Transcript Ch.11 Massive star death

Chapter 11
The Death of High Mass Stars
a star’s mass determines its life
story
1 Msun
25 Msun
Life Stages of High-Mass Stars
• high-mass stars are similar to low-mass stars:
– Hydrogen core fusion (main sequence)
– Hydrogen shell burning (supergiant)
– Helium core fusion (subgiant)
• They are also different..
– H-->He via CNO cycle not p-p chain
– Core much hotter
– Eventually fuse C, O into heavier elements
– He core is not degenerate
– no He flash!
– Lose a lot of mass
High-mass stars make the
elements necessary for life!
Big Bang made 90% H, 10% He – stars make everything else
Helium fusion can make only carbon in low-mass stars
Helium Capture occurs only in
high-mass stars
•
High core T, P allow helium to fuse with
heavier elements
Helium capture builds C into O, Ne, Mg, …
Total # of P+N = Multiples of 4!
Evidence for
helium
capture:
Higher
abundances of
elements with
even numbers
of protons
Advanced Nuclear Burning
•
Core temperatures in stars with >8MSun
allow fusion of elements up to iron
Si, S, Ca, Fe, etc. can only be made in high-mass stars
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Structure of massive
stars
Fusion releases energy only when the mass of the products
< mass of the reactants
• Iron is “ash” of fusion: nuclear reactions
involving iron do not release energy
• Iron-56 has lowest mass per nuclear
particle
• Highest “binding energy” of all the
elements
QuickTime™ and a
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How does a high-mass star die?
Iron builds up in core
until degeneracy
pressure can no longer
resist gravity
Supernova Explosion
• Core degeneracy
pressure cannot
support degenerate
core of > 1.4 Msun
• electrons forced into
nucleus, combine
with protons
• making neutrons,
neutrinos and LOTS
of energy!
Collapse only takes very short amount of time
(~seconds)
Supernova!
Energy and neutrons released in supernova explosion cause elements
heavier than iron to form, including Au and U
Neutron Stars & Supernova Remnants
• Energy released by
collapse of core
drives outer layers
into space
• The Crab Nebula is
the remnant of the
supernova seen in
A.D. 1054
Supernova 1987A
•
The first visible supernova in 400 years
Tycho’s supernova of 1572
Expanding at 6 million mph
Kepler’s supernova of 1609
Supernovae are 10,000 times more
luminous than novae!
Massive star supernova: (Type II)
Massive star builds up 1.4 Msun core and collapses into
a neutron star, gravitational PE released in explosion
White dwarf supernova: (Type I)
White dwarf near 1.4 Msun accretes matter from red
giant companion, causing supernova explosion
light curve shows how luminosity changes with time
A neutron star:
A few km in
diameter,
supported
against gravity
by degeneracy
pressure of
neutrons
Discovery of Neutron Stars
• Using a radio telescope in 1967, Jocelyn Bell
discovered very rapid pulses of radio emission
coming from a single point on the sky
• The pulses were coming from a spinning neutron
star—a pulsar
Pulsar at center
of Crab Nebula
pulses 30 times
per second
Why does a neutron star spin so rapidly?
Conservation of angular momentum!!
X-rays
Visible light
Pulsars
What happens if the neutron star has
more mass than can be supported by
neutron degeneracy pressure?
There is nothing to prevent it from collapsing infinitely:
BLACK HOLE!!
• Neutron degeneracy pressure can no longer
support a neutron star against gravity if its mass is
> about 3 Msun
Black Holes: Gravity’s Ultimate Victory
A black hole is
an object whose
gravity is so
powerful that not
even light can
escape it.
Escape Velocity
Initial Kinetic
Energy
=
Final Gravitational
Potential Energy
1 2 GmM
mv 
2
r
Where m is your mass,
M is the mass of the object that you are trying to escape from, and
r is your distance from that object

“Surface” of a Black Hole
• The “surface” of a black hole is the distance at which
the escape velocity equals the speed of light.
• This spherical surface = event horizon.
• The radius of the event horizon is known as the
Schwarzschild radius.
How does the radius of the event horizon change
when you add mass to a black hole?
A. Increases
B. Decreases
C. Stays the same
Neutron star
The event horizon of a 3 MSun black hole is a few km
A black hole’s mass strongly warps space
and time in vicinity of event horizon
Light waves take extra time to climb out of a deep hole in
spacetime, leading to a gravitational redshift
Time passes more slowly near the event horizon
Tidal forces near the
event horizon of a
3 MSun black hole
would be lethal to
humans
Tidal forces would be
gentler near a
supermassive black
hole because its radius
is much bigger
Do black holes really exist?
Black Hole Verification
•
•
Need to measure mass
— Use orbital properties of companion
— Measure velocity and distance of orbiting gas
It’s a black hole if it’s not a star and its mass
exceeds the neutron star limit (~3 MSun)
Some X-ray binaries contain compact objects of mass
exceeding 3 MSun which are likely to be black holes
Cygnus X-1: Black hole candidate
If the Sun shrank into a black hole, its
gravity would be different only near the
event horizon
The end
Some extra slides follow…
High mass stars : CNO Cycle
• H fusion is faster
because C, N and O
act as catalysts
• Same net result: 4 H
become 1 He.
• No total gain or loss
of C, N, O
How does the total energy produced
during one CNO cycle compare to
that of the proton-proton chain?