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Warm Up 6/9/08
Another name for the interstellar matter that will eventually
form a star is ____.
a. supernova
c. red giant
b. nebula
d. black hole
2) A star is said to be born when ____.
a. a dark, cool interstellar cloud begins to contract
b. a protostar reaches a temperature high enough for nuclear
fusion to begin
c. pressure within a protostar becomes so great that a
supernova occurs
d. a red giant collapses on itself and becomes a black hole
3) Which force is most responsible for the formation of a star?
a. interstellar force
c. electromagnetic force
b. nuclear force
d. gravity
Answers: 1) b. 2) b. 3) d.
1)
The Universe
Chapter 25, Section 3
The Milky Way Galaxy
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Galaxies – groups of stars, dust, and gases held
together by gravity
The Milky Way is a large spiral galaxy whose
disk is about 100,000 light-years wide and about
10,000 light-years thick at the nucleus
It has at least three distinct spiral arms, the sun
lies about 2/3 of the way from the center on one
of these arms and orbits the nucleus about every
200 million years
There may be more than 100 billion stars in our
galaxy alone
The Milky Way Galaxy
Structure of the Milky Way
Concept Check
How big is the Milky Way Galaxy?
 The Milky Way Galaxy is about 100,000
light-years wide and 10,000 light-years
thick at the nucleus.
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Types of Galaxies
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Spiral Galaxies – like our galaxy, these galaxies
have multiple arms that sweep out from a central
nucleus
Elliptical Galaxies – do not have spiral arms,
makes up ~60% of known galaxies, can range
from round to oval
Irregular Galaxies – consist mostly of younger
stars, appear as clouds of stars
In addition to shape and size, one of the major
differences among different galaxies is the age
of their stars
Galaxy Cluster – a system of galaxies
containing from several to thousands of member
galaxies
Spiral Galaxy
Elliptical Galaxy
Irregular Galaxy
Galaxy Cluster
The Expanding Universe
When a light source is moving away from
an object, its light appears red
 Hubble discovered that the light from
distant galaxies was shifted to the red
spectrum (red shift), implying that these
galaxies are moving away from us
 Hubble’s Law – galaxies are retracting
from us at a speed that is proportional to
their distance
 The red shifts from distant galaxies
indicate that the universe is expanding
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Red Shift
Raisin Bread Analogy
Concept Check
What relationship did Hubble discover
between red shifts and the distances of
galaxies from Earth?
 Galaxies that have the greatest red shifts
are the most distant.
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The Big Bang
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Big Bang Theory – The universe began as a
violent explosion from which the universe
continues to expand, evolve, and cool
The big bang theory states that at one time, the
entire universe was confined to a dense, hot,
supermassive ball. Then, about 13.7 billion
years ago, a violent explosion occurred, hurling
this material in all directions
Supporting evidence – Hubble’s discovery that
the galaxies are speeding away from each other
The Big Bang
Concept Check
What evidence supports the big bang?
 The red shift of distant galaxies and
cosmic background radiation support the
big bang.
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Assignment
Read Chapter 25 Section 3 (pg. 715-721)
 Do Chapter 25 Assessment #1-31 (pg. 725726)
 Turn in Lectures (total of 8)
 Study for the UNIT TEST!!!
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