Transcript Hewitt/Lyons/Suchocki/Yeh, Conceptual Integrated Science

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Chapter 27:
STARS AND GALAXIES
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This lecture will help you
understand:
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•
•
•
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Observing the Night Sky
The Brightness and Colors of Stars
The Hertzsprung-Russell Diagram
The Life Cycles of Stars
Black Holes
Galaxies
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Observing the Night Sky
Observing the Night Sky:
constellations are groups of stars named over
antiquity
familiar constellation is Ursa Major, the Great
Bear.
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Observing the Night Sky
The monthly constellations seen in the night
sky change as Earth’s path around the Sun
progresses.
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Observing the Night Sky
Can you see that during a solar eclipse, the
darkened daytime sky would show
constellation positions as normally seen six
months earlier or later?
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Observing the Night Sky
The Big Dipper is a well-known
constellation. The pairs of stars at the end of
its bowl point to Polaris, the North Star.
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Observing the Night Sky
The seven stars of the Big Dipper are at
very different distances from Earth.
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Observing the Night Sky
A time-exposure of the night sky shows
streaks of stars from our “carousel Earth.”
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Observing the Night Sky
CHECK YOUR NEIGHBOR
Knowing the names of the constellations tells us much
about the
A.
B.
C.
D.
stars that comprise them.
people in the cultures that named them.
difference between stars and planets.
All of the above.
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Observing the Night Sky
CHECK YOUR ANSWER
Knowing the names of the constellations tells us much
about the
A.
B.
C.
D.
stars that comprise them.
people in the cultures that named them.
difference between stars and planets.
All of the above.
Explanation:
The names of constellations tell us nothing about the makeup of
the stars that compose them. They are more interesting
historically.
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The Brightness and Colors
of Stars
A star’s color indicates its temperature:
• a red star is cooler than a blue star
• a blue star is almost twice as hot as a red
star (blue light has almost twice the
frequency of red light in accord with   T)
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The Brightness and Colors of Stars
CHECK YOUR NEIGHBOR
Which of these stars radiates light of the longest
wavelength?
A.
B.
C.
D.
Red star.
Yellow star.
Blue star.
Violet star.
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The Brightness and Colors of Stars
CHECK YOUR ANSWER
Which of these stars radiates light of the longest
wavelength?
A.
B.
C.
D.
Red star.
Yellow star.
Blue star.
Violet star.
Explanation:
The longest wavelength is emitted by the star with the lowest
frequency, the red-hot star. (If the question had asked for the
highest frequency, that would be emitted by the violet star.)
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The Brightness and Colors of Stars
We measure the brightness of a star in two
ways:
• apparent brightness—the brightness as it
appears to us
• luminosity—the intrinsic brightness, independent
of how bright it appears
The luminosity of stars is compared to that of the
Sun, which is noted LSun.
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The Brightness and Colors of Stars
CHECK YOUR NEIGHBOR
We measure the Sun’s luminosity as LSun. If we were on a
spaceship twice as far away from the Sun, its apparent
brightness would appear
A.
B.
C.
D.
the same.
half as much.
one quarter as much.
four times as much.
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The Brightness and Colors of Stars
CHECK YOUR ANSWER
We measure the Sun’s luminosity as LSun. If we were on a
spaceship twice as far away from the Sun, its apparent
brightness would appear
A.
B.
C.
D.
the same.
half as much.
one quarter as much.
four times as much.
Explanation:
In accordance with the inverse-square law, twice as far away
means one quarter the brightness.
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The Hertzsprung-Russell Diagram
Graph of intrinsic brightness versus surface
temperature for stars
Note: positions that form a main sequence for average stars, and exotic
stars above or below the main sequence.
The H-R diagram is to an astronomer what the Periodic Table is to a
chemist.
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The Hertzsprung-Russell Diagram
CHECK YOUR NEIGHBOR
On the H-R diagram, the Sun is
A.
B.
C.
D.
an average star.
seen to be special.
a low-temperature star.
especially bright.
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The Hertzsprung-Russell Diagram
CHECK YOUR ANSWER
On the H-R diagram, the Sun is
A.
B.
C.
D.
an average star.
seen to be special.
a low-temperature star.
especially bright.
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The Hertzsprung-Russell Diagram
CHECK YOUR NEIGHBOR
A dying star that has collapsed to a small size and is
cooling off would appear in which part of the H-R diagram?
A.
B.
C.
D.
Lower left.
Upper left.
Lower right.
Upper right.
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The Hertzsprung-Russell Diagram
CHECK YOUR ANSWER
A dying star that has collapsed to a small size and is
cooling off would appear in which part of the H-R diagram?
A.
B.
C.
D.
Lower left.
Upper left.
Lower right.
Upper right.
Explanation:
Such a star would be a white dwarf.
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The Life Cycles of Stars
Life cycle of stars:
• begins as a nebula
• advances to a protostar
• becomes a star when fusion in its core occurs
Depending on its mass, the star may become a
red giant and then burn out to become a white
dwarf.
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The Life Cycles of Stars
White dwarf:
• cools for eons until it is too cold to emit
light
• if part of a binary, pulls matter from its
partner, which can lead to a nuclear blast
(nova)
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The Life Cycles of Stars
Final stage of more massive stars is collapse, then
an explosion called a supernova.
Remnant of a supernova is the Crab Nebula.
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The Life Cycles of Stars
CHECK YOUR ANSWER
The source of energy in the Sun and stars is
A.
B.
C.
D.
chemical reactions.
thermonuclear reactions.
Both of the above.
None of the above.
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The Life Cycles of Stars
CHECK YOUR ANSWER
The source of energy in the Sun and stars is
A.
