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Stars & Galaxies
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A star is born

A star is made up of a large amount of
gas in a relatively small volume.

A nebula, on the other hand, is a large
amount of gas and dust spread out in an
immense volume.

All stars begin their lives as parts of
nebulas.
A star is born
Gravity can pull some of the gas and
dust in a nebula together.
 The contracting cloud is then called a
protostar.
 Proto means earliest stage of a star’s
life.
 A star is born when the contracting gas
and dust become so hot that nuclear
fusion starts.

Lifetimes of Stars
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Before they can tell how old a star is,
astronomers must determine its mass.
How long a star lives depends on how much
mass it has.
Stars with more mass have shorter lives than
those with less mass.
Small stars use up their fuel more slowly than
large stars, so they have much longer lives.
Generally, stars that have less mass than the
sun use their fuel slowly, and can live for about
200 billion years.
Medium- mass stars like the sun live for about
10 billion years.
Death of Stars

When a star begins to run out of fuel, the
center of the star shrinks and the outer part of
the star expands. The star becomes a red
giant or supergiant.
 All main sequence stars eventually become
red giants or supergiants. However, what
happens next depends on the mass of the
stars.
 When a star runs out of fuel, it becomes a
white dwarf, a neutron star, or a black hole.
White Dwarf

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Small and medium stars become red giants
and then white dwarfs.
Their outer layers expand to become red
giants.
Eventually, the outer parts grow bigger still
and drift out into space.
The blue white hot core is left behind causing
a white dwarf.
When there is no more energy it becomes a
black dwarf.
Neutron Stars

A dying giant or supergiant star can suddenly explode.
Within hours, the star blazes millions of times brighter.

The explosion is called a supernova.

After a star explodes, some material from the star is left
behind. This material may become part of a nebula.
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This material may form a neutron star.

Neutron stars are even smaller and denser than white
dwarfs.
Black Holes

The most massive stars- those having more
than 40 times the mass of sun- become black
holes when they die.

After this kind of star becomes a supernova,
more than five times the mass of the sun may
be left.

The gravity of this mass is so strong that the
gas is pulled inward, packing it into a smaller
and smaller space.
Lives of stars
Stars form in a nebula
from collapsing clouds
of interstellar gas and
dust.
The rest of the life
cycle depends on the
mass of the star
Sun-like and Smaller Stars
Due to the mass of the star, fusion occurs (hydrogen is
fused together releasing light and heat)
Stars the size of our sun can undergo fusion for 10 billion
years.
After all of the helium undergoes fusion, it becomes a
white dwarf.
As the fuel continues to dwindle the star eventually
becomes a black dwarf.
http://www.daviddarling.info/encyclopedia/B/blackdwarf.html
As the hydrogen runs out and helium undergoes fusion,
the star turns into a red giant.
Huge Stars
(1.5-3X the mass of the sun)
• The remnants of the explosion could become
a neutron star
• Stars this size usually undergo fusion for less
time than the sun-like stars.
• As a supergiant can suddenly explode into a
supernova.
• As the hydrogen runs out and helium
undergoes fusion, the star turns into a super
giant.
Giant Stars
(over 3X the mass of the sun)
A dying supergiant can explode into a
supernova.
As the hydrogen runs out helium and
undergoes fusion, the star turns into a
supergiant.
Due to the immense mass, the material
may be pulled by gravity inward,
packing the gas so tightly a black hole is
formed.
Quasars
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Quasar is a distant
galaxy with a black
hole at its center.
 As enormous
amounts of gas
revolve around such
objects as a black
hole, the gas heats
up and shines
brightly.
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http://www.classzone.com/book
s/earth_science/terc/content/vis
ualizations/es2807/es2807page
01.cfm?chapter_no=visualizatio
n
Elliptical Galaxies
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Not all galaxies have
spiral arms.
Elliptical galaxies look
like flattened balls.
These galaxies contain
billions of stars but have
little gas and dust
between the stars.
Because of the lack of
gas and dust, new stars
cannot form in most
elliptical galaxies and
most contain only old
stars.
Types of Galaxies
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Some galaxies do
not have regular
shapes and are
known as irregular
galaxies.
Spiral Galaxy
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Our galaxy is a
spiral galaxy.
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http://www.classzone.com/books/earth_sci
ence/terc/content/visualizations/es2809/es
2809page01.cfm?chapter_no=visualizatio
n
The Universe
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Universe is space and everything in it.
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http://www.classzone.com/books/earth_science/terc/content/visualizations/es0401
/es0401page01.cfm?chapter_no=visualization – solar system animation
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http://www.classzone.com/books/earth_science/terc/content/visualizations/es2808
/es2808page01.cfm?chapter_no=visualization – Milkyway at different scales