• The farther away from Earth a galaxy is, the
  faster it is moving.
• Hubble Constant - A value of approximately 70
  kilometers per second per megaparsec

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Elliptical Galaxy
This is an elliptical galaxy. Galaxies that are
not flattened into disks and do not have spiral
arms. Their shapes range from round ellipticals
to very elongated ellipticals. Possible example
Virgo A
Please copy illustration
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Elliptical Galaxy cont.
Elliptical galaxies are divided into subclasses
based on the ratio of their major and minor axes.
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Spiral Galaxy
This is a spiral galaxy. Disklike galaxy with
spiral arms consisting of interstellar matter and
young star clusters. A bulge consists of an old
stellar population. There are normal and barred
spirals. Possible example Milky Way.
Please copy illustration
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Irregular Galaxy
This is an irregular galaxy. These galaxies do
not fit the elliptical or spiral classification.
They typically have no distinct shape. Possible
example Large and Small Magellanic Clouds.
Please copy illustration
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Galaxy Cluster
This is a galaxy cluster. These groups of
galaxies may have from a few to hundreds of
member galaxies and may range in sizes up to 30
million ly. In a cluster, most of the inner
region galaxies are ellipticals. Galaxies in the
outer portions are a mix of ellipticals and
spirals. The galaxies often merge to form
strangely shaped galaxies with more than one
nucleus. Possible example Local Group.
Please copy illustration
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Galaxy Clusters cont.
Most galaxies in the inner region of a large
cluster are ellipticals.
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Galaxy Shapes
An Sc galaxy is a normal spiral with loosely
wound arms and a small, dim nucleus.
M74 Spiral Galaxy
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Galaxy Shapes Cont.
Galaxy shapes Galaxy shapes
a Tightly wound arm and large, bright nucleus
c Loosely wound arms and a small, dim nucleus
E7 Very elongated elliptical
EO Round elliptical
Irr Irregular galaxy
S Normal spiral
SB Barred spiral
SO Flat disks that do not have spiral arms
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Quiz 31.2 Other Galaxies in the Universe
(10pts)
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31.3 Cosmology

• Objectives
• Explain the different theories about the
  formation of the universe.
• Describe the possible outcomes of universal
  expansion.

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Models of the Universe
Steady-state theory - Proposes that the universe
looks the same on large scales to all observers
and that it has always looked that way

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Cosmic background radiation
Has a wavelength of approximately 1 mm, which
makes it microwave radiation.

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Cosmic background radiation
Provides information about conditions very early
in the expansion of the universe.

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The Hubble Constant
Recent observations show that the rate of
expansion of the universe is speeding up.

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Models of the Universe cont.
Big Bang Theory - States that the universe began
as a point and has been expanding ever since

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The Big Bang cont.
The theory does not suggest an explosion into
space, but instead that there is an expansion of
space with matter going along for the ride.

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Models of the Universe cont.
Inflationary Universe Theory - Model says the
universe began as a fluctuation in a vacuum and
expanded very rapidly for a fraction of a second
before settling into a more orderly expansion

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Inflationary Universe Theory cont.
A flat universe was first predicted in the 1980s
by the inflationary universe model.
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The Critical Density
Average density and Critical Density are factors
in the expansion of the universe.
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The Critical Density cont.
The average density is the total matter of the
universe.
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The Critical Density cont.
Critical density is the dividing point between a
closed or an open universe.

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The Critical Density cont.
If the average density is higher than the
critical density, the universe is closed.

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The Critical Density cont.
If the average density is lower than the
critical density, the universe is open.

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The Critical Density cont.
If the average density equals the critical
density, the universe is flat.

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Quiz 31.2 Other Galaxies in the Universe
(10pts)
Quiz 31.3 Cosmology (10pts)