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Table of Contents
Stars and Galaxies
Section 1 • Observing the Universe
Section 2 • Evolution of Stars
Section 3 • Galaxies and the Milky Way
Section 4 • Cosmology
Section
1
Observing the Universe
Constellations
• Long ago, people named patterns of stars after
characters in stories, animals and even tools.
• Many of the names given to
these star patterns by ancient
cultures survive today and
are called constellations.
Section
1
Observing the Universe
Telescopes
• Many stars are visible with the unaided eye.
• However, to see some stars and other distant objects
better, you need a telescope.
• Optical telescopes are used to study objects in visible
light, and radio telescopes are used to study objects in
the radio wavelengths.
Section
1
Observing the Universe
Optical Telescopes
• There are two basic types of optical telescopes.
• One type uses only lenses to collect and focus light and
the other uses lenses and mirrors.
• The distance from the objective lens to the focus is the
focal length of the telescope.
Section
1
Observing the Universe
Refracting Optical Telescopes
• A refracting telescope uses a convex lens.
• Light passes through the objective lens and the
eyepiece lens.
• The eyepiece
then magnifies
the image.
Section
1
Observing the Universe
Reflecting Optical Telescopes
• A reflecting telescope uses a mirror as an
objective to collect and focus light.
• The figure shows how
light passes through the
open end of a reflecting
telescope and strikes a
concave mirror at the
base of the telescope.
Section
1
Observing the Universe
Focal Point and Focal Length
• You can calculate the magnifying power (Mp) of a
telescope by dividing the focal length of the
objective (fo) by the focal length of the eyepiece
(fe).
Mp = fo/fe
Section
1
Observing the Universe
Adaptive Optics
• The most recent innovations in optical telescopes
involve adaptive optics.
• In an adaptive optics system, the light from the
objective mirror strikes a small, deformable mirror
before it is focused, which reduces distortion.
Section
1
Observing the Universe
Radio Telescopes
• A telescope that collects and amplifies radio waves
is a radio telescope.
• Because radio waves have long wavelengths, a radio
telescope must be built with a very large objective,
usually some form of dish antenna.
• The very large array near Socorro, New Mexico has 27
radio telescopes that detect and amplify radio waves
from distant objects in space.
Section
1
Observing the Universe
Space Telescopes
• Earth’s atmosphere limits what ground-based
telescopes can achieve.
• For this reason, astronomers use space-based
telescopes, such as the Chandra X-Ray Observatory
and the Spitzer Space Telescope.
• Large distances in space are measured in a unit called
a light-year, the distance that light travels in one year.
Section
1
Observing the Universe
The Light-Year
• Light travels at a speed of 300,000 km/s in space.
• It takes millions to billions of years for light from distant
objects to reach Earth.
Section
1
Observing the Universe
Spectroscopes
• A spectroscope uses a prism or diffraction grating to
disperse the light into its component wavelengths.
• The separated wavelengths are called the spectrum of
the star.
• The spectrum can determine a star’s chemical
composition, its surface temperature, and whether it is
moving away from or toward Earth.
Section
1
Section Check
Question 1
A refracting telescope uses a _______ as an objective.
A. mirror
B. wave
C. convex lens
D. laser
Section
1
Section Check
Answer
The answer is C. Convex lens are curved outward
like the surface of a ball.
Section
1
Section Check
Question 2
What does a spectroscope do?
Answer
A spectroscope disperses the light from a star or other
celestial object collected by a telescope into an
electromagnetic spectrum.
Section
1
Section Check
Question 3
What is the distance that light travels in one year?
A. 15 million km
B. 9.5 trillion km
C. 12 billion km
D. 2 million km
Section
1
Section Check
Answer
The answer is B. Large distances in space are
measured in a unit called a light-year, which is equal to
9.5 trillion km.
Section
2
Evolution of Stars
How do stars form?
• Stars form from a large cloud of gas, ice, and dust
called a nebula.
• The nebula contracts, and a protostar forms in
the center of the cloud.
Section
2
Evolution of Stars
H-R Diagram
• In the early 1900s, Ejnar Hertzsprung and Henry
Russell studied the relationship between the brightness
and temperature of stars.
