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LESSON 5
GALAXIES AND BEYOND
Chapter 8
Astronomy
OBJECTIVES
• Classify galaxies according to their
properties.
• Explain the big bang and the way in which
Earth and its atmosphere were formed.
MAIN IDEA
The Milky Way is one of billions of galaxies that
are moving away from each other in an
expanding universe.
VOCABULARY
• galaxy
• Milky Way
• spectrum
• expansion redshift
• big bang
• background radiation
WHAT ARE GALAXIES?
• A galaxy is a group of star clusters held
together by gravity.
• Stars move around the center of their galaxy
in the same way that planets orbit a star.
• Galaxies differ in size, age, and structure.
• Astronomers place them in three main groups
based on shape.
• spiral
• elliptical
• irregular
SPIRAL GALAXY
• Spiral galaxies look like whirlpools.
• Some are barred galaxies.
• Have bars of stars, gas, and dust
throughout its center.
• Spiral arms emerge from this bar.
ELLIPTICAL GALAXY
• An elliptical galaxy is shaped a bit like a
football.
• It has no spiral arms and little or no dust.
IRREGULAR GALAXY
• Has no recognizable shape.
• The amount of dust varies.
• Collisions with other galaxies may have
caused the irregular shape.
THE MILKY WAY GALAXY
• During the summer at night, overhead, you
will see a broad band of light stretching
across the sky.
• You are looking at part of the Milky Way.
• The Milky Way is our home galaxy.
• The Milky Way is a spiral galaxy.
• The stars are grouped in a bulge around a core.
• All of the stars in the Milky Way including our
sun orbit this core.
• The closer a star is to the core the faster its
orbit.
• Several spiral arms extend from the core.
• Our solar system is located on one of these spiral
arms.
• We can not see the center of the Milky Way
between us and the core.
• However, we can see more stars in the direction of
the core from Earth than any direction.
• To find the center of the galaxy, look in the direction
of the constellation Sagittarius, the archer.
QUICK CHECK
• Fact and Opinion
• “The Milky Way is a spiral galaxy.”
• Is this statement a fact or an opinion? Why?
• The Milky Way is in fact a spiral galaxy. It is
based on observations; it contains dust, and spiral
arms.
• Critical Thinking
• How are the three types of galaxies similar?
• How are they different?
• All contain stars that move around a core.
• A spiral galaxy has a whirlpool type shape with
arms and lots of dust.
• An elliptical galaxy has a globe or football shape
and little or no dust.
• An irregular galaxy has no recognizable shape and
the amounts of dust and gas vary.
WHAT WAS THE BIG BANG?
• When light passes through water droplets it
turns into a band of colors.
• White light is typically all the colors of the
rainbow and it’s called a spectrum.
• The heated gases of stars produce light.
• As light passes through a star’s outer
atmosphere, some of the light is absorbed by
the stars atmosphere.
• When scientists look at a spectrum of this
starlight, they see that the absorbed light has
“dropped out” of the spectrum, forming dark
lines called absorption lines.
• When we look at a spectrum from a galaxy, the
absorption line patterns don’t appear at the
same point in the spectrum as they were
formed here on Earth.
• The position of the pattern shifted.
• This is because the galaxies are all
moving away from each other as the
space between them expands.
• If the absorption lines of a spectrum shift to
the blue end, a blueshift occurs.
• Which causes the galaxy to move
toward us.
• If the line shifts to the red end, a redshift
occurs.
• Most lines are redshifted.
• Space expands the absorption.
• Lines show an expansion redshift.
• There is no center to this expansion.
• Observers consider themselves at the center
would see the galaxies moving away.
• Astronomers think the galaxies must have been
closer to each other in the past.
• The early universe was very dense and its
temperature was high.
• At the beginning moment, the universe was
extremely tiny, hot, and dense.
• From this it grew rapidly.
• This expansion, called the big bang, sent
maytter out in all directions.
• According to the big bang theory, the universe is
expanding and its density and temperature are
decreasing.
• Gravity has caused matter to collect into clumps
forming stars and galaxies.
• The galaxies continue to move outward.
• Evidence for the big bang theory comes from
background radiation, radiation that is left over from
the beginning moments of the universe and is
coming from all directions in space.
FORMATION OF THE
SOLAR SYSTEM
• Scientists believe that the big bang created
our universe.
• This formed protostars and protoplanets.
QUICK CHECK
• Fact or Opinion
• “Scientists theorize that the universe was
very hot and dense in its first moments.”
• Is this a fact or an opinion?
• Scientists theorize that the universe was very hot
and dense is a fact; the idea itself is a theory, not
a fact.
• Critical Thinking
• What do astronomers think caused the
background radiation found in space?
• The background radiation was caused by the
events that occurred at the beginning of the
universe.
HOW OUR SOLAR SYSTEM
FORMED
• Scientists think that Earth is about 4.6 billion
years old.
• Astronomers think that Earth and its
atmosphere developed in a series of stages.
• The process began in the nebula that formed
the Sun.
• Dust and ice were colliding within the nebula.
• Clumps of particles grew until they became the
young Earth or proto-Earth.
• The protoplanet’s larger mass and gravity
attracted smaller bodies to it.
• Collisions increased.
• Over time, proto-Earth became large enough
that its gravity could hold an atmosphere.
• Earth’s original atmosphere was mostly
hydrogen and helium.
• The heat of the molten planet and impacts of
space objects blew away much of that
atmosphere, leaving water vapor, sulfur, carbon
dioxide, and nitrogen released by volcanic
eruptions.
• Atmospheric oxygen developed as a waste
product of photosynthesis.
QUICK CHECK
• Fact and Opinion
• “Plants did not exist on Earth in its first
years, because the atmosphere lacked
oxygen.”
• Is this statement a fact or an opinion? Why?
• Fact. Oxygen was not added to Earth’s
atmosphere until plants began producing it
through photosynthesis.
• Critical Thinking
• How did Earth’s original atmosphere evolve into
its present one?
• The original hydrogen and helium atmosphere
was lost and replaced by volcanic gases of
water vapor, carbon dioxide, and nitrogen.
Then, oxygen was added after plants appeared
and produced the oxygen as a waste product of
photosynthesis.