Astronomy
Stars, Galaxies, and the Universe
What is Astronomy?
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Astronomy is the
study of the moon,
stars, and other
object in space
Astronomers study
the Universe using
telescope, satellites,
probes, as well as
manned and
unmanned space
flights
The Universe
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Astronomers define the Universe as all of space
and everything in it
What’s found in the Universe
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Galaxies
Nebulas
Stars
Solar Systems (planets, dwarf planets, moons)
Asteroids, comets, meteors, meteoroids
Dark matter
 Does not give off electromagnetic energy and can
not be seen directly
 Estimated to make up 23% of the Universe’s mass
The Origin of the Universe
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Astronomers believe that
billions of years ago all
the matter and energy in
the Universe was
concentrated into single
hot dense point called a
singularity
Tremendous amounts of
heat and pressure made
this point so unstable that
it exploded
The Big Bang Theory
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According to the Big Bang Theory , the
Universe began to expand after an
enormous explosion of concentrated
matter and energy
As it expanded, the Universe cooled
 Atoms formed after a few hundred million
years
 The first stars and galaxies formed after about
200 million years
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In Support of the Big Bang
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Moving Galaxies
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Edwin Hubble discovered that almost all galaxies are
moving away from us and from each other
Hubble’s Law- the farther away a galaxy is, the faster
it is moving away from us
Cosmic Background Radiation
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The electromagnetic radiation (thermal energy)
leftover from the big bang
Detected with a radio telescope in 1965 by Arno
Penzias and Robert Wilson
Age of the Universe
Can be determined based on
measurements of how fast distant galaxies
are moving away from us as and from the
cosmic background radiation
 Astronomers estimate
that the Universe is
13.7 billion years old
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Future of the Universe
Background radiation, left over from the
Big Bang, has led astronomers to two
possible fates for the Universe
 Closed Universe- A universe that would
expand then collapse back in on itself
 Open Universe- A universe that would
continue to expand
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A Closed Universe
A possible future for the Universe in which
the force of gravity would pull the galaxies
back together
 Would result in a “reverse big bang” or a
“big crunch”
 After many billions of years, all the matter
and energy in the Universe would be
concentrated in an enormous black hole
 The final result could be another “Big
Bang” billions of years from now
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An Open Universe
A possible future where the galaxies
continue racing outward, expanding the
Universe
 Stars would eventually run out of fuel and
burn out, leaving the Universe cold and
dark
 New evidence leads astronomers to
believe that dark energy is causing the
expansion of the Universe to accelerate
and that it will likely expand forever
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Galaxies
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A galaxy is a large
system of stars and
other cosmic bodies
Galaxies are the
major features of the
Universe
There may be more
than 100 billion major
galaxies
There are three main
types of galaxies
Spiral Galaxies
Have a bulge in the middle and arms that
spiral outward, like pinwheels
 Bright, central nucleus is made up of
millions of stars
 Spiral arms contain millions of bright,
young stars as well as dust and gas
 Example: our Milky Way Galaxy
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The Milky Way Galaxy is approximately 100,000 light years across.