• As stars form, they can be plotted on the HertzsprungRussell (H-R) diagram.
Section
2
Evolution of Stars
H-R Diagram
• About 90 percent of
all stars fall on a
region from the
upper left to the
lower right of the HR diagram called
the main
sequence.
Section
2
Evolution of Stars
How do stars evolve?
• A protostar continues to collapse until nuclear fusion
begins.
• Equilibrium is the balance between outward pressure
exerted by fusion and inward pressure due to gravity.
Section
2
Evolution of Stars
Main Sequence
• Once equilibrium is reached, the star becomes a main
sequence star.
• When a star uses
up all the
hydrogen in its
core, it is no
longer in a state
of equilibrium.
Section
2
Evolution of Stars
Giants and Dwarfs
• When hydrogen in a star’s core is used up, its outward
pressure is overcome by gravity.
• Its core contracts and increases in temperature.
• The outer layers expand and cool.
• In this late stage of its life cycle, an average star like our
Sun is called a giant star.
Section
2
Evolution of Stars
Giants and Dwarfs
• Now the star is enormous and its surface is much
cooler.
• When the core temperature reaches 100 million K,
helium fuses, forming carbon.
• Its outer layers escape into space leaving behind the
hot, dense core that continues to contract.
Section
2
Evolution of Stars
Giants and Dwarfs
• A white dwarf forms as the core of a giant star
collapses and the star’s outer layers escape into
space.
• A white dwarf is hot with a dense core.
Section
2
Evolution of Stars
Supergiants, Neutron Stars, and Black Holes
• When the core of stars over eight times more massive
than our Sun reach temperatures high enough to cause
fusion that produce heavier elements, the star expands
into a supergiant.
• Fusion reactions end when iron accumulates in the
star’s core.
Section
2
Evolution of Stars
Supergiants, Neutron Stars, and Black Holes
• A supernova is a gigantic explosion in which the
temperature in the collapsing core reaches 10 billion K
and atomic nuclei are split into neutrons and protons.
• A collapsing star can also evolve into a neutron star
when protons and electrons in the star’s core collide to
form neutrons.
Section
2
Evolution of Stars
Supergiants, Neutron Stars, and Black Holes
• Very massive stars, with masses greater than 25 times
the mass of the Sun collapse to form a black hole.
• A black hole is an area of space that is so dense that
nothing can escape the inward pull of gravity.
Section
2
Evolution of Stars
The Sun—A Main Sequence Star
• The Sun’s interior can be divided into several distinct
layers: the core, the radiation zone, and the
convection zone.
Section
2
Evolution of Stars
The Sun’s Interior
• The innermost layer of the
Sun is the core. The
temperature inside the core
is about 15 million degrees
K. This is where fusion
occurs.
• The layer of the Sun just
above the core is the
radiation zone.
Section
2
Evolution of Stars
The Sun’s Interior
• Thermal energy produced by nuclear fusion in the core
is transferred through the radiation zone to the
convection zone.
• Columns of hot material form convection cells as they
rise to the surface, cool, and sink back down.
Section
2
Evolution of Stars
Surface Features of the Sun
• The surface of the Sun is called the photosphere.
• This is the layer of the Sun that gives us light.
• The atmosphere above the photosphere is
composed of the chromosphere and the corona.
Section
2
Evolution of Stars
Granules and Sunspots
• The Sun’s photosphere, or surface is at the top of the
convection zone and has a mottled appearance, called
granulation.
• These darker areas of the Sun’s photosphere, called
sunspots are cooler than surrounding areas.
• Sunspots are not permanent features of the Sun.
• They appear and disappear over periods of days,
weeks, or months.
Section
2
Evolution of Stars
Prominences and Flares
• Intense magnetic fields associated with sunspots can
cause huge arching columns of gas called prominences
to erupt.
• Gases near a sunspot sometimes brighten
suddenly, shooting gas outward at high speed in
what are called solar flares.
Section
Evolution of Stars
2
CMEs
• Sometimes large bubbles of electrically-charged gas
are emitted from the Sun. These are known as CMEs
(coronal mass ejections).
