Chapter 11 The Death of High Mass Stars

a star’s mass determines its life story

1 M sun 25 M sun

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 >8

M

Sun 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

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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 M sun • 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 M sun core and collapses into a neutron star, gravitational PE released in explosion

White dwarf supernova: (Type I)

White dwarf near 1.4 M sun 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

M

sun

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

mv

2 

GmM 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

M

Sun 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

M

Sun 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

M

Sun )

Some X-ray binaries contain compact objects of mass exceeding 3

M

Sun 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?