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Ch 11--Life Cycle of Stars
2 Nov 2000
ASTR103, GMU, Dr. Correll
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What do you think?
• How do stars form?
• Are stars forming today?
• Do stars with greater or lesser mass shine longer
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Stellar Evolution
• Birth
• Evolution to main
sequence line (youth)
• Main sequence
(adulthood)
• Maturation off the main
sequence towards giant
stars (retirement)
• Death of Stars
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Origins of Star Formation
• Radio telescopes (and
optical) reveal interstellar
medium--giant clouds of gas
and dust lying between
existing stars
– By number: 90% Hydrogen,
9% Helium, less than 1% other
– By mass: 74% H, 25% He,
1% other
– Other includes, heavier
elements, molecules (H2, CO,
H2O, NH3, H2CO, etc) and dust
– about 1 hydrogen atom per
cubic centimeter of space
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Interstellar Medium (cont.)
• Visible light efficiently blocked
and scattered by gas and
dust, limiting observing range
– Pleides appear blue due to
preferential scattering of blue
light from thick dust in
surrounding IM
• Radio waves from CO
molecules (giant molecular
clouds, ~1000 hydrogen
atoms per cubic cm) travel
further through gas and dust,
allowing us to observe IM to
great distance
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ASTR103, GMU, Dr. Correll
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The Horsehead Nebula
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Star Formation
• Shock wave (from supernova
or colliding gas clouds)
causes a local region of the
IM to begin gravitational
collapse
– If nebula too hot, gas pressure
prevents further collapse (or
must wait until nebula cools)
– If nebula cool enough, Jeans
instability allows gravity to
overtake thermal energy
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ASTR103, GMU, Dr. Correll
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Protostars
• The collapsing gas and dust form a sphere
• As the sphere accretes mass, and collapses the
temperature raises, stalling the collapse
• At this stage the sphere, now a protostar, can
radiate very much thermal energy
• As the protostar radiates away energy, it
gradually becomes more compact with a higher
central temperature and pressure
• Eventually, nuclear fusion (hydrogen burning)
can begin
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Pre-Main-Sequence Stars
• After about 100,000 years of
accretion and collapse, nuclear
fusion begins--the protostar
becomes a pre-main-sequence
star
• PMS Stars evolve toward Main
Sequence
– Collapse continues somewhat
– Stellar structure stabilizes
– Nebular cloud push away by stellar
wind
– Rate of evolution depends on
mass--higher mass, faster
evolution!
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Masses of Stars
• Upper limit--about 100 MSun
• Lower limit--about 0.08 MSun
(brown dwarves)
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Emission Nebula
• The gas in star forming
regions typically glows from
the hot, newborn stars
– Hydrogen in nebula ionized
(HII)by UV light from the stars
– Occasional capture of an
electron by H II causes emission
of pinkish light
– These areas are referred to as
emission nebula
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Star Cluster Formation
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Star Clusters on an H-R Diagram
• Plotting the stars in a
cluster on an H-R
diagram tell us how
old a cluster is
– massive stars reach
the main sequence
first
– lower mass stars take
10s, 100s, 1000s of
millions of years to
reach the main
sequence phase
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ASTR103, GMU, Dr. Correll
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Main Sequence Stars
• Zero Age Main Sequence (ZAMS) is where stars have
reached an equilibrium configuration and begin their
lives on the main sequence
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Giant Stars
• Hydrogen burning leaves a
central core of inert Helium
• When hydrogen burning stops
(due to insufficient
temperature and pressure)
then
– Core of star collapses
• Helium fusion may begin-slowly, or in a helium flash for
massive stars
– Outer layers of star expand
• Thus surface of star becomes
cooler but MUCH larger
– Giant stars are formed
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Evolution Off the Main Sequence
• As stars enter giant
phase, their size
increases greatly, their
surface temperature
drops slightly, thus
their luminosities
migrate upwards
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Globular Clusters
• An H-R census of the Globular
cluster stars reveals the age of
the cluster
– since the globular cluster stars are
gravitationally bound close
together, they are the same
distance from us
• use apparent magnitude
– Youngest clusters are metal rich
• Population I stars (such as our
Sun)
– Oldest clusters are metal poor
• Population II stars
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Variable Stars
• As stars leave the
main sequence, their
structure and
luminosity becomes
unstable and variable
– but often in predictable
ways--Cepheid
variables
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Cepheid Variables
• The fixed relationship
between period and
luminosity allows
Cepheid variable stars
to be used as distance
candles!
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Close Binary Stars
• Close binary stars can exchange mass, greatly
altering the evolution process for the stars!
2 Nov 2000
ASTR103, GMU, Dr. Correll
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What do you think?
• How do stars form?
– Stars form from gas and dust inside giant molecular clouds
• Are stars forming today?
– Yes. Astronomers have seen stars that have just arrived
on the main sequence, as well as infrared images of gas
and dust clouds in the process of forming stars
• Do stars with greater or lesser mass shine longer?
– More massive stars live shorter lives because the
gravitational force creates higher temperatures and greater
pressures which accelerate the pace of nuclear fusion in
their cores
2 Nov 2000
ASTR103, GMU, Dr. Correll
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Questions for Thought
• Describe the formation of stars within a cluster and
explain a method for estimating the age of the
cluster.
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