B.
C.
D.
chemical reactions.
thermonuclear reactions.
Both of the above.
None of the above.
Explanation:
The Sun and other stars are balls of plasma, much too hot for
chemical reactions to occur.
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Stars Sizes
Planets
Sun
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Vega
Stars Sizes
Vega
Sun
Arcturus
(Red Giant)
Alpha Ceti
(Red Giant)
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Stars Sizes
Vega
Sun
Arcturus
Alpha Ceti
Betelgeuse A
(Red Supergiant)
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Stars Sizes
Betelgeuse A
Alpha Ceti
Arcturus
Sun
Vega
Cephei A
(Red Hypergiant)
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Stars Sizes
Betelgeuse A
Alpha Ceti
Arcturus
Sun
Vega
All these stars are within 3000 light-years of
Earth, which means they are our immediate
neighbors within the Milky Way Galaxy.
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Cephei A
(Red Hypergiant)
Black Holes
Black hole:
• what remains when a supergiant star
collapses into itself
• named because gravitation at its surface is
so intense that even light cannot escape
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Black Holes
Black hole
Why gravitation at the surface of a star
increases when it collapses
star shrinks to half its radius  gravitation at its
surface increases by 4 (inverse-square law)
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Black Holes
CHECK YOUR NEIGHBOR
When a star collapses to one-tenth size, gravitation at its
surface becomes
A.
B.
C.
D.
one tenth as much.
the same.
10 times as much.
100 times as much.
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Black Holes
CHECK YOUR ANSWER
When a star collapses to one-tenth size, gravitation at its
surface becomes
A.
B.
C.
D.
one tenth as much.
the same.
10 times as much.
100 times as much.
Explanation:
This follows from the inverse-square law introduced in Chapter 4.
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Black Holes
CHECK YOUR NEIGHBOR
When a giant star collapses to become a black hole, gravity
is greatly increased
A.
B.
C.
D.
at it surface.
at its center.
in all surrounding space.
All of the above.
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Black Holes
CHECK YOUR ANSWER
When a giant star collapses to become a black hole, gravity
is greatly increased
A.
B.
C.
D.
at its surface.
at its center.
in all surrounding space.
All of the above.
Explanation:
It is important to know that gravitation increases mainly at the
surface of the collapsed star. Gravity at the surface before
collapse is the same at that same distance from the center of the
black hole after collapse.
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Black Holes
CHECK YOUR NEIGHBOR
If the Sun collapsed to become a black hole, the orbit of
Earth would
A.
B.
C.
D.
remain unchanged.
be pulled inward toward the black hole.
spiral outward away from the black hole.
be a straight-line path.
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Black Holes
CHECK YOUR ANSWER
If the Sun collapsed to become a black hole, the orbit of
Earth would
A.
B.
C.
D.
remain unchanged.
be pulled inward toward the black hole.
spiral outward away from the black hole.
be a straight-line path.
Explanation:
F = G(m1  m2)/d2. Letting this equation guide our thinking, we see that
none of its terms differ. Although the density of the black hole has greatly
increased, its mass is the same before and after collapse. Because the
mass of Earth and the solar black hole are the same, and distance
between centers is the same, the force holding Earth in orbit wouldn’t
change. Equations nicely guide thinking!
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Galaxies
Galaxy:
• huge assemblage of stars, interstellar gas,
and dust
• most familiar—Milky Way
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Galaxies
Three types of galaxies:
• elliptical
• irregular
• spiral
This is a giant elliptical galaxy M87
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Galaxies
A pair of irregular galaxies—
the Large Magellanic Cloud and
neighboring Small Magellanic Cloud
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Galaxies
This is Spiral Galaxy M83, thought to be much like
our Milky Way.
Galaxies are not the largest things in the universe. There
are clusters of galaxies, and then galaxy superclusters—
larger than can be imagined!
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Galaxies
Some galaxies are known as active galaxies and
are emitting huge amount of energy.By
comparison, these active galaxies emit many
orders of magnitude more energy than our own
Milky Way! Two examples of active galaxies
include:
• Starburst galaxies
• Galaxies with an active galactic nucleus
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Galaxies
Starburst galaxies form stars at a very high rate.
They result from violent disturbances, such as the
collision between two galaxies.
This image shows the aftermath of the collision of two
spiral galaxies. Areas in blue are regions of rapid star
formation.
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Galaxies
Some active galaxies have supermassive black
holes in their centers into which large amounts of
matter is falling, sometimes causing jets that extend
thousands of light years from the galactic center
(called an active galactic nucleus, or AGN).
This jet coming from M87 consists of charged particles being
accelerated to velocities near the speed of light.
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Galaxies
CHECK YOUR NEIGHBOR
The motion of individual stars in a galaxy normally follow
A.
B.
C.
D.
elliptical orbits around the center of the galaxy.
completely random paths.
straight lines for the most part.
circular orbits around the center of the galaxy.
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Galaxies
CHECK YOUR ANSWER
The motion of individual stars in a galaxy normally follow
A.
B.
C.
D.
elliptical orbits around the center of the galaxy.
completely random paths.
straight lines for the most part.
circular orbits around the center of the galaxy.
Explanation:
Within galaxies, stars move in elliptical orbits around the center of
the galaxy.
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Clusters and Superclusters
The Milky Way Galaxy and its neighboring galaxies are
known as the Local Group.
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Clusters and Superclusters
Our local group is situated between the Virgo and Eridanus
clusters, which all together make our Local Supercluster.
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Clusters and Superclusters
Our Local Supercluster is part of a network of superclusters.
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Clusters and Superclusters
As far as we can see, superclusters hold together like a foam
within which there are bubbles of super large voids.
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