Stellar Evolution The lives of low-mass stars And the lives of massive stars
Stars of High Mass High mass stars fuse H He, but do so in a different reaction: the CNO cycle Carbon is a catalyst, speeding fusion rate and increasing the luminosity of star.
Why does CNO cycle fusion require a higher temperature than p-p fusion in the Sun? A. Carbon nuclei contain neutrons, hydrogen nuclei do not. B. Carbon nuclei contain neutrinos, hydrogen nuclei do not. C. Carbon nuclei have more mass than hydrogen nuclei. D. Carbon nuclei have more electric charge than hydrogen nuclei.
Very high core temperature: Fast fusion rate means short life High pressure: core contracts less Fusion rate doesn t change as much, luminosity nearly constant Stars of High Mass
Evolution of Massive Stars Very high core temperatures mean that core fuses through heavier and heavier elements: S, Si-fusing layer to Fe,Ni, O-fusing layer to S, Si, Ne-fusing layer to O, Mg, C-fusing layer to Ne, Na, Mg, He-fusing layer to C, H-fusing layer to He, and outer layers of H & He.
Evolution of Massive Stars What happens after formation of iron? All of the previous reactions release energy because the reacting nuclei had more mass than the nuclei that were formed. Iron has the lowest mass of any nucleus. Combining two iron nuclei absorbs energy rather than releasing it. Uh oh
Rushing toward the End The fusion of heavier elements releases a smaller amount of energy. To release enough energy to support the star, the fusion rate sky-rockets. Each stage of fusion proceeds faster and faster The fusion of silicon takes only 2 days. The temperature has also reached 8 billion Kelvin!
The Death of a Massive Star While silicon fuses to form heavier elements, the extreme temperature (8 billion Kelvin) in the center makes collisions so violent that nuclei break up: Fe nucleus 13 He nuclei + 4 neutrons He nucleus 2 protons + 2 neutrons Disintegration uses up energy, so the core cools and begins to collapse. Densities rise and protons and electrons collide!
The Death of a Massive Star The reaction of protons and electrons creates neutrons and neutrinos: This further robs the core of energy and triggers the collapse of the core at 25% of lightspeed. The inner core (about the size of Earth) contains a few times the Sun s mass. In less than 1 second, the inner core shrinks to the size of a city and reaches densities higher than an atomic nucleus.
A Really Big Bounce At this incredible density, gravitational collapse is stopped neutrons can t be squeezed any tighter. But the rest of the star still crashes down onto the collapsed core and bounces back outward again.
And a Really Big Flash The proton neutron conversion in the core has also released a flood of neutrinos. They carry away energy equal to about 20% of the mass-energy of the inner core of the dying star. Normally, the neutrinos would stream out of the star and escape to space. But the density in the core is so high, even the neutrinos have a hard time getting out. One of every 1000 neutrinos collides with particles in the star s outer layers, speeding the explosion.
A Core-Collapse Supernova The release of energy in the form of light is so dramatic that a supernova explosion can rival an entire galaxy about 10 billion Sun s luminosity. In 1987, such a supernova appeared in the Large Magellanic Cloud about 170,000 light-years away:
A Core-Collapse Supernova However, if we could observe the supernova event in neutrinos we would measure 100 luminosity compared to the light output! In fact, when Supernova 1987A went off, about a dozen neutrinos were captured by the Kamioka neutrino experiment in Japan. This was the first proof of neutrinos emitted by any astronomical object other than the Sun. And the first proof that the preceding story is right.
A Bright Star in Taurus On the night of July 4, 1054 CE, astronomers in several cultures around the world noticed a new star among the stars of the constellation Taurus. This new star was visible in the daytime sky for almost two weeks and visible at night for 2 years. We now recognize this new star was a supernova that created the Crab nebula. It is only 6500 light-years away, so we can study it in detail with telescopes like Hubble and Chandra.
The Crab Supernova Remnant Hubble mosaic by Jeff Hester & Allison Loll (ASU)
The Crab Supernova Remnant After nearly 1000 years of expansion, the debris has expanded to a size of 11 light-years. It is still moving outward at 1500 km/s. At the heart of the remnant is the Crab pulsar, a neutron star spinning 33 times a second. The Crab pulsar was one of the first pulsars discovered. It emits radio waves, visible light, and x-rays. (We ll talk more about neutron stars next week.)
The Vela Supernova Remnant The Vela remnant is much closer, much larger, and much older than the Crab nebula. Estimates indicate the Vela remnant is about 800 light-years away, about 100 light-years across, and exploded about 11,000 years ago. The Vela remnant also contains a pulsar, the first such direct evidence that supernovas and pulsars were related. The Vela pulsar also emits radio waves, visible light, and x-rays.
The Vela Supernova Remnant Astrophoto by Robert Gendler
The Ultimate Fate What happens if the mass of the inner core grows so even the neutrons cannot provide support? GRAVITY WINS In this case, the collapse of the star is complete and the matter that makes up the core is hidden from view. A black hole is formed. A black hole is an object that is so compact that nothing can escape it not even light.
The Ultimate Fate No example of a neutron star in a binary has a mass greater than 2 solar masses. It appears that neutron stars do not form with masses larger than about 2 solar masses. Any star s core that grows to more than this mass apparently collapses to form a black hole. We ll talk more about black holes next week
High-mass star #1 #2 #3a #3b
What is the end result of the evolution of the Sun? A. The Sun will leave behind a black hole. B. The Sun will leave behind a white dwarf. C. The Sun will leave behind a neutron star. D. The Sun will leave nothing behind.
How big must a star be to produce a supernova? A. More than 0.5 solar mass B. More than 1 solar mass C. More than 4 solar masses D. More than 8 solar masses
What is a neutron star made of? A. hydrogen & helium B. carbon & oxygen C. neutrons & a few protons D. neutrinos & dark matter