We are located about 25,000 light years from the center, out on one
of the spiral arms
Elliptical Galaxies
Vary in shape from nearly spherical to
flattened disks
 Most of the stars are close to the center
 Have no arms
 Contains billions of stars but little gas or
dust
 Stars are no longer forming and are
generally older than those in the other
galaxies
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Irregular Galaxies
Have no definite shape
 Stars are spread unevenly
 Typically smaller than other
types of galaxies
 Generally have many bright
young stars and lots of gas
and dust to form new stars
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Quasars (quasi-stellar objects)
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Very bright, distant
objects
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Many 10 billion light
years away
Look almost like stars
Believed to be active
young galaxies with
enormous amounts of
gas revolving around
a giant black in their
centers
Stars
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There may be as many as 200 quintillion stars in
the universe (200,000,000,000,000,000,000)
Stars are huge spheres of glowing gas
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Made up mostly of hydrogen
Produce energy by nuclear fusion
Stars differ in size,
mass, color, composition
temperature, and
brightness
Color & Temperature of Stars
The color of stars can be used to determine
their surface temperature
 Blue
= 35,000°C
 White = 10,000°C
 Yellow = 6,000°C
 Red-orange = 5,000°C
 Red = 3,000°C
The temperature at the center
of a star is much greater than
at its surface
Stars are divided into 5 main
groups according to size
Smallest
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Largest
Neutron stars- the smallest stars
(avg. diameter = 16 km)
White dwarfs- smaller than the Earth
(ex. Van Maanen’s star)
Medium-sized stars- vary in size from
1/10 to 10 times the size of the Sun
(ex. the Sun & Sirius)
Giant stars- 10 to 100 times as large as
the Sun (ex. Aldebara)
Supergiant stars- up to 1000 times larger
than the Sun (ex. Rigel & Betelgeuse)
Composition of Stars
Spectroscopes are used to determine the
composition of stars
 Almost all stars have the same
composition
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 60
- 80% hydrogen
 20 – 30% helium
 2% other elements
Brightness of Stars
The brightness of a star depends on its
size, its surface temperature, and its
distance from Earth
 Apparent magnitude- the brightness of a
star as it appears from Earth
 Absolute magnitude- the amount of light
a star actually gives off
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Hertzsprung-Russell Diagram
A chart that shows the relationship
between the absolute magnitude and the
surface temperature of stars
 Astronomers use H-R diagrams to classify
stars and to understand how stars change
over time
 As the absolute magnitude of main
sequence stars increases, the temperature
increase as well
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Hertzsprung-Russell Diagram
Measuring Distances to Stars
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Astronomers use light years to measure
distances between stars
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A light year is the distance light travels in a year, 9.5
trillion kilometers
Parallax is used to measure distances to nearby
stars
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Parallax is the apparent change in
position of an object when you look at
it from different places
The star is viewed when Earth is on
one side of the Sun and then six months
later when Earth is on the other side of the Sun
Evolution of Stars
Stars evolve, or change, over time
 The amount of mass a star begins with is
the main factor that determines its
evolution
 The different kinds of stars in the sky
represent the various stages in the life
cycle of stars
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Stages in the Life Cycle of Stars
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All stars are created from the gases in a nebula
When the contracting gas and dust from a nebula
become so dense and hot that nuclear fusion begins,
the protostar begins to shine
When a star begins to run out of fuel, its core shrinks
and its outer portion expands
The evolutionary path of a star depends on its mass
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Medium-sized stars Expand to red giants  white dwarfs
 dims to a black dwarf or dead star
Massive stars  Expand to red giants or supergiants 
explode in a supernova  become a neutron star or a black
hole (depending on initial mass)
Possible Evolutionary Paths for Stars
Constellations
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Groups of stars that form a pattern
The revolution of the Earth around the Sun
cause different constellations to be seen at
different times of the year
Stars located above the north and south poles,
called circumpolar stars, appear to move in
circles above the horizon each night
Astronomers use constellations as landmarks to
locate other objects in the sky
Constellations in the Autumn
Sky
Optical Telescopes
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Tools that use large lenses or mirrors to gather
rays of light from a star and focus it in one spot
The light-gathering power depends on the area
of its lens or mirror (the greater the area, the
more light it can gather)
Have three basic functions
1.
2.
3.
Collect more light than the naked eye
Separate distant objects from one another
Magnify images
Refracting Telescopes
Use convex lenses to gather light
 The lenses
refract (bend)
the rays of
light to form
an image
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Reflecting Telescopes
Use mirrors to gather and focus light
Types of Optical Telescopes
How Light Behaves
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Light travels in straight-line paths called rays
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Reflection- When light strikes
a surface, some of it bounces
back
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Refraction- The bending of
light due to a change in speed
Mirrors Reflect Light
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Mirrors are classified based on the shape of
their surface
Plane mirrors
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Concave mirrors
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Perfectly flat surfaces
Image appears to be on the other side of the mirror
Surface curves inward
Reflect light rays to the same point in front of the mirror
(focal point)
Convex mirrors
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Surface curves outward
Reflected rays spread out from the surface of the mirror
Types of Mirrors
Lenses Refract Light
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Convex lenses- Lenses that are thicker in the
center than at the edges
Bend rays of light toward the thicker center
of the lens
 The amount of refraction depends on how
much the lens curves (the greater the curve,
the more the light is refracted)
 Converge light rays at the focal point
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Concave Lenses- Lenses that are thinner in the
center than at the edges
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Bend rays of light outward toward the
thicker ends of the lens
Diverge rays of light
Types of Lenses