NOTES: The Lives of Stars

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NOTES The Lives of Stars Gestation, Birth,
and Youth 1. The womb Stars are born in dense
molecular clouds. --The interstellar medium
must be dense enough so H atoms can
collide and form H2 molecules. This also
is facilitated on dust--for other molecules as
well. It increases gravitation enough for stars
to form in reasonable time. --Different sized
clumps form stars of differing mass. --Disk
with central sphere (protostar) formed. Gravity
heats by Helmholtz contraction. Disk
forms solar system. --Stability when gravity
balances gas pressure (overlay).
(Fully developed fetus) --Star draws a
womb of dust around it. It glows in the IR. 2.
Birth A star is born when its cores temperature
reaches 10 million K. This happens for masses gt
0.08 M(Sun). --the star blasts away its womb of
dust and shines. --T Tauri Stars variable
brightness (like contractions). Low mass stars
just about to move to the main sequence.
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How do we find the mass of a star?
The mass--luminosity relation (a line in a
logarithmic plot --main sequence stars only)
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The womb Stars are born in dense molecular
clouds. --The interstellar medium must be dense
enough so H atoms can collide and form H2
molecules. This also is facilitated on dust--for
other molecules as well. It increases gravitation
enough for stars to form in reasonable time.
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The cloud starts to contract.
The cloud fragment is about 104 AU
in size
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A protostar has condensed in the middle. The
protostar is about 1 AU in size the whole
picture is about 100 AU in size
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Protostar evolution different tracks for
different mass.
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A protostar has a womb of dust--it is an infrared
black body.
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2. Birth A star is born when its cores
temperature reaches 10 million K. This happens
for masses gt 0.08 M(Sun).
--the star blasts away its womb of dust and
shines. --T Tauri Stars variable brightness
(like contractions), low mass, strong magnetic
fields, large sunspots.
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Infancy --Jets of gas may heat the
interstellar medium --Herbig Haro objects or YSOs
(Young Stellar Objects).
Bipolar outflow.
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--Mass less than .08 M(sun) but larger than
Jupiter failed star or brown
dwarf (large planet).
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IONIZATION STATE OF ATOMS Each state for a
given element has a unique spectrum.
Number of electrons removed
H
He
O
--------------------------------------------------
------------------------ 0 (neutral atom)
HI HeI.. OI
1 HII HeII.
OII 2
HeIII
OIII 6

OVII
Number of electrons removed roman numeral 1.
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Hot O and B stars form HII regions - Stromgren
Spheres.
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Chain Reaction Star Formation Massive star
formation triggers nearby regions to become new
star formation regions. Shockwaves from
ionization and supernovae bunch up material to
form stars.
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'Working Years'--Main Sequence--H burning phase.
Lasts 9 billion years for the Sun. Moves very
slightly up and to the right in H-R diagram.
As H in core is depleted, star contracts
slightly and Luminosity increases a little. He
has less gas pressure than H.
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Midlife Crisis'--Red Giant Phase 1. Stops
burning H in the core, contracts, starts burning
H in a shell around the core (Shell
H-Burning).
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The heat expands the outer envelope of the star.
It moves way up in the H-R diagram for a 1
solar mass star, stays at the same luminosity,
but gets redder for a 5 Msun star.
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Giant phase evolutionary track varies with mass.
Mass loss as Red Giant is as much as 10-6
msun/year!
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The Red Giant contracts and Helium begins to burn
in Helium flash with electron degeneracy
holding up core in 1 Msun star.
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He continues to burn to C by triple alpha
process. In larger mass stars, alpha particles
are added one by one, creating elements with an
even atomic number. Sometimes this is called the
triple alpha process as well, even though more
than three alpha particles are involved.
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Shell He-burning. He and H rekindles around
core. 1 Msun star expands to Red Giant again and
5 Msun redder and lower temp. (To right in H-R.)
5 Msun or more undergoes thermal
pulsations (Cepheids and RR Lyrae
variables --are on the instability strip on H-R
Diagram).
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'Retirement' 1. Stars starting with less than
about 2 Msun finish burning to carbon, become
unstable as they burn H and He in a shell and
shuck off a shell of 10-20 of their mass,
becoming a planetary nebula, glowing because
they are ionized by the hot UV core.
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2. Stars with more than 2 Msun burn to whatever
element is the largest possible for
their temperature.
In very large stars (over 10 Msun), core burns
to iron(Fe).
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Overview heavy element nucleosynthesis
process conditions timescale site
s-process(n-capture, ...) T 0.1 GKtn 1-1000 yr, nn107-8/cm3 102 yrand 105-6 yrs Massive stars (weak)Low mass AGB stars (main)
r-process(n-capture, ...) T1-2 GKtn ms, nn1024 /cm3 lt 1s Type II Supernovae ?Neutron Star Mergers ?
p-process((g,n), ...) T2-3 GK 1s Type II Supernovae
The s (slow) and r (rapid) process elements
heavier than Fe are formed
by addition of neutrons and then beta
decay (see overlay). The s process adds one
neutron at a time, the r process
many at a time. Ex. of s process 114Cd
1n --gt 115Cd --gt 115In e- ? .
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'Death' of stars 1. Supernova Type II A
star of over 2 solar masses
burns to all it can, collapses as supporting
radiation turns off, gets hot,
produces neutrinos by combining protons
and electrons, and rebounds, and explodes.

Supernova 1987A in Large Magellanic cloud
detected by Ian Shelton--new star on plate.
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The Crab Nebula in Taurus is a supernova II
remnant. It exploded almost a thousand years ago.
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The Anasazi (native americans) recorded the event
in Chaco canyon. (The Chinese read their
manuscripts at night in the light of a night-time
sun.)
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3. Nova and Supernova Type Ia A binary with a
white dwarf and a red giant creates an
explosion. Mass from
the red giant is pulled onto the surface of the
white dwarf until it
reaches 1.43 solar massescritical mass.
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The heating creates an explosion Supernova Type
I, if the white dwarf is destroyed, Nova if it
is not.
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Supernovae type Ia are standard candles--of same
peak luminosity. Which means we automatically
know their what?
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White Dwarf death state of low mass stars about
earth-sized, held up by electron pressure.
Fusion has ceased. Hot at first on
surface--20,000 K then cool to black dwarf (a
carbon cinder in space) in tens of billions of
years.
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Chandrasekar Limit--white dwarfs form with
remnant under 1.3 Msun.
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Think very thick styrofoam coating on golf
balls styrofoam is like electron cloud of
H atom, golf ball is like proton.
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Put these styrofoam balls in a pile with electron
clouds touching and you have white dwarf material.
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Squeeze the styrofoam into the golf ball and you
have An analogy to neutron star matter.
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It is this process backwardsinverse beta
decay, or squeezing and electron into a proton to
make a neutron.
Beta decay forward, Inverse beta decay backward.
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A black hole is like putting the golf balls into
the ultimate trash compactor the neutrons are
squeezed into a pointthe black hole singularity.
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Cons. of angular momentum--rapid spin, strong
magnetic fields and synchotron
radio radiation as electrons are spun out along
field lines.
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As NS axis wobbles, the beams may be detected as
pulsars, with a period of milliseconds to
seconds. Not all neutron stars are pulsars.

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Jocelyn Bell discovered the first pulsar in
1967. It was at first thought to be a
signal from an alien civilization and
had a period of about 1 s.
Her thesis advisor, Anthony Hewish made an
effective radio dish by stringing wires in a
grape arbor.
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The Crab Nebula has a pulsar in its core with
period 1 sec.