Download Leaving the Main Sequence

A Star
Is Born
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After a period that ranges from a few tens of thousand to hundreds of
million years depending on mass, the protostar finally collapses enough to
yield a core temperature where hydrogen burning can begin (a few-10
million degrees).
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After a brief period as a T Tauri star, hydrostatic equilibrium is finally
achieved, the dust around the protostar evaporates, and we have a
newborn star.
Stellar Evolution:
After the main Sequence
Beyond hydrogen:
The making of the elements
Leaving the main Sequence
Main Sequence Stars
• Slowly fuses hydrogen into helium(hydrogen
burning) at the core.
• In a state of hydrostatic equilibrium
– Inward gravity is balanced by outward pressure due
to Hydrogen burning
 Size does not vary over time
• In a state of Thermal equilibrium
Temperature remains constant over time
Changes in the Solar
composition
Leaving the main Sequence
• In main sequence stars, the core temp. is not
high enough to “burn” helium.
• Eventually, the hydrogen becomes depleted
at the core
– the nuclear fire there ceases, and the location of
principle burning moves higher layers of the
core - shell burning hydrogen.
– In inner core - the temp. still not high enough to
ignite helium.
Leaving the main Sequence
• Without the nuclear reactions to maintain,
the outward pressure weakens in the helium
core, and the core begins to contract under
gravity.
• This collapse causes the core temperature
to rise from about 15 million kelvin to about
100 million kelvin.
• Rising heat in the contracting core creates
pressure that causes outer layers to expand.
Leaving the main Sequence
• During this post-main-sequence phase, the
star’s outer layers expand to many time its
original size while the core contracts.
• The expansion of the outer layers, causes
the these layers to cool down
– This will give the star a red color
• The star is then referred to as a red giant
• Red giants are former main sequence stars,
now at a different stage of its evolution.
Red Giant stars
Life of a Sun-like star
1. Protosun forms - cloud collapses and heats
(energy from gravitational collapse)
– characterized by bipolar flow and jets from
star’s poles
2. Main sequence star (Sun today)
– Nuclear fusion starts. Hydrogen burns to
helium in core and star is in hydrostatic
equilibrium, AS ARE ALL MAIN
SEQUENCE STARS!
Life of a Sun-like star (cont.)
3. Red giant – Core begins to run out of hydrogen fuel, begins to
contract and heats. Remaining hydrogen burns faster
in the shell around core and generates extra energy,
disrupting hydrostatic equilibrium and causing outer
regions to expand and cool. Star turns red.
– Core (helium) becomes a degenerate gas (quantum
mechanical state) - conducts heat easily and is
incompressible due to degenerate electron pressure.
– Helium flash - eventually core is heated enough for
helium to ignite all at once, explosively. Degeneracy
ends - Helium burning starts.
Helium Burning?
•Helium Burning - triple alpha process:
4He + 4He  8Be
8Be + 4He  12C + (photons)
• Some of the carbon nuclei can fuse with a
Helium nuclei to form oxygen
12C + 4He  16O + (photons)
Stages in stellar evolution
Life of a Sun-like star (cont.)
•
The gases inside a star behaves like an ideal
gas under most circumstances
–
Pressure  (density), (Temperature)
What is a degenerate gas?
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•
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In a star with mass less than 3M the gas in
the core behaves differently - degenerate gas.
The free electrons in this highly compressed
core is prohibited from getting any closer to
each other by the Pauli exclusion principle.
A gas in such a state is referred to as a
degenerate gas and the pressure that exist due
to this resistance to compression is called
degenerate-electron pressure.
What is a degenerate gas?
• Temperature rises to a required level He
burning.
• Degenerate-electron pressure is independent
of temperature.
• He heats up, He burning happens faster.
• Without having a “ pressure safety valve”,
temperature becomes too high to make the
electrons no longer degenerate.
• Star’s core ends up in helium flash.
Life of a Sun-like star (cont.)
4. Yellow giant - Helium is burned into Carbon in
the core. Star shrinks, turns yellow, and
pulsates Analogy: pot with lid on it.
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Pulsating variable stars
Helium Burning - triple alpha process:
4He + 4He  8Be
8Be + 4He  12C + (photons)
12C + 4He  12O
Life of a Sun-like star (cont.)
5. Red giant (again) - core runs out of helium
fuel, helium burns in shell, core contracts,
heats up, outer regions expand. Star is more
luminous than during previous red giant stage.
Life of a Sun-like star (cont.)
