UW Astronomy Chp. 17 - Star Stuff

White dwarf as a companion

A WD can steal mass from a red giant and become 1A Supernova

What are the life stages of a high-mass star?

A high-mass star lives a much shorter life than a low-mass star, fusing hydrogen into helium via the CNO cycle. After exhausting its core hydrogen, a high-mass star begins hydrogen shell burning and then goes through a series of stages, burning successively heavier elements. The furious rate of fusion makes the star swell in size to become a supergiant.

How does a star's mass determine its life story?

A star's mass determines how it lives its life. Low-mass stars never get hot enough to fuse carbon into heavier elements in their cores, and they end their lives by expelling their outer layers and leaving white dwarfs behind. High-mass stars live short but brilliant lives, ultimately dying in supernova explosions.

Main form of pressure in inert helium core?

Degeneracy pressure

CNO cycle

Faster chain of hydrogen fusion reactions

How do high-mass stars make the elements necessary for life?

In its final stages of life, a high-mass star's core becomes hot enough to fuse carbon and other heavy elements. The variety of different fusion reactions produces a wide range of elements - including all the elements necessary for life - that are then released into space when the star dies.

Hydrogen shell burning

Inert helium core and the surronding shell of hydrogen, hydrogen fusion in a shell around the core

What does mass determine?

Pressure and temp

Helium capture

Process of getting 2 protons

Life of a high-mass star:

Protostar, blue main-sequence, red supergiant, helium-burning supergiant, multiple shell-burning supergiant, supernova, neutron star or black hole

Life of a low-mass star:

Protostar, yellow main-sequence, red giant, helium-burning star, double shell-burning red giant, planetary nebula, white dwarf

Planetary nebula

Radiation ionizes the gas in the expanding shell, making it glow brightly as a planetary nebula

Fusion rate of high mass star?

Rapid fusion

Helium-capture reactions

Reactions in which a helium nucleus fuses with some other nucleus, helium capture reactions can change carbon into oxygen, oxygen into neon, etc.

How does a low-mass star die?

The core again shrinks after core helium burning ceases. Helium shell burning begins around the inert carbon core beneath the hydrogen-burning shell. The outer layers expand again, making the star into a double shell-burning red giant. The star's energy generation never reaches equilibrium during this time; instead, the star experiences a series of thermal pulses and ultimately expels its outer layers into space as a planetary nebula. The remaining "dead" stellar core is a white dwarf.

Thermal pulses

The fusion rate spikes upward every thousand years

Helium flash

The rising temperature causes the helium fusion rate to rocket upward in what is called helium flash

Helium fusion

When atoms slams together much higher speed than those needed for hydrogen fusion, at a higher temperature

How are the lives of stars with close companions different?

When one star in a close binary system begins to swell in size at the end of its hydrogen-burning life, it can begin to transfer mass to its companion. This mass exchange can change the remaining life histories of both stars.

Type 1A

White dwarf

Type 2

Young hot stars (red giants)

How does a high-mass star die?

A high-mass star dies in the cataclysmic explosion of a supernova, scattering newly produced elements into space and leaving a neutron star or a black hole behind. The supernova occurs after fusion begins to pile up iron in the high-mass star's core. Because iron fusion cannot release energy, the core cannot hold off the crush of gravity for long. In the instant that gravity overcomes degeneracy pressure, the core collapses and the star explodes. The expelled gas may be visible for a few thousand years as a supernova remnant.

What are the life stages of a low-mass star?

A low-mass star spends most of its life generating energy by fusing hydrogen in its core via the proton-proton chain. When core hydrogen is exhausted, the core begins to shrