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Chapter 15
Stars, Galaxies, and the Universe
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Section 1 Stars
Section 2 The Life Cycle of the Stars
Section 3 Galaxies
Section 4 Formation of the Universe
Concept Map
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Chapter 15
Section 1 Stars
Bellringer
List ways that stars differ from one another.
How is the sun like other stars? How is it different?
Write your answers in your Science Journal.
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Chapter 15
Section 1 Stars
What You Will Learn
• Stars differ in size, temperature, composition,
brightness, and color.
• Distances between stars are very large and are
measured in light-years.
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Chapter 15
Section 1 Stars
Color of Stars
• The color of a star indicates the star’s temperature.
• Red stars are the coolest, and blue stars are the
hottest.
• If two stars differ in color, you can conclude that they
differ in temperature too.
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Chapter 15
Section 1 Stars
Composition of Stars
• Astronomers use an instrument called a
spectroscope to separate a star’s light into a
spectrum (plural, spectra).
• A spectrum is the band of colors produced when
white light passes through a prism.
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Chapter 15
Section 1 Stars
Composition of Stars, continued
• A continuous spectrum shows all of the colors, while
an absorption spectrum shows which wavelengths of
light are absorbed.
• The spectrum of a star is an absorption spectrum
because the atmosphere of the star absorbs certain
portions of the light produced by the star.
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Chapter 15
Section 1 Stars
Composition of Stars, continued
• When a chemical element emits light, only some
colors in the spectrum show up. The colors that
appear are called emission lines.
• Every element has a unique set of emission lines that
act like a fingerprint for that element.
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Chapter 15
Section 1 Stars
Composition of Stars, continued
• The pattern of lines in a star’s absorption spectrum is
unique to that star.
• A star’s absorption spectrum can be used to
determine the elements in that star’s atmosphere and
the stage the star occupies in its life cycle.
• Stars are made of mostly hydrogen and helium
gases.
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Chapter 15
Stars, Galaxies, and the Universe
Composition of Stars, continued
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Chapter 15
Section 1 Stars
Classifying Stars
• Stars are now classified by how hot they are.
• Temperature differences between stars result in color
differences that can be seen. For example, class O
stars are blue—the hottest stars.
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Chapter 15
Section 1 Stars
Classifying Stars, continued
• Magnitude is used to compare the brightness of one
object with the brightness of another object.
• To express the brightness of stars, astronomers use
a system of magnitudes.
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Chapter 15
Section 1 Stars
Classifying Stars, continued
• Positive magnitude numbers represent dim stars.
Negative magnitude numbers represent bright stars.
• The brightest star in the night sky, Sirius, has a
magnitude of -1.4.
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Chapter 15
Stars, Galaxies, and the Universe
Classifying Stars
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Chapter 15
Section 1 Stars
How Bright Is That Star?
• The apparent magnitude is the brightness of a star
as seen from Earth. The absolute magnitude is the
brightness that a star would have at a distance of
32.6 light-years from Earth.
• If all stars were the same distance away, their
absolute magnitudes would be the same as their
apparent magnitudes.
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Chapter 15
Stars, Galaxies, and the Universe
Absolute and Apparent Magnitude
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Chapter 15
Section 1 Stars
Distance to the Stars
• Because stars are so far away, astronomers use a
unit called a light-year to measure the distance from
Earth to the stars.
• A light-year is the distance that light travels in one
year, about 9.46 trillion kilometers.
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Chapter 15
Section 1 Stars
Distance to the Stars, continued
• Parallax is an apparent shift in the position of an
object when viewed from different locations.
• Astronomers use parallax and trigonometry to find
the actual distance to stars that are close to Earth.
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Chapter 15
Section 1 Stars
Motions of Stars
• All of the stars in the sky appear to make one
complete circle around Polaris every 24 h.
• This apparent motion of the stars is due to Earth’s
rotation and its revolution around the sun.
• Each star is actually moving in space. But because
stars are so distant, their actual motion is hard to see.
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Chapter 15
Stars, Galaxies, and the Universe
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Chapter 15
Section 2 The Life Cycle of Stars
Bellringer
List 3 stages in the life cycle of an insect.
