Transcript Death of Massive Stars

The Death of Massive Stars
• What about the final iron cores of massive stars?
H → He
He → C, O
C → Ne, Na, Mg, O
Ne → O, Mg
O → Si, S, P
O burning:
3.97 ms
23:59:59.99997
25 MSUN: 0.5 yr
H → He
He → C, O
C → Ne, Na, Mg, O
Ne → O, Mg
O → Si, S, P
Si → Fe, Co, Ni
Si burning:
0.03 ms
25 MSUN: 1 yr
The final 0.03
msec!!
Core Collapse Supernovae
• When the Iron core reaches its Chandrasekhar mass (about 1.4 MSUN),
the pressure of the degenerate electron gas cannot hold the core up against
Gravity – the iron core starts to collapse
Si > Fe
Fe
Iron Core
Fe
500km
Fe
• As the iron core collapses, the electrons and Iron nuclei get squashed together.
Electrons combine with protons in the nuclei to form more neutrons.
- this decreases the pressure and speeds up the collapse
• Collapse takes only a few thousandths of a second
Core Collapse Supernovae
• When the core reaches a size of about 50 km, the material is a degenerate
gas of about 70% neutrons, 30% protons, and the temperature is about
100,000,000,000 K
• Pressure of this gas halts the collapse. The core is now a proto-neutron star.
• The outer layers are still falling in.
• A vast stream of neutrinos are flowing out from the proto-neutron star
core collapse supernova mechanism
1.
pre SN star
2.
Fe core
proto neutron star
infalling outer core
inner core
outgoing shock from
rebounce
3.
4.
proto neutron star
infalling outer core
proto neutron star
matter flow gets reversed
- explosion
stalled shock
neutrinos
revived shock
neutrino heated
layer
5
Core Collapse Supernovae
• The outer layers of the star are blown away by the enormous energy released
during the collapse in the form of neutrinos (99% of the energy) and light (the
remaining 1%).
• Luminosity of supernova is 10 billion times greater than that of the Sun
• Can outshine its galaxy for a few days
• Core collapse supernovae occur at a rate of about 1 per century per galaxy
Core Collapse Supernovae in our Galaxy
• 1054 A.D. – Chinese astronomers observe ‘guest star’,
visible during the day
• We see the remnant of the supernova as the Crab nebula in Taurus
Core Collapse Supernovae in our Galaxy
• Tycho’s supernova
1572 A.D. in Cassiopeia
Core Collapse Supernovae in our Galaxy
• Kepler’s supernova
1605 A.D. in Ophiuchus
Supernova 1987a
Core Collapse Supernovae in other galaxies
Core Collapse Supernovae in other galaxies
Core Collapse Supernovae in other galaxies
Supernova 1994D in NGC 4526
Supernova Lightcurves
Supernova classification:
• Type II : Hydrogen lines seen in spectrum
• Type I : No hydrogen lines seen
Type II are all the result of iron core-collapse
Heavy Element Production in Supernovae
• Stars can only manufacture elements up to iron through fusion
reactions in their core
• Enormous temperatures and energy release in supernovae can
cause fusion of heavier elements through the r-process – rapid
capture of neutrons by nuclei
• Most heavy elements around us are manufactured this way
What about the left over collapsed core?
Possibility 1: for stars less than 25 to 30 MSUN, the core
stabilises and becomes a neutron star
Neutron Star Vital Statistics
•Radius: 10km
•Mass: 1.4 MSUN
• Composition:
A fluid of about 90%
neutrons, 10% protons
• Temperature: about
10,000,000,000 K at
birth, cools rapidly
Neutron Star Vital Statistics
• Average density 1014-15g/cc
• Equivalent density:
– Take all humans on Earth….
.... And compress to the size of a sugar cube
• Gravitational force at surface:
100,000,000,000 times greater than Earth
Pulsars
• Discovered 1967 by Jocelyn Bell
by accident
• Pulsed radio emission detected
from source beyond our solar system
• Received a burst of radio waves
once every 1.337 seconds
• Nicknamed LGM1
• Actually a rotating neutron star
Pulsars
• Very strong magnetic field accelerates particles towards the magnetic poles, causing
Emission of radio waves in beams out from the poles
• Poles are not aligned with rotation axis:
• If we are in the line-of-sight of the beam, we see a pulse of radio waves every time
the star rotates
PSR B0329+54 1.4 times per second
VELA
11 times per second
CRAB 30 times per second
PSR J0437-4715 174 times per second
PSR B1937+21 176 times per second
Black Holes
• At the end of the lives of stars greater than 25-30 MSUN,
neutrons in the collapsing core eventually cannot provide
enough pressure to support the proto-neutron star against
its gravity, and it collapses further. What happens next?
