Transcript 55-80 BIOGRAPHY_of_A STAR

Our Sun’s Story
…and that of heavy stars
mass of a star determines its core pressure and temperature:
our sun’s low mass  cooler core and slower fusion rate
lower internal temperature, and external (yellow)
smaller luminosity
longer lifetime (10 B y)
high mass stars (> 8 msun) have higher core temperature
more luminous and higher external temperature (blue)
more rapid fusion
shorter-lived (30 M y)
High-Mass Stars
> 8 MSun
IntermediateMass Stars
Low-Mass Stars
< 2 MSun
Brown Dwarfs
famous HerztsprungRussell diagram
sun is stable…
as long as it
has hydrogen
in its core to
fuse into
helium
Thought Question
What happens when a star no longer has enough
hydrogen in its core to fuse to helium?
i.e. after 10 B years
A.
B.
C.
D.
Core cools off
Core shrinks and heats up
Core expands and heats up
Helium fusion immediately begins
Thought Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off
Core shrinks and heats up
Core expands and heats up
Helium fusion immediately begins
Life Track after Main Sequence
Observations of
stars in clusters, all
born at the same
time, show that a
star becomes larger,
redder, and more
luminous after
fusing all the H in
its core
as inert He core contracts,
H in a shell
around the He core
begins burning
luminosity increases 1,000 x
too hot for life on Earth
radius grows 100 x,
out to Earth
increased fusion rate in the
H shell does not stop
He core from contracting
H shell burns for ~1 B years
luminosity of a sun? same as for any black body…
L  Area  T4
Stefan-Boltzman law
even though T down  2, A up (100)2  for red giant,
Helium fusion does not begin until heated by collapse
requires 100 MK since charge (+2)2 leads to 4 x greater
repulsion than with 2 protons
Fusion of 2 helium nuclei doesn’t work (8Be unstable),
helium fusion must combine 3 He nuclei to make carbon
Thought Question
What happens in a low-mass star when core temperature rises
enough for helium fusion to begin?
A. Helium fusion slowly starts up
B. Hydrogen fusion stops
C. Helium fusion (triple alpha) starts very sharply
Hint: this is a strong reaction (no neutrinos)
once the temperature is hot enough to overcome
Coulomb barrier
Thought Question
What happens in a low-mass star when core temperature rises
enough for helium fusion to begin?
A. Helium fusion slowly starts up
B. Hydrogen fusion stops
C. Helium fusion rises very sharply
Helium Flash
Core temperature rises rapidly when helium fusion begins
Helium fusion rate skyrockets until thermal pressure
takes over and expands core again
Helium burning stars neither shrink nor grow,
core He burns to C for 100 M years,
then expand again in a second red giant phase
Thought Question
What happens when the star’s core runs out of helium?
A. The star explodes
B. Carbon fusion begins
C. The core cools off
D. Helium fuses to C in a shell around a heavier carbon
core
Thought Question
What happens when the star’s core runs out of helium?
A.
B.
C.
D.
The star explodes
Carbon fusion begins
The core cools off
Helium fuses in a shell around the core
Double Shell Burning
After core helium used up,
He fuses into carbon in a shell around the inert
carbon core
H fuses to He in a shell around the fusing helium layer
double-shell burning stage never reaches equilibrium—
fusion rate periodically spikes upward in a series of
thermal pulses
With each spike, convection dredges carbon up from core
and transports it to surface
Our Sun’s Dregs: a Planetary Nebula
after few M years,
double-shell burning ends in a pulse,
ejecting H, He, C out into space
a planetary nebula
(but nothing to do with planets)
white dwarf, carbon core left behind
…two example pix from Hubble
for our sun, C is the end of the fusion trail
fusion progresses no further in a low-mass star
mass too small for gravity to collapse it further,
and heat it up even more
electron degeneracy pressure
supports white dwarf against gravity
(e-’s approach speed c if m > 1.4 msolar )
temperature never grows hot enough (400 M K)
for fusion to heavier elements
e.g. for He to fuse with C to make oxygen
Life stages
of a lowmass star
like the Sun
Life Path of a Sun-Like Star
How different are life stages of high-mass (e.g. 25 m๏) star?
similar to those of low-mass stars, like our sun, but each is faster
Hydrogen core fusion, ~ M years
not pp, but much faster CNO cycle, higher luminosity
making N and O as well as He
becomes a red supergiant when core H exhausted
Hydrogen shell burning around a He core
Helium core fusion to carbon, lasting ~ 100,000 years
Carbon burning (0.6 B ºK) for ~ 100 years
What are the life stages of a high-mass star?
CNO Cycle
High-mass fuse H to He
at a higher rate using
carbon catalyst, CNO cycle
Greater core temperature
heavy nuclei overcome
greater Coulomb repulsion
How do high-mass stars make the
elements necessary for life?
Big Bang made 75% H, 25% He – stars make everything else
3 Helium fusion makes carbon in low-mass stars
CNO cycle changes C into N and O
Helium Capture by O and Ne
higher core temperatures from successive gravitational collapses gives
helium the energy to thwart
ever stronger Coulomb barriers (zZ) of heavier elements
Advanced Nuclear Burning
•
Core temperatures in stars with >8MSun
allow fusion of elements as heavy as iron
Multiple Shell Burning
• Advanced nuclear
burning proceeds in
a series of nested
shells
Iron = dead end for fusion
nuclear reactions of
iron release no energy
Fe has lowest mass
per nucleon
signature of helium
capture
nucleosysthesis:
highest abundances
are elements with
even numbers of
protons
Iron builds up
in core until
degeneracy
pressure can no
longer resist
gravity
Core then
suddenly
collapses,
creating
supernova
explosion
Energy and neutrons released in supernova explosion enable elements
heavier than iron to form, including Au and U
What causes a supernova collapse?
Core degeneracy pressure disappears
electrons combine with protons,
making neutrons and neutrinos
kT + me + mp > mn
kT + 0.5 + 938.3 > 939.6 MeV
Ethermal = kT @ 10 BK = 1 MeV,
k = Boltzman’s constant
Neutrons collapse to the center,
forming a much smaller (~10km~Boston)
neutron star (me/mn ~ 1/2000)
… then collapsing to a black hole if  12 msun
Supernova Remnant
energy released by core
collapse
drives outer layers into space
Crab Nebula the remnant of
supernova of AD 1054
…and its neutron star
our picture of a pulsar (neutron star)
during collapse…
angular momentum conserved
big spin up
magnetic fields pinched
very strong
but in chaos of explosion,
magnetic  rotational axis
beams of radiation escape
along magnetic axis
“lighthouse” beam sweeps periodically past earth…
spinning so fast it can only be from a compact source, r ~ 10 km
Supernova 1987A
closest supernova in the last four centuries