The Universe…
…is space and everything in it.
Important Key Terms
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star
Protostar
planetary nebula
barred-spiral galaxy
absolute magnitude
elliptical galaxy
spiral galaxy
irregular galaxy
luminosity
Hertzsprung-Russell diagram
apparent magnitude
main sequence star
universe
red giant star
red supergiant star
white dwarf star
neutron star
black dwarf star
black hole
galaxy
stellar nebula
Galaxies
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A galaxy is a cluster of
stars, gas, and dust that
are held together by
gravity.
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There are three main
types of galaxies:
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Irregular
Elliptical
Spiral
Types of Galaxies – Irregular…
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Some galaxies do
not have definable,
regular shapes and
are known as
irregular galaxies.
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They contain young
stars, dust, and gas.
Types of Galaxies – Elliptical...
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Elliptical galaxies look like
flattened balls.
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These galaxies contain billions
of stars, but have little gas and
dust between the stars.
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Because of the lack of gas and
dust, new stars cannot form in
most elliptical galaxies, and so
they contain only old stars.
Types of Galaxies –Spiral…
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Spiral galaxies consist of a
flat, rotating disk with stars,
gas, and dust and a central
concentration of stars, known
as the bulge. The bulge is
surrounded by a much fainter
halo of stars.
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Contains middle aged stars
Reflection Questions
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What are galaxies?
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What is a nebula?
What force helps a star begin to form?
 What are the types or categories for
galaxies?
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Types of Galaxies – Barred Spiral…
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Our Milky Way galaxy has
recently (in the 1990s)
been confirmed to be a
type of spiral galaxy known
as a barred spiral galaxy.
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Our Sun is located at the
edge of the Milky Way
galaxy.
Nebulae (plural for Nebula)
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Stellar Nebula- A
gigantic cloud of gas and
dust from which stars are
made; sometimes called
a “Stellar Nursery”
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Planetary Nebula- When
a star runs out of fuel to
burn, its outer layers of
gas are blown outward in
the shape of a ring.
Stars
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A star contains hydrogen gas that forms helium
through a process known as nuclear fusion. Stars
form when gravity pulls together gas and dust
from stellar nebula.
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It releases enormous amounts of energy which
causes the star to become very hot and glow.
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Vary in mass, size, and temperature
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Held together by gravity.
A Star is Born…
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A star is made up of a large amount of gas,
in a relatively small volume.
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A stellar nebula, on the other hand, is a
large amount of gas and dust,
spread out in an immense
volume.
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All stars begin their lives as parts of stellar
nebulas.
A Star is Born…
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Gravity can pull some of the gas and dust
in a nebula together.
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The contracting cloud is
then called a protostar.
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A star is born when the contracting gas
and dust become so hot that nuclear
fusion begins.
Lifetimes of Stars…
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Before they can tell how old a star is,
astronomers must determine its mass.
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Medium mass stars, such as the Sun, exist
for about 10 billion years.
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These are known as
main sequence stars.
Lifetimes of Stars…
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Stars with more mass have shorter lives
than those with less mass.
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Small stars use up their fuel more slowly
than large stars, so they have much longer
lives, about 200 billion years.
Lifetimes of Stars…
Stars form in a stellar
nebula, from collapsing
clouds of interstellar gas
and dust. This is called a
protostar.
When a star runs out of
fuel, it will become a white
dwarf, neutron star, or
black hole.
The rest of the life
cycle depends on
the mass of the
star.
Main Sequence Stars…
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Main Sequence is the stage in which stars
spend the majority of their lifetime.
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Small and medium stars first
become red giants.
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Their outer layers expand during the red
giant phase.
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Eventually, the outer parts grow bigger still
and drift out into space.
Main Sequence Stars...
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The blue, white, hot core is left
behind causing a white dwarf.
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When there is no more energy, it
becomes a black dwarf. It is the remaining
burnt-out cinder left, as the star goes out.
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A dying giant, or supergiant star, can
suddenly explode. Within hours, the star
blazes millions of times brighter.
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The explosion is called a supernova.
Neutron Stars…
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After a star explodes, some material
from the star is left behind. This
material may become part of a
planetary nebula.
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The core will compress
and form a neutron star.
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Neutron stars are even
smaller and more dense
than white dwarfs.
Reflection Questions
What forms black holes?
 What kind of stars become white dwarfs?
 What is the outcome of a star running
out of fuel?
 What is the closest star to the Earth?