List 3 stages in the life cycle of a human.
List any stages you may know of in the life cycle of a
star.
Write your answers in your Science Journal.
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Chapter 15
Section 2 The Life Cycle of Stars
What You Will Learn
• During star formation, gravity pulls dust and gas into
a sphere, and when the sphere gets dense enough,
nuclear fusion begins.
• The stages of a star’s life cycle and the star’s
properties at each stage can be plotted on the H-R
diagram.
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Chapter 15
Section 2 The Life Cycle of Stars
Types of Stars
• Stars can be classified by mass, size, brightness,
color, temperature, composition, and age.
• A star is classified differently as its properties change.
Its properties often relate to whether the star can
generate energy by nuclear fusion.
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Chapter 15
Section 2 The Life Cycle of Stars
The Life Cycle of Sunlike Stars
• New stars form from gas and dust, which are pulled
into a sphere by gravity.
• As the sphere becomes denser, hydrogen combines
into helium and energy is released in a process
called nuclear fusion.
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Chapter 15
Section 2 The Life Cycle of Stars
The Life Cycle of Sunlike Stars, continued
• After a sunlike star forms, it enters the main
sequence stage, the longest stage of its life cycle.
The main sequence is the location on the H-R
diagram where most stars lie.
• During the main sequence stage, energy is
generated in the core of the star from nuclear fusion.
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Chapter 15
Section 2 The Life Cycle of Stars
The Life Cycle of Sunlike Stars, continued
• When a main-sequence star uses all of the hydrogen
in its core, the atmosphere of the star grows very
large and cools. The star may become a red giant or
red supergiant.
• In the final stage of its life cycle, a sunlike star
becomes a white dwarf. A white dwarf no longer
generates energy by nuclear fusion.
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Chapter 15
Section 2 The Life Cycle of Stars
A Tool for Studying Stars
• The H-R diagram is a tool for studying the life cycles
of stars. The H-R diagram is a graph that shows the
relationship between the star’s surface temperature
and absolute magnitude.
• The stages of a star’s life cycle and its properties at
each stage can be plotted on an H-R diagram.
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Chapter 15
Stars, Galaxies, and the Universe
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Chapter 15
Stars, Galaxies, and the Universe
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Chapter 15
Stars, Galaxies, and the Universe
H-R Diagram
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Chapter 15
Section 2 The Life Cycle of Stars
The Aging of Massive Stars
• Massive stars use their hydrogen very fast and tend
to have shorter lives than other stars do.
• A supernova is a gigantic explosion in which a
massive star collapses and throws its outer layers
into space.
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Chapter 15
Section 2 The Life Cycle of Stars
The Aging of Massive Stars, continued
• Following a supernova, the center of the collapsed
star may contract into a very small dense ball of
neutrons called a neutron star.
• If a neutron star is spinning, it is called a pulsar.
Pulsars send out beams of radiation that sweep
across space.
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Chapter 15
Section 2 The Life Cycle of Stars
The Aging of Massive Stars, continued
• If the center of a collapsed massive star has a mass
that is several times the mass of the sun, the star
may contract further and leave a black hole.
• A black hole is an object that is so dense and
massive that light cannot escape its gravity.
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Chapter 15
Section 3 Galaxies
Bellringer
What do you think galaxies are made of?
What is the name of the galaxy in which we live?
Draw a sketch of what you think a spiral galaxy looks
like when viewed from outside that galaxy.
Record your answers in your Science Journal.
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Chapter 15
Section 3 Galaxies
What You Will Learn
• The three types of galaxies are spiral galaxies,
elliptical galaxies, and irregular galaxies.
• Galaxies are composed of stars, planetary systems,
nebulas, and star clusters.
• Looking at distant galaxies reveals what young
galaxies looked like.
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Chapter 15
Section 3 Galaxies
Types of Galaxies
• A galaxy is a collection of stars, dust, and gas bound
together by gravity.
• Astronomers classify a galaxy as spiral, elliptical, or
irregular according to its shape and rate of star
formation.
