Chapter 17 - Red Giants and Star Death

How are supernovae and planetary nebulae similar? How are they different?

A Planetary nebula is the complex structure of layers of gases that have been shed from a low-to-medium mass star in the final stages of its energy-producing life. The star starts to collapse on itself, but this heats the core and the layers surrounding it. The remnant collapses into a very dense object known as a White dwarf. A supernova is a stellar explosion. Once the core becomes too heavy to support itself (often because the core has accumulated large amounts of iron) it collapses. This triggers a core-collapse supernova.

Neutron Star

A supernova explosion leaves behind balls of neutron called a neutron star.

What type of stars end up as black holes? What type end up as neutron stars? What type end up as white dwarfs?

High mass stars end up as black holes or neutron stars and low-mass stars end up as white dwarfs

Explain what happens to the interior of a high-mass star as it runs out of hydrogen in its core.

Hydrogen fusion proceeds at a fast rate via the CNO cycle and begin to run low on core hydrogen fuel. It responds much like a low-mass star but much faster, developing a hydrogen- fusion shell and outer layers expanding outwards to turn into a supergiant. The core contracts and gravitational contraction releases energy that raises core temp to become hot enough to fuse heloum into carbon.

Iron Core

Iron core is not possible to generate any kind of nuclear energy.

What is the basic process by which heavier elements up to iron are created within a star?

Mass per nuclear particle tends to decrease as we go form light elements to iron, which means that fusion of light nuclei into heavier nuclei generates energy. I

Helium Fusion

Occurs only when the nuclei slam int one another at much higher speeds than those needed for helium fusion, which means it requires higher temperatures. 3 He(4) ==> 1 C(12) + Energy

Carbon Fusion

Only possible at temps above 600 million K and can only undergo in high-mass stars

Helium Capture

Reactions in which a helium nuclear fuses with some other nucleus, changing carbon into oxygen and son.

White Dwarf

Small in radius and often white hot. They are small because they are the exposed cores of dead stars supported against the crush of gravity by degeneracy pressure They are often hot because some of them were only recently the centers of stars.

Supernova

The gravitational collapse of the core of a high mass star released an enormous amount of energy and it drives the outer layers into space in an explosion called a supernova.

How are white dwarfs and neutron stars similar? How are they different?

The major difference is due to the way in which they are formed. 1. White dwarfs are formed from the collapse of low mass stars, On the other hand, neutron stars are formed in the collapse of the core of a massive star. 2. A white dwarf is supported by electron degeneracy pressure, a neutron star by neutron degeneracy pressure 3. A white dwarf has a larger radius 4. A neutron star has a stronger gravitational field 5. Finally, neutron stars have higher temperatures at birth, spin faster, and have stronger magnetic fields, among other things.

What happens when a low-mass star completely exhausts it's hydrogen supply.

When the star's core hydrogen depletes, nuclear fusion will cease. With no fusion to replace the energy, the core will no longer be able to resists the inward pull of gravity and it will begin to shrink. The hydrogen shell will become hot enough for hydrogen shell fusion, causing a build up of thermal pressure, pushing surface outward until luminosity rises and becomes huge red giant. Helium fusion will than begin.

Explain what happens to the interior of a low-mass star as it runs out of hydrogen in its core.

As numbers of particle drops, the core must shrink and heat up in order to keep pressure in balance with gravity. The slight but continual rise in core temp slowly raises the fusion rate and the luminosity.

Hydrogen shell burning

Because gravity shrinks both the inert helium core and the surrounding shell of hydrogen, the hydrogen shell becomes hot enough for hydrogen shell fusion. The shell becomes so hot that the fusion will proceed at a higher rate than core fusion. Increase in energy output will cause buildup of thermal pressure inside the sun which will push its surface outward until the luminosity rises to match the elevated fusion rate. That is why the sun will become a huge red giant.

Why are even-numbered elements more common than odd-numbered elements in the universe?

Because helium capture reactions add two protons and two neutrons at a time, we expect nuclei with even numbers of protons to outnumber those with odd numbers of protons.

Helium Flash

The rising temperature causes the helium fusion to spike drastically in a helium flash. It releases an enormous amount of energy into the core and thermal pressure again becomes dominant and degeneracy pressure is no longer important.

Why is the evolution of a high-mass star different from the evolution of a medium or low-mass star? At what point do the two evolutionary sequences diverge?

They diverge when they both become red giants/supergiants and begin helium core fusion. However, high mass stars become supergiants where as low-mass stars only become red giants. They differ in the fact that high-mass stars undergo multiple shell fusion whereas low mass undergo double shell fusion.

Planetary Nebula

Through winds and other processes, the sun will eject its outer layers into space, creating a huge shell of gas expanding way from the inert carbon core. The exposed core will be very hot and still emit intense UV radiation, the radiation will ionize the gas in an expanding shell as a planetary nebula.