6. Planetary nebula- Outer region of star is so
cool that helium, carbon and oxygen flakes
condense. High luminosity (photons) push
these flakes off, which drag gas as they go,
stripping Star down to a carbon core.
7. White dwarf - this is all that is left of the Star.
Core of carbon is very hot, but is no longer
burning anything, so will eventually cool to a
cinder.
Structure of Sun-like stars after the main
sequence
• Interiors of Sun-like
stars layered like
onions. Each layer
further down
consists heavier
element, created as
the layer above
burns.
Life of a high mass star
1. Protostar: Interstellar cloud collapses
gravitationally, heats and glows.
– Characterized by bipolar flow and jets of matter
from star’s poles
2. Main sequence massive star - burns hydrogen
to helium in core and is in hydrostatic
equilibrium, AS ARE ALL MAIN
SEQUENCE STARS! Burns very luminously
(quickly).
Life of a high mass star (cont.)
3. Yellow giant (pulsating) - core begins to run
out of hydrogen and contracts, heats up.
Helium begins to burn (no helium flash
necessary - temperature is high enough -100
million K for He burning - in massive star’s
cores), then carbon, oxygen, etc. The most
massive stars will have cores of iron.
Nothing past iron is created. (To create iron
requires 2 billion K.)
Life of a high mass star (cont.)
4. Red giant - Later stages of burning – hotter, core
burning heavier elements, means more heat
produced, which means greater luminosity and
expands outer regions of star, which cools.
Life of a high mass star (cont.)
5. Supernova explosion - Core of iron grows until
it can not support itself under its own weight.
So compressed that protons and electrons join
to form neutrons. Core shrinks instantaneously.
Rest of star falls in, then rebounds off of
neutron star or black hole created in core
collapse. Rebound is outward explosion.
6. Neutron star or black hole - after explosion,
this is all that is left.
Nucleosynthesis: making heavy
elements from light ones
• Helium: created as hydrogen is burned.
• Light elements (carbon, oxygen): created as
helium is burned in low mass stars.
• Heavier elements up to iron (Fe): created by
burning of carbon, oxygen, etc. in more massive
stars. Iron can not be fused and release energy.
• Elements heavier than Fe: Created mostly in
supernova!
• We are stardust!
Structure of massive stars after the main
sequence
• Interiors of massive
stars layered like
onions. Each layer
further down
consists heavier
element, created as
the layer above
burns.
Main sequence lifetime of stars
and the importance of gravity
• The more massive a star, the faster it burns
hydrogen in its core, and the shorter its life
(despite there being so much more fuel
available).
– More gravity (more massive star) means star
must create more energy (outward pressure) to
support its weight. So massive stars burn fuel
much faster than Sun-like stars, and have much
hotter cores.
Main sequence Lifetimes
• 0.5 M  0.03L - 200 billion years on
main sequence
• 1 M  1L (Sun) - 10 billion years on
main sequence
• 3 M  60L - 1/2 billion years
• 25 M  80,000L - 3 million years
Evidence of these processes
• We have observed
– planetary nebula (the end of one Sun-like star,
the beginning of new interstellar clouds)
– supernova remnants (the end of one massive
star, the beginning of new interstellar clouds)
Stellar evolution
after the main
sequence
Zero-age main
sequence (ZAMS)
- newborn stars lie
on this line.
Star clusters on the HR diagram
and stellar evolution
Star clusters on the HR diagram
and stellar evolution
Star clusters on the HR diagram
and stellar evolution
Star clusters on the HR diagram
and stellar evolution
How do we know?
• These evolutionary tracks are fine, but how
do we know they are right?
– The physics works
– all the observed types of stars are explained
• Star clusters
– A cluster of stars forms when a large gas cloud
collapses into many stars of many different
masses. Each cluster is a snapshot of stellar
evolution.
Young Star Cluster
Open Cluster: The Hyades
cluster. About 600 million
yrs. old
Old Star Cluster
Globular Cluster M80:
About 12 billion yrs. old
Star Populations
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Population I stars: .
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Population II stars:
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Relatively young stars
Metal rich
Ancient stars
Metal poor
During the last stages of population II stars,
they produce “metals”, and when they die,
these metals are expelled into space.
Population I stars are second generation stars
formed from these metal rich nebulae.
Check questions
• What determines a main sequence star?
• What makes a star move off the main
sequence?
• How does stellar mass influence location on
the main sequence? Subsequent evolution?
• Name the steps in the evolution of a low
and high mass star, and the energy source at
each step.