• Earth’s atmosphere protects from CMEs.
• Auroras take place when high-energy particles in CMEs
interact with Earth’s magnetic field.
Section
2
Section Check
Question 1
How do stars form?
Answer
Stars form from a large cloud of gas, ice, and dust. Once
the temperature inside a contracting nebula reaches 10
million, fusion begins.
Section
2
Section Check
Question 2
Which is NOT a layer of the Sun’s interior?
A. the core
B. the radiation zone
C. the convection zone
D. sunspots
Section
2
Section Check
Answer
The answer is D. The Sun’s interior can be divided into
several distinct layers or zone: the core, the radiation
zone, and the convection zone.
Section
2
Section Check
Question 3
What is a sunspot?
Answer
Sunspots are dark, cool areas in the photosphere where
the Sun’s magnetic field has weakened.
Section
3
Galaxies and the Milky Way
Galaxies
• A galaxy is a large group of stars, gas, and dust held
together by gravity.
• Our galaxy, called the Milky Way Galaxy contains 200
billion and 400 billion stars, by most recent estimates,
including the Sun.
Section
3
Galaxies and the Milky Way
Spiral Galaxies
• Spiral galaxies are disk-shaped and have spiral arms
that radiate outward from the galaxy’s center.
• These spiral arms are star forming regions that contain
clouds of ice, dust, and gas.
• Spiral galaxies have a central bulge, or nucleus, where
stars are closer together.
• They range in size from 20,000 to 200,000 light-years
across.
Section
3
Galaxies and the Milky Way
Elliptical Galaxies
• Elliptical galaxies are round and have shapes that
range from spherical to football-shaped.
• Less star formation occurs in an elliptical galaxy
because they contain less gas, ice, and dust.
Section
3
Galaxies and the Milky Way
Irregular Galaxies
• Galaxies that are not elliptical or spiral are considered
irregular galaxies.
• They take many different shapes and contain 100
million to 10 billion stars, making them larger than
dwarf ellipticals but smaller than spirals.
Section
3
Galaxies and the Milky Way
The Local Group
• Just as stars are grouped together within galaxies,
galaxies are grouped into clusters.
• Our Milky Way galaxy belongs to a cluster called the
Local Group.
• It is a relatively small cluster containing about 50
galaxies spread out over a diameter of 10 million lightyears across.
Section
3
Galaxies and the Milky Way
How do galaxies form?
• Astronomers hypothesize that the first galaxies began
to form 13.7 billion years ago.
• Astronomers believe that the first galaxies that formed
tended to be irregular in shape, smaller, and closer
together than galaxies today.
Section
3
Galaxies and the Milky Way
The Milky Way
• The Milky Way galaxy measures about 100,000 lightyears in diameter. The Sun lies about 28,000 lightyears from the galactic center on the edge of one the
spiral arms.
• The oldest stars in
the Milky Way are
thought to be 9 to 10
billion years old.
Section
3
Galaxies and the Milky Way
The Nuclear Bulge
• Stars are much closer together in the central region of
a spiral galaxy compared to its arms.
• The region where stars are closely clustered is called
the nuclear bulge.
Section
3
Galaxies and the Milky Way
The Halo and Galactic Center
• The halo is a spherical region that surrounds the
nuclear bulge.
• The galactic center of the Milky Way emits a
tremendous amount of energy and could be a black
hole.
Section
3
Section Check
Question 1
Which is NOT a type of galaxy?
A. elliptical
B. irregular
C. round
D. spiral
Section
3
Section Check
Answer
The answer is C. “Round” is not recognized as a major
type of galaxy.
Section
3
Section Check
Question 2
How do galaxies grow?
A. by producing new stars
B. by emitting light
C. by absorbing other galaxies
D. by absorbing stars that don’t belong to other galaxies
Section
3
Section Check
Answer
The answer is C. The Milky Way has been gobbling up
the Sagittarius dwarf elliptical galaxy for 2 billion years.
Section
3
Section Check
Question 3
Where is the Sun located in the Milky Way galaxy?
