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Stellar Lifecycles
• The process by which stars
are formed and use up their
fuel.
• What exactly happens to a
star as it uses up its fuel is
strongly dependent on the
star’s mass.
The Orion Nebula - Birthplace of stars
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A Star is Born
A cluster of massive, hot blue stars
have formed still surrounded by
clouds of gas that may form new stars.
• Stars form from huge, cold,
clouds of gas and dust.
• At some point this cloud
collapses on itself.
• Its own gravity causes clumps
of material to form. These
clumps heat up as material
continues to fall upon them.
• Eventually temperatures are
high enough in the center of
these clumps to allow nuclear
fusion reactions to occur.
• Often several large clumps can
form within the cloud. Clusters
of stars can all form at the same
time from the same cloud.2
Main Sequence Stars
• Once nuclear fusion has
begun, pressures in the
core grow high enough to
stop the stars from
collapsing any further. It
is then in Hydrostatic
Equilibrium.
• These are now Main
Sequence stars
• Their position along the
line of the Main Sequence
depends on their mass.
The H-R diagram showing the
• Almost the entire lifetime
Main Sequence line (in purple).
of a star is spent on the
More massive stars are to the upper left, Main Sequence.
less massive stars to the lower right.
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Differences Between High Mass and
Low Mass Stars
• Stars that are more massive than
the Sun have stronger
gravitational forces.
• These forces need to be
balanced by higher internal
pressures.
• These higher pressures result in
higher temperatures which drive
a higher rate of fusion reactions.
A star like our Sun will remain • The Hydrogen within the core
of a high mass star therefore
on the Main Sequence for about
gets used up much faster than in
10 billion years. A very massive
the Sun and “ages” faster.
star may only be on the Main
Sequence for a few million years. • Low mass stars “age” slower.
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When the Sun Leaves the Main Sequence
• When a star uses up the
Hydrogen in its core it can no
longer support itself against
gravity.
• The core compresses and
temperatures begin to rise.
• Temperatures may get high
enough outside the core to begin
The life cycle of a star like the Sun
Hydrogen fusion there instead.
• The pressure from this shell
around the core pushes the outer
layers of the star out.
• These outer layers cool and get
redder.
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The Last Years of the Sun
• During this Red Giant
stage the core of the Sun
will continue to contract
and heat up.
• Eventually temperatures
will be high enough for
the fusion of Helium in
the core. The Sun then
converts Helium into
Carbon & Oxygen. The
surface temperature of the
Sun increases and it
The motion of the Sun through the
becomes a Yellow Giant.
H-R diagram as the Sun ages. Notice • This stage lasts as long as
that the Sun spends most of its life on
there is Helium available
the Main Sequence.
in the core.
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The Sun’s Planetary Nebula
• As the core exhausts its
Helium fuel it begins to
contract and heat
causing the Helium to
get used even faster.
The Sun increases its
luminosity. The outer
layers of the Sun
expand, cool and
redden again.
• The outer layers of the
Sun start streaming
Except for the core, the rest of the Sun
away from the core.
will eventually be dispersed into space
This material forms a
forming a planetary nebula like this one. nebula surrounding the
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Sun.
White Dwarf Stars
• The core of the Sun
eventually stops all nuclear
fusion but remains
extremely hot.
• The core will form a White
Dwarf star, a very dense,
small object about the size
of the Earth.
• Over time the White Dwarf
will cool and dim.
• By measuring the
temperature of white
dwarfs you can estimate
how long ago they formed.
White Dwarf stars are very hot
but also very small. They appear
in the lower left corner of the H-R Diagram.
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How do High Mass Stars Evolve?
• For stars greater than 8 times
the Sun’s mass after Helium
fuel is exhausted the core of
the star contracts, heats up
and starts to fuse Carbon &
Oxygen into Neon and
Silicon.
• Helium and Hydrogen fusion
continue in shells around the
core.
• As long as the star can raise
its core temperature high
enough it can continue to fuse
new elements. Until iron is
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created.
High Mass Stars Evolve Quickly
Compare the duration of the stages of a high mass star’s life to
that of a solar-mass star (two slides back).
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Supernova
• The formation of iron actually absorbs rather
than releases energy.
• Nuclear fusion at the core stops and it begins
to collapse.
• The pressures of the surrounding layers are so
high that the atoms of the iron core are
crushed, smashing the electrons into the
protons forming neutrons.
• Once neutrons are formed the collapse stops,
the surrounding gas is heated and explodes off
the core. This is a supernova explosion.
• The explosion is so energetic that it can
outshine the combined light of a galaxy!
• Heavy elements are formed in the material
blown off the star. These elements are
dispersed into space where they can be used to
form planets and new stars.
• Depending on its mass the core may become a
neutron star or collapse further to a black
hole.
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Testing Stellar Evolution Theories
Open Clusters: Young Stars
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Pleiades HR Diagram:
A Young Cluster
Blue Dots: Pleiades
Orange crosses: Coma Bernice
Gray Circles: Hyades
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Hyades:
Older than the Pleiades
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M67: Open Cluster
Getting older
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Globular Clusters: Old Stars
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47 Tucanae: Globular Cluster
Red Giant Stars
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47 Tucanae: Globular Cluster
Blue Straggler Binaries
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