Discussions for week 9/10/11
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27_Lecture_Outline.pptxChapter 27: Stars and Galaxies
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Chapter 27 Lecture
1
This lecture will help you understand:
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
stars that comprise them.
people in the cultures that named them.
difference between stars and planets.
All of the above.
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B. people in the cultures that named them.
Observing the Night Sky CHECK YOUR ANSWER
Knowing the names of the constellations tells us much about the
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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B. people in the cultures that named them.
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 )
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Which of these stars radiates light of the longest wavelength?
Red star.
Yellow star.
Blue star.
Violet star.
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The Brightness and Colors of Stars CHECK YOUR NEIGHBOR
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A. Red star.
The Brightness and Colors of Stars CHECK YOUR ANSWER
Which of these stars radiates light of the longest wavelength?
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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A. Red star.
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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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
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 NEIGHBOR
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C. one quarter as much.
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
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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C. one quarter as much.
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
an average star.
seen to be special.
a low-temperature star.
especially bright.
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A. an average star.
The Hertzsprung–Russell Diagram CHECK YOUR ANSWER
On the H–R diagram, the Sun is
an average star.
seen to be special.
a low-temperature star.
especially bright.
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A. an average star.
A dying star that has collapsed to a small size and is cooling off would appear in which part of the H–R diagram?
Lower left.
Upper left.
Lower right.
Upper right.
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The Hertzsprung–Russell Diagram CHECK YOUR NEIGHBOR
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A. Lower left.
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?
Lower left.
Upper left.
Lower right.
Upper right.
Explanation:
Such a star would be a white dwarf.
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A. Lower left.
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 NEIGHBOR
The source of energy in the Sun and stars is
chemical reactions.
thermonuclear reactions.
Both of the above.
None of the above.
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B. thermonuclear reactions.
The Life Cycles of Stars CHECK YOUR ANSWER
The source of energy in the Sun and stars is
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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B. thermonuclear reactions.
Stars Sizes
Planets
Sun
Vega
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Stars Sizes
Sun
Arcturus
Alpha Ceti
Vega
(Red Giant)
(Red Giant)
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Stars Sizes
Sun
Arcturus
Alpha Ceti
Vega
Betelgeuse A
(Red Supergiant)
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Stars Sizes
Sun
Arcturus
Alpha Ceti
Vega
Betelgeuse A
(Red Hypergiant)
Cephei A
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Stars Sizes
Sun
Arcturus
Alpha Ceti
Vega
(Red Hypergiant)
Cephei A
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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Betelgeuse A
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
one tenth as much.
the same.
10 times as much.
100 times as much.
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D. 100 times as much.
Black Holes CHECK YOUR ANSWER
When a star collapses to one-tenth size, gravitation at its surface becomes
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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D. 100 times as much.
When a giant star collapses to become a black hole, gravity is greatly increased
at it surface.
at its center.
in all surrounding space.
All of the above.
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Black Holes CHECK YOUR NEIGHBOR
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A. at it surface.
Black Holes CHECK YOUR ANSWER
When a giant star collapses to become a black hole, gravity is greatly increased
at it 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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A. at it surface.
Black Holes CHECK YOUR NEIGHBOR
If the Sun collapsed to become a black hole, the orbit of Earth would
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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A. remain unchanged.
Black Holes CHECK YOUR ANSWER
If the Sun collapsed to become a black hole, the orbit of Earth would
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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A. remain unchanged.
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 NGC 6744, 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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The motion of individual stars in a galaxy normally follow
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 NEIGHBOR
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A. elliptical orbits around the center of the galaxy.
Galaxies CHECK YOUR ANSWER
The motion of individual stars in a galaxy normally follow
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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A. elliptical orbits around the center of the galaxy.
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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