A. 28,000 light-years from the center of the galaxy
B. at the edge of one of the spiral arms
C. 100,000 light-years from the center
D. 1,000 light-years from the center
Section
3
Section Check
Answer
The answer is B. The Sun lies about 26,000 light-years
from the center of the galaxy on the edge of one of the
spiral arms.
Section
4
Cosmology
The Expanding Universe
• The study of the universe—how it began, how it
evolves, and what it is made of—is called
cosmology.
Section
4
Cosmology
Evidence of the Big Bang
• The theory that all matter and energy in the universe
were compressed into a single point which began to
expand outward is called the big bang theory.
• It states that the universe started with a big bang, or
explosion, and has been expanding ever since.
• The big bang is not like an explosion of matter into
empty space; it is the rapid expansion of space.
Section
4
Cosmology
Cosmic Microwave Background Radiation
• Cosmic background radiation is the residual radiation
from the formation of the universe.
• Data from the Wilkinson Microwave Anisotropy Probe
indicate that the big bang occurred 13.7 billion years
ago.
Section
4
Cosmology
The Doppler Effect
• The motion of the stars within the Milky Way can be
detected by using the Doppler effect.
• Doppler shifts occur in light as well as sound.
Section
4
Cosmology
The Doppler Effect
• If a star approaches Earth, its wavelengths of light are
compressed, causing a blue shift. If a star moves away,
its wavelengths are stretched, causing a red shift.
Section
4
Cosmology
What is the universe made of?
• Visible or otherwise detectable mass, called regular
matter, appears to make up only a very small amount
of the known universe.
• Dark matter is an invisible form of matter that does
not emit any detectable electromagnetic radiation.
• Observations show that there is about five to six times
as much dark matter in the universe as regular matter.
Section
Cosmology
4
Dark Energy
• Data indicate that the expansion of the universe is
accelerating.
• Explaining this acceleration is difficult.
• Dark energy is an invisible form of energy that causes
a repulsive force causing the universe to accelerate
faster.
Section
4
Section Check
Question 1
When did the universe begin?
A. 13.7 billion years ago
B. 5 million years ago
C. 25 trillion years ago
D. 14 trillion years ago
Section
4
Section Check
Answer
The answer is A. The Wilkinson Microwave Anisotropy
Probe team proposed that the universe began about 13.7
billion years ago with a big bang.
Section
4
Section Check
Question 2
What causes the Hubble redshift?
Answer
The Hubble redshift is caused by the expansion of space,
not the movement of galaxies.
Section
4
Section Check
Question 3
______ might be causing accelerated expansion of the
universe.
A. Kinetic energy
B. Potential energy
C. Dark energy
D. Thermal energy
Section
4
Section Check
Answer
The answer is C. Dark energy explains the accelerated
expansion of the universe.
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Chapter Resources
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Image Bank
Video Clips and Animations
Chapter Summary
Chapter Review Questions
Standardized Test Practice
Image Bank
Click on individual
thumbnail images to
view larger versions.
Image Bank
Constellations
THUMBNAILS
Image Bank
Refracting Telescope
THUMBNAILS
Image Bank
Reflecting Telescope
THUMBNAILS
Image Bank
Hertzsprung-Russell Diagram
THUMBNAILS
Image Bank
Main Sequence Stars
THUMBNAILS
Image Bank
The Sun
THUMBNAILS
Image Bank
Milky Way
THUMBNAILS
Image Bank
Doppler Effect
THUMBNAILS
Video Clips and Animations
Reviewing Main Ideas
Observing the Universe
• Constellations are patterns of stars that resemble
things familiar to the observer.
• Optical telescopes collect and focus visible light and
magnify viewed objects.
Reviewing Main Ideas
Observing the Universe
• A refracting telescope uses lenses to collect light and
magnify the image, and a reflecting telescope uses a
mirror to collect light and a lens to magnify the image.
• A radio telescope collects and amplifies radio waves.
Reviewing Main Ideas
Evolution of Stars
• Stars form from a large cloud of gas, ice, and dust,
called a nebula. When the temperature inside the
contracting nebula reaches 10 million K, fusion begins,
and a star is born.
• Stars are classified as main sequence stars, giant stars,
and white dwarfs on the H-R diagram.
