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

Total # of P+N = Multiples of 4! 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

9 7 http://physics.gmu.edu/~jevans/astr103/CourseNotes/Text/Lec05/Lec05_pt5_txt_stellarPostMSEvol.htm

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

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 Crab1b.mov

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 Final Gravitational Potential Energy Initial Kinetic = 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

The event horizon of a 3 MSun black hole is a few km Neutron star Fill in your favorite city map here. 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

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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?