Transcript Slide 1

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 > 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.
How do we find the mass of a star?
The mass--luminosity relation (a line in a logarithmic plot
--main sequence stars only)
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.
The cloud starts to contract.
The cloud fragment is about 104 AU in size
A protostar has condensed in the middle.
The protostar is about 1 AU in size;
the whole picture is about 100 AU in size
Protostar evolution: different tracks for different mass.
A protostar has a womb of dust--it is an infrared black body.
2. Birth: A star is born when its cores temperature reaches
10 million K. This happens for masses > 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.
Infancy:
--Jets of gas may heat the interstellar medium
--Herbig Haro objects or YSOs (Young
Stellar Objects).
Bipolar outflow.
--Mass less than .08 M(sun) but larger than Jupiter:
failed star or brown dwarf (large planet).
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.
Hot O and B stars form HII regions - Stromgren Spheres.
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.
'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.
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).
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.
Giant phase
evolutionary track
varies with mass.
Mass loss as Red Giant
is as much as
10-6 msun/year!
The Red Giant contracts and Helium begins to burn
in Helium flash with electron degeneracy holding up core
in 1 Msun star.
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.
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).
'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.
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).
Overview heavy element nucleosynthesis
process
conditions
timescale
site
s-process
(n-capture, ...)
T~ 0.1 GK
tn~ 1-1000 yr, nn~107-8/cm3
102 yr
and 105-6 yrs
Massive stars (weak)
Low mass AGB stars (main)
r-process
(n-capture, ...)
T~1-2 GK
tn ~ ms, nn~1024 /cm3
< 1s
Type II Supernovae ?
Neutron Star Mergers ?
p-process
((g,n), ...)
T~2-3 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 --> 115Cd --> 115In + e- + ν .
'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.
The Crab Nebula in Taurus is a supernova II remnant.
It exploded almost a thousand years ago.
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.)
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 masses—critical mass.
The heating creates an explosion: Supernova Type I,
if the white dwarf is destroyed, Nova if it is not.
Supernovae type Ia are
standard candles--of same peak luminosity.
Which means we automatically know their what?
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.
Remnant:
< 1.3 MO
1.3 MO<M<3.0 MO
> 3.0 MO
End State: Supporting Pressure:
White Dwarf Electron degeneracy
Neutron Star
Neutron
Black Hole
None
Chandrasekar Limit--white dwarfs form
with remnant under 1.3 Msun.
Think very thick styrofoam coating on golf balls:
styrofoam is like electron cloud of H atom,
golf ball is like proton.
Put these styrofoam balls in a pile with electron clouds
touching and you have white dwarf material.
Squeeze the styrofoam into the golf ball and you have
An analogy to neutron star matter.
It is this process backwards—inverse beta decay,
or squeezing and electron into a proton to make
a neutron.
Beta decay forward,
Inverse beta decay backward.
A black hole is like putting the golf balls into
the ultimate trash compactor the neutrons are squeezed into
a point—the black hole singularity.
. Neutron Star:
--Electrons squeezed into protons make neutrons.
--Gravity sufficient to make neutrons 'touch'.
--10's of km in diameter with some surface electrons.
Cons. of angular momentum--rapid spin, strong
magnetic fields and synchotron radio radiation as
electrons are spun out along field lines.
As NS axis wobbles, the beams may be detected as pulsars,
with a period of milliseconds to seconds. Not all neutron stars
are pulsars.
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.
The Crab Nebula has a pulsar in its core with period 1 sec.