Reviewing Main Ideas
Evolution of Stars
• When a star reaches stellar equilibrium it is considered
a main sequence star. When the hydrogen fuel is
depleted, a star loses equilibrium and evolves into a
giant stars or supergiant.
Reviewing Main Ideas
Evolution of Stars
• After losing its outer layers, a giant star becomes a
white dwarf. A supergiant can evolve into a neutron
star or a black hole.
• The Sun’s energy is produced at its core by nuclear
fusion.
Reviewing Main Ideas
Galaxies and the Milky Way
• A galaxy is a large group of stars, gas, and dust held
together by gravity. The Local Group of galaxies is a
cluster that contains the Milky Way galaxy.
• The three main types of galaxies form by absorbing or
merging with smaller objects. They continue to evolve
by colliding or merging with other galaxies.
• The Milky Way galaxy is about 100,000 light-years
across and the Sun lies about 28,000 light-years from
its center.
Reviewing Main Ideas
Cosmology
• The Big Bang theory is the most accepted theory of
how the universe began.
• The universe is 13.7 billion years old and appears to be
expanding faster now than in the past.
• The Hubble redshift is caused by the expansion of
space, not the movement of galaxies.
Chapter Review
Question 1
Which type of galaxy is most common?
A. barred galaxy
B. dwarf elliptical
C. irregular
D. spiral
Chapter Review
Answer
The answer is B. Dwarf elliptical galaxies can be over 9
million light-years across and contain trillions of stars.
Chapter Review
Question 2
What type of telescope is shown here?
A. reflecting
B. refracting
C. radio
D. laser
Chapter Review
Answer
The answer is A. A reflecting telescope uses a mirror
as an objective to reflect light to the focus.
Chapter Review
Question 3
According to the H-R diagram, which is the largest
group of stars?
A. giants
B. white dwarfs
C. supergiants
D. main sequence
Chapter Review
Answer
The answer is D. As long as the star’s gravity balances
outward pressures, the star remains on the main
sequence. Stars spend most of their life cycle on the
main sequence.
Chapter Review
Question 4
What is produced when the core of a star collapses, and
the outer portion of the star explodes?
A. giant
B. supernova
C. dwarf
D. blackhole
Chapter Review
Answer
The answer is B. A supernova is a gigantic explosion in
which the temperature in the collapsing core reaches 10
billion K and atomic nuclei are split into neutrons and
protons.
Chapter Review
Question 5
Which is the most accepted theory of how the universe
formed?
A. oscillating
B. collision
C. steady state
D. Big Bang
Chapter Review
Answer
The answer is D. The Big Bang theory states that the
universe started with a big bang, or explosion, and has
been expanding ever since.
Standardized Test Practice
Question 1
How do stars change?
Answer
Once fusion begins, a star enters stellar equilibrium and
becomes a main sequence star. When the hydrogen fuel
is depleted, a star evolves into a giant star, or a
supergiant.
Standardized Test Practice
Question 2
Which is a feature of the Sun that can reach 100 million
K?
A. CME
B. solar flare
C. prominence
D. sunspot
Standardized Test Practice
Answer
The answer is B. Temperatures within a solar flare can
reach 100 million K.
Standardized Test Practice
Question 3
Which of the following is NOT a true statement about
stars?
A. Stars are composed of similar chemical elements.
B. Stars differ in age.
C. Stars differ in size.
D. Stars are the same temperature.
Standardized Test Practice
Answer
The answer is D. The temperature of a star changes
as it progresses into new stage of its life cycle.
Standardized Test Practice
Question 4
What layer of the Sun gives us light?
A. corona
B. convection zone
C. core
D. photosphere
Standardized Test Practice
Answer
The answer is D. The photosphere is the surface of the
Sun and gives us light.
Standardized Test Practice
Question 5
What occurs in the core
and radiation zone of the
Sun’s interior?
Standardized Test Practice
Answer
The core is the innermost layer and is where fusion
occurs. The layer of the Sun just above the core is the
radiation zone. In this layer, gases are completely
ionized. This layer of the Sun is transparent to radiation.
Help
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