The Death of Massive Stars

The Death of Massive Stars

• What about the final iron cores of massive stars?

H → HeHe → C, O

C → Ne, Na, Mg, ONe → O, Mg

O burning:

3.97 ms 23:59:59.99997

O → Si, S, P

H → HeHe → C, O

C → Ne, Na, Mg, ONe → O, Mg

Si burning:

0.03 ms

The final 0.03 msec!!

O → Si, S, PSi → Fe, Co, Ni

25 MSUN: 0.5 yr

25 MSUN: 1 yr

Iron Core

Si > Fe

Fe

Fe

Fe

• 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

• 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

500km

Core Collapse Supernovae

Core Collapse Supernovae• When the core reaches a size of about 50 km, the material is a degenerategas of about 70% neutrons, 30% protons, and the temperature is about100,000,000,000 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

5

core collapse supernova mechanism

Fe core

inner core

pre SN star1.

infalling outer core

outgoing shock from rebounce

proto neutron star2.

infalling outer coreproto neutron star

stalled shock

3.

revived shock

proto neutron star

matter flow gets reversed- explosion

4.

neutrinos

neutrino heatedlayer

Core Collapse Supernovae• The outer layers of the star are blown away by the enormous energy releasedduring 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 supernova1572 A.D. in Cassiopeia

Core Collapse Supernovae in our Galaxy

• Kepler’s supernova1605 A.D. in Ophiuchus

Supernova 1987a

Core Collapse Supernovae in other galaxies

Core Collapse Supernovae in other galaxies

Core Collapse Supernovae in other galaxiesSupernova 1994D in NGC 4526

Supernova Lightcurves

Supernova classification:• Type II : Hydrogen lines seen in spectrum• Type I : No hydrogen lines seenType II are all the result of iron core-collapse

Heavy Element Production in Supernovae• Stars can only manufacture elements up to iron through fusionreactions in their core• Enormous temperatures and energy release in supernovae cancause fusion of heavier elements through the r-process – rapidcapture 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 corestabilises 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: about10,000,000,000 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 Bellby accident• Pulsed radio emission detectedfrom 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, causingEmission 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 timethe 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 provideenough pressure to support the proto-neutron star againstits gravity, and it collapses further. What happens next?

Relativity• 1905: Einstein publishes special theory of relativity• 1915: Einstein publishes general theory of relativity

Speed of light = 300,000 km/s (180,000 miles/s)

1. Speed of light is a constant and nothing can travel faster

Relativity• 1905: Einstein publishes special theory of relativity• 1915: Einstein publishes general theory of relativity

Speed of light = 300,000 km/s (180,000 miles/s)

1. Speed of light is a constant and nothing can travel faster

2. The faster something moves, the slower time appears to move for it-For every second that passes on Earth, 0.99999999967 seconds passon 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.

Relativity3. 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 clockson 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

Relativity3.. 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 becomesredder; light falling in becomes bluer. This is gravitational red/blue shift

Relativity3.. 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 VelocityHow fast do you have to throw a ball up for it to escapeThe Earth completely?• Depends on the mass of the Earth and your distancefrom 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 radiusAs 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 VelocityEarth: 11 km/sSun: 620 km/sWhite Dwarf: 6,500 km/sNeutron Star: 165,000 km/s

• 1783: Rev. John Mitchell noticed a strange consequence of Newton’s laws of motionand gravity: an object with the same density as the Sun but 500 times its radius wouldhave 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 tocollapse 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 speedof light.

Sun: 3 kmEarth: 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 provideenough pressure to support the proto-neutron star againstits gravity, and it collapses further. What happens next?

• Answer: it collapses inwards until all of its matter is withinThe 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 theboundary 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 theplanets 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 beunable to escape.