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The Life Cycle of Stars
How are stars born, and how do they die?
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Star Birth
Learning Goals How do stars form?
How massive are newborn stars?
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Stars are born in molecular clouds Consist of molecular hydrogen (H2) gas and dust grains.
Masses of ~105 to 106 times the Suns mass.
Sizes of ~150 light years in diameter.
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Stars are born in molecular clouds
Clouds are very cold: ~10-30 K. (recall 273 K = water freezes)
Stars form when gravity overcomes thermal pressure.
Then gas clumps begin to collapse.
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Earliest stages of star birth
Dense cores of gas in the
larger molecular cloud
collapse due to self-gravity.
Cloud heats up as it
contracts due to conservation
of energy: gravitational
potential energy is convertedto thermal energy (heat).
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Rotation is an importantfactor during the star birth
process (part 1)
As gravity forces a dense coreto become smaller, it spins
faster and faster.
This is due to conservation ofangular momentum. Dense cores have a small
amount of initial rotation.
As the cores get smaller, theymust spin up to conserve
angular momentum.
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Rotation is an important
factor during the star birth
process (part 2)
Collisions between gas particles incloud gradually reduce random
motions and up+down motions.
Collisions flatten the cloud into a disk.
The result is a rotating protostar with
a rotating disk of gas & dust.
The orderly motions of our solar
system today are a direct result of the
solar systems birth in a spinning,
flattened cloud of gas.
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Angular momentum leads to:
Rotation of protostar
Disk formation
And sometimes
Jets from protostar Fragmentation into a
binary star.
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Outflows (jets)from protostars
Young proto-stars(
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Outflowanimation
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Protostar to Main-sequence Star
Protostar contracts and heats until core temperatureis sufficient for hydrogen fusion.
Contraction ends when energy released by hydrogen
fusion balances energy radiated from surface.
Takes 50 million years for star like Sun from birth toreach the main sequence. (Takes less time for more
massive stars).
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Star birth is a dynamic process
First generation of stars to form can affect
the molecular cloud from which it wasborn.
Can trigger birth of a new generation of
stars. Can lead to destruction of the original
molecular cloud.
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Star birth is best-studied using long wavelengths
Much on-going star formation is invisible:
Interstellar dust grains in molecular clouds
absorb most optical photons.
Easiest to study star birth at long wavelengths
Infrared wavelengths
Millimeter/radio wavelengths
These wavelengths are good for 2 reasons:
Less absorption due to dust
Radiation from young stars can heat up the dust,
causing it to emit at IR/millimeter wavelengths.
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Interstellar dust blocks optical light
Optical image
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Interstellar dust blocks optical light
Infrared image
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Summary of
Star Birth Process
Gravity causes dense cores
in molecular clouds tocollapse.
Collapsing cores heats up.
When core gets hot enough,
fusion begins and stops the
shrinking.
New star achieves long-
lasting state of balance.
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Clusters of many stars can form from a
single molecular cloud: wide range of stellar masses
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Mass ranges of stars
Temperature
L
uminosity
Very
massivestars are
rare.
Low-mass
stars arecommon.
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Mass ranges of stars
Temperature
L
uminosity
Stars more
massivethan
100 MSunwould blow
themselvesapart.
Stars less
massive than0.08 MSuncant sustain
core fusion.
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Life Cycle of Stars captured
in a single view of HST
Giant Molecular Cloud
Starburst cluster (O-type
stars)
Proplyds
Pillar
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NGC 3603
Circumstellar ring
ProplydsPillar (sculpted by
ionizing radiation)
O-star cluster
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What prevents protostars fromcontinually collapsing ever smaller?
If M > 0.08 MSun, then gravitational contraction
heats the core until fusion begins. Energy
generated by fusion provides thermal
pressure to stop the collapse (star).
If M < 0.08 MSun, electron degeneracypressure stops gravitational contraction
before fusion can begin (brown dwarf).
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Degeneracy Pressure:
Laws of quantum mechanics (the Pauli ExclusionPrinciple) prohibit two electrons from occupying same
state in same place
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Thermal Pressure:
Depends on heat
content
The main form of
pressure in most stars
Degeneracy Pressure:
Electrons cant be in same
state in same place.
Doesnt depend on heat
content.
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Brown Dwarf An object less massive than 0.08 MSun.
Radiates infrared light.
Has thermal energy from gravitationalcontraction.
Never reaches a high enough temperature in
the core to ignite nuclear fusion. Electron degeneracy pressure eventually
stops contraction.
These failed stars then cool off over time.
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Brown dwarfs: Between planets and stars
Stars
Mass > 80 Mjuphot objects
Planets
Mass < 15 Mjupcold objects
Brown dwarfs
Mass = 15-80Mjup
start hot, get cold
(failed stars)
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What have we learned?
How do stars form? Stars are born in cold, relatively
dense molecular clouds.
As a cloud fragment collapsesunder gravity, it becomes a
protostar surrounded by a spinningdisk of gas.
The protostar may also fire jets of
matter outward along its poles.Protostars rotate rapidly, and somemay spin so fast that they split toform close binary star systems.
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What have we learned? How massive are newborn stars?
Newborn stars come in a range of masses, butcannot be less massive than 0.08MSun.
Below this mass, degeneracy pressure
prevents gravity from making the core hotenough for efficient hydrogen fusion, and the
object becomes a failed star known as a
brown dwarf.
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Life as a Low-Mass Star
Learning goals What are the life stages of a low-mass star?