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Chapter 15
Section 3 Galaxies
Types of Galaxies, continued
• Spiral galaxies have a bulge at the center and spiral
arms.
• The spiral arms are made of gas, dust, and new stars
that have formed.
• The galaxy in which we live is a spiral galaxy called
the Milky Way.
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Chapter 15
Section 3 Galaxies
Types of Galaxies, continued
• Most elliptical galaxies are round or oval and seem to
have stopped making new stars.
• Elliptical galaxies are among the largest galaxies in
the universe.
• Irregular galaxies have no definite shape and form
new stars slowly.
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Chapter 15
Section 3 Galaxies
Contents of Galaxies
• Some galaxies contain nebulas and star clusters.
• A nebula is a large cloud of gas and dust in
interstellar space.
• Nebulas are a region in space where stars are born.
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Chapter 15
Section 3 Galaxies
Contents of Galaxies, continued
• A globular cluster is a highly concentrated group of
up to 1 million stars that looks like a ball.
• Open clusters are groups of 100 to 1,000 stars that
are close together relative to other stars.
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Chapter 15
Stars, Galaxies, and the Universe
Contents of Galaxies
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Chapter 15
Section 3 Galaxies
Quasars
• Quasars are among the most distant objects in the
universe and are located in the centers of galaxies.
• Quasars generate energy at a high rate and are
among the most powerful energy sources in the
universe.
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Chapter 15
Section 3 Galaxies
Origin of Galaxies
• Because light takes time to travel through space,
looking through a telescope is like looking back in
time.
• Looking at distant galaxies reveals what early
galaxies looked like.
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Chapter 15
Section 4 Formation of the
Universe
Bellringer
Write a paragraph that describes how you think the
universe formed.
Write your answers in your Science Journal.
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Chapter 15
Section 4 Formation of the
Universe
What You Will Learn
• The big bang theory is the standard theory to explain
the formation of the universe.
• Gravity controls the size and shape of the universe.
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Chapter 15
Section 4 Formation of the
Universe
The Big Bang Theory
• Cosmology is the study of the origin, structure,
processes, and evolution of the universe.
• Careful measurements have shown that most
galaxies are moving away from each other and that
the universe is expanding. With these findings,
scientists have worked backward in time to figure out
how the universe formed.
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Chapter 15
Section 4 Formation of the
Universe
The Big Bang Theory, continued
• The big bang theory is the theory that all matter and
energy in the universe was compressed into an
extremely small volume that about 14 billion years
ago exploded and began expanding in all directions.
• Scientists believe that radio “noise” coming from all
directions in space is cosmic background radiation
left over from the big bang.
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Chapter 15
Stars, Galaxies, and the Universe
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Chapter 15
Section 4 Formation of the
Universe
Gravity and the Universe
• After the big bang, gravitational attraction caused the
matter distributed throughout the universe to form
galaxies.
• Because gravity acts over such great distances,
gravity controls the size and shape of the universe.
• Every object in the universe is part of a larger system
and gravity acts within each system.
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Chapter 15
Stars, Galaxies, and the Universe
Structure of the Universe
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Chapter 15
Section 4 Formation of the
Universe
How Old Is the Universe?
• Scientists use white dwarfs to estimate the age of the
universe. The oldest white dwarfs are 12 billion to 13
billion years old.
• It took about 1 billion years after the big bang for the
first white dwarf to form. So, the universe must be
about 14 billion years old.
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Chapter 15
Section 4 Formation of the
Universe
A Forever-Expanding Universe?
• The universe is composed of matter, dark matter, and
dark energy.
• Dark matter does not give off light but has gravity and
can be detected. Dark energy seems to be
accelerating the expansion of the universe.
• Scientists think that the universe may keep growing
and expanding forever.
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Chapter 15
Stars, Galaxies, and the Universe
Concept Map
Use the terms below to complete the concept map
on the next slide.
spectra
absolute magnitude
brightness
color
temperature
spectroscope
stars
emission lines
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Chapter 15
Stars, Galaxies, and the Universe
Concept Map
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Chapter 15
Stars, Galaxies, and the Universe
Concept Map
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