Relativity
• 1905: Einstein publishes special theory of relativity
• 1915: Einstein publishes general theory of relativity
1. Speed of light is a constant and nothing can travel faster
Speed of light = 300,000 km/s (180,000 miles/s)
Relativity
• 1905: Einstein publishes special theory of relativity
• 1915: Einstein publishes general theory of relativity
1. Speed of light is a constant and nothing can travel faster
Speed of light = 300,000 km/s (180,000 miles/s)
2. The faster something moves, the slower time appears to move for it
-For every second that passes on Earth, 0.99999999967 seconds pass
on the international space station (orbiting at about 77,000 mph)
- adds up to about 4 seconds difference per year
-GPS satellites need to correct for this otherwise they will give the wrong
position by about 10km after one day!
• This is called time dilation.
Relativity
3. Gravity is not a force like any other: it is really the curvature of space-time
• For two objects, the one in the stronger gravitational field will see time
running slower for the object in the weaker gravitational field.
• We are in a stronger gravitational field than the GPS satellites, so clocks
on the GPS satellite will appear to run slower. If this is not taken into account,
errors of about 10km per day would accumulate.
• This is gravitational time dilation
Relativity
3.. Gravity is not a force like any other: it is really the curvature of space-time
• Light moving outwards in a gravitational field will lose energy – it becomes
redder; light falling in becomes bluer. This is gravitational red/blue shift
Relativity
3.. Gravity is not a force like any other: it is really the curvature of space-time
• If an object is rotating, it will drag space-time around with it like a whirlpool.
Escape Velocity
How fast do you have to throw a ball up for it to escape
The Earth completely?
• Depends on the mass of the Earth and your distance
from the center of the Earth.
• At Earth’s surface: 25,000 mph (11km/s)
• From the top of a 1000 mile high tower, 22,000 mph (10km/s)
• From the surface of the Sun? (Mass 1MSUN, radius 700,000km):
1,400,000 mph (620km/s)
• From the surface of a white dwarf (Mass 1MSUN, same radius
As Earth): 14,600,000 mph (6,500 km/s)
• From the surface of a neutron star (Mass 1MSUN, radius 10km):
370,000,000mph (165,000 km/s)
Escape Velocity
Earth: 11 km/s
Sun: 620 km/s
White Dwarf: 6,500 km/s
Neutron Star: 165,000 km/s
• 1783: Rev. John Mitchell noticed a strange consequence of Newton’s laws of motion
and gravity: an object with the same density as the Sun but 500 times its radius would
have an escape velocity greater than the speed of light
Speed of light = 300,000 km/s (180,000 miles/s)
• Escape velocity of a neutron star is already close! How much further would it have to
collapse before the escape velocity exceeded the speed of light? Down to 3 km
If an object has an escape velocity greater then the speed of light,
then nothing, not even light, can escape its surface. From the outside,
We would never see anything come from that object. It is a Black Hole.
Schwarzchild Radius
• How small an object must become before its escape velocity exceeds the speed
of light.
Sun: 3 km
Earth: 0.9 cm (1/3 of an inch)
A human: 10-23m (10,000,000 times smaller than a proton)
Black Holes
• At the end of the lives of stars greater than 25-30 MSUN,
neutrons in the collapsing core eventually cannot provide
enough pressure to support the proto-neutron star against
its gravity, and it collapses further. What happens next?
• Answer: it collapses inwards until all of its matter is within
The Schwarzchild radius (3km). It is then a black hole.
Black Holes
• Any event that takes place inside the Schwarzchild radius of a black hole can
never be seen by the outside universe. The Schwarzchild radius is effectively the
boundary of the black hole (its surface), and is called the event horizon.
• The only things we can tell about the matter inside the black hole are
• Its mass
• Its spin
• Its electric charge
• Far from the black hole, gravity appears to be normal
• If the Sun were to suddenly collapse into a black hole, the orbits of the
planets would be unaffected (we wouldn’t be ‘sucked in’)
• Very close to the black hole, gravity is extremely strong. If you get closer than 3
times the Schwarzchild radius (9 km for a solar-mass black hole), you would be
unable to escape.