How does a low-mass star die?
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Different classes of stars
High-Mass Stars
> 8 MSun
Low-Mass Stars
< 2 MSun
Intermediate-Mass
Stars (~2-8 Msun)
Brown Dwarfs(no H-burning)
< 0.08 MSun
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Overview of stellar evolution
In general, the story of stellar evolution is the ongoing
struggle of stars to generate internal energy in order to
resist collapsing under their own self-gravity.
A star remains on the main sequence as long as it can
fuse hydrogen into helium in its core.
About 90% of total lifetime spent on main sequence Stable energy generation during this phase.
Once the core H fuel is gone, the star evolves rapidly off
the main-sequence, spending much of the remaining
time as a cooler & more luminous star.
Exact fate will depend on its original mass.
In the end, gravity always wins.
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Life and Death of a Low Mass Star (Like the SUN)
1. Protostar
2. Main sequence star
3. Red giant star
4. Helium-burning star
5. Double-shell burning star
6. Planetary nebula
7. White dwarf
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Stage #2, QUESTION: What happenswhen a star can no longer fuse hydrogen
to helium in its core?
A. Core cools off.
B. Core shrinks and heats up.
C. Core expands and heats up.
D. Helium fusion immediately begins.
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Stage #2, QUESTION: What happenswhen a star can no longer fuse hydrogen
to helium in its core?
A. Core cools off.
B. Core shrinks and heats up.
C. Core expands and heats up.D. Helium fusion immediately begins.
Remember gravitational equilibrium: outward pressure fromcore energy generation balances the inward crush of gravity.Without the energy generation, the core will be compressedand will heat up.
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Step #2, QUESTION: What happens as a
stars inert helium core starts to shrink?
A. Hydrogen fuses in shell around coreB. Helium fusion slowly begins
C. Helium fusion rate rapidly rises
D. Core pressure sharply drops
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Step #2, QUESTION: What happens as a
stars inert helium core starts to shrink?
A. Hydrogen fuses in shell around coreB. Helium fusion slowly begins
C. Helium fusion rate rapidly rises
D. Core pressure sharply dropsThe pressure from the outer portion of the starcompresses the interior enough that H-burning startsin a narrow shell around the (inert) helium core.
H-burning shell generates much more energy thanduring the main sequence. The star swells up due to
this energy generation.
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Stage #2: Red giant phase
H-burning shell with inert He core
After 10 billion years,
Suns core H is used up.
Left with inert He core: not
hot enough to burn (yet).
He core shrinks in size
along with inner layers. Surrounding shell of
hydrogen gets hot enough
to begin fusion.
H-burning shell producesmore energy than main-
sequence phase.
Star expands and cools:
red giant phase.
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Stage #2: Red giant phase
H-burning shell with inert He core
Main sequence star
(core H burning)
Red giant star
(H-shell burning)
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Stage #2: Red giant phase
the broken thermostat During main sequence,
fusion controlled by
solar thermostat.
During H-shell burning
phase, no such thermostat
working.
H-burning shell makesmore He, which is dumped
on inert core.
Core contracts further due
to added weight, increasingthe pressure.
H-shell burning accelerates
with time.
Sun grows to 100x in size.
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Helium fusion
Cant fuse just 2 He nuclei, need 3 to make carbon.
Helium fusion requires higher temperatures than
hydrogen fusion because helium nucleis greater
electrical charge leads to greater repulsion(100 million K, compared to 15 million K).
Eventually the core becomes hot enough to burn He.
Ignition of the core happens very quickly (helium flash)
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Helium Flash
Thermostat is broken in low-mass red giant becausedegeneracy pressure supports core
Core temperature rises rapidly when helium fusion
begins
Helium fusion rate skyrockets until thermal pressuretakes over and expands core again
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Stage #3: Helium-burning star
He-burning in core (making carbon) with a surrounding
hydrogen-burning shell.
Energy generation in this phase is steady, b/c the internalthermostat is temporarily fixed.
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Stage #4, QUESTION: What happenswhen the stars core runs out of helium?
A. The star explodes
B. Carbon fusion begins
C. The core cools off
D. Helium fuses in a shell around the core
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Stage #4, QUESTION: What happenswhen the stars core runs out of helium?
A. The star explodes
B. Carbon fusion begins
C. The core cools off
D. Helium fuses in a shell around the core
The core is not hot enough to burn the produced
carbon. So analogous to the red giant phase, thecore shrinks and the surrounding layers getdenser. Get double-shell burning:
(1) H-burning outer shell, and(2) He-burning inner shell.
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Stage #4: Double-shell burning star
Similar to the red giant phase
Inert core of carbon
Surrounding shells: burning H and He
Star becomes very cool & luminous, expanding in size.
Gravity is very weak at stellar surface, leading tomass loss through a strong stellar wind (Phase #5).
Continuing contraction of the core leads to greater &
greater luminosity. However, never gets hot enough to
burn carbon core: end of the line.
Star collapses to a dense, small, hot object: white dwarf.
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Phase #5: Planetary nebula.
A low-mass star dies by puffing off its outer layers.
C t l i i i hit d f it hi h
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Central remaining core is a white dwarf: its very high
temperature leads to very ionized gas shell
C t l i i i hit d f it hi h
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Central remaining core is a white dwarf: its very high
temperature leads to very ionized gas shell
Central remaining core is a white dwarf: its very high
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Central remaining core is a white dwarf: its very high
temperature leads to very ionized gas shell