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Black Holes…
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The most massive stars may have more
than 40 times the mass of the Sun. One
might have more than five times the
mass of the Sun left, after it becomes a
supernova.
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The gravity of this mass is so strong that
the gas is pulled inward, packing it into a
smaller and smaller space. These
massive stars become black holes when
they die.
Star Life Cycle
Generally speaking, there are
two main life cycles for stars.
The factor which determines
the life cycle of the star is its
mass.
1 solar mass = size of our Sun
Any star less than about three
solar masses will spend almost
all of its existence in what is
called the “Main Sequence”.
Main Sequence Stars
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Space may seem empty, but
actually it is filled with thinly spread
gas, mostly hydrogen, and dust.
The dust is mostly microscopic
grains of carbon and silicon. In
some places, this material is
collected into a big cloud of dust
and gas, known as a nebula.
Stars form from collapsing clouds
of gas and dust. All stars begin in
a nebula.
Stellar Nebula (a star nursery)
ke Stars
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Some gas and dust is pulled by gravity to
the core. As the region of condensing
matter heats up, it begins to glow. This is
called a protostar.
 Temperature rises, and nuclear fusion
begins. This is the “birth” of the star.
Nuclear fusion is the atomic reaction that
fuels stars. Fusion in stars is mostly
converting hydrogen into helium.
 Stars that are up to 1.5 times the mass of
the Sun are called “Main Sequence” stars
and will burn for a long time.
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A red giant is a large star that is
reddish or orange in color.
 It represents the phase in a star's
life when its supply of hydrogen has
been exhausted and helium is being
fused into carbon. This causes the
star to collapse, raising the
temperature in the core. The outer
surface of the star expands and
cools, giving it a reddish color.
 Red giants are very large, reaching
sizes of over 100 times the star's
original size.
Red Giant
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Planetary nebulae form when a main
sequence star grows into a red giant
and throws off its outer layers and the
core collapses.
The term "planetary" comes from the
19th century, when astronomers saw
what looked like a new planet in their
primitive telescopes.
This was a time before people knew that
there were different types of galaxies.
The name has stuck ever since.
lanetary Nebula
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The collapsed core left when a red giant
loses its outer layers is called a white
dwarf.
It is made of pure carbon that glows white
hot with leftover heat from the spent fuel. It
will drift in space while it slowly cools.
It is the size of Earth, but very dense. A
teaspoon of the material would weigh as
much as an elephant.
White Dwarf
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A black dwarf is a white dwarf star
that has cooled completely and does
not glow.
 It will drift in space as a frozen lump
of carbon. The star is considered
“dead”.
Black Dwarf
Massive Stars
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All stars form from
collapsing clouds of
gas and dust found in
a nebula.
Stellar Nebula (a star nursery)
ve Stars
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Massive stars are stars that are between 1.5
to 3 times the mass of the Sun.
 A star with a much greater mass will form,
live, and die more quickly than a main
sequence star.
 Massive stars follow a similar life cycle as
small and medium stars do, until they reach
their main sequence stage.
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This occurs because the gravity squeezes
the star's core and creates greater
pressures, resulting in a faster fusion rate.
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A red supergiant glows red because its
outer layers have expanded, producing the
same amount of energy over a larger
space. The star becomes cooler.
Red stars are cooler than blue or white
stars. A supergiant has the pressure
needed to fuse carbon into iron.
This fusion process takes energy, rather
than giving it off. As energy is lost, the star
no longer has an outward pressure equal to
gravity pushing in. Gravity wins, and the
core collapses in a violent explosion.
Red Supergiant
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A supernova is an explosion
of a massive star at the end
of its life; the star may briefly
equal an entire galaxy in
brightness.
At this point, the mass of the
star will determine which way
it continues in the life cycle.
Supernova
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Neutron Star
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Black Hole
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If the star is at least 1.5 but
less than 9 times larger
than the Sun, the core left
after the supernova will
collapse into a neutron
star. This is a star
composed only of
neutrons.
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If the star is at least 9 or
more times larger than the
Sun, the core will continue
to collapse into a black
hole, an extremely dense
area with a strong
gravitational pull that light
can not escape.
Neutron Star or Black Hole?
Reflection Questions
What is the relationship between mass
and the lifespan of a star?
 Where is our Sun located in the Milky
Way galaxy?
 What size star is our Sun?
 What are the stages of a massive star’s
life?
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Our Sun is a medium
sized, main sequence
star.
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It is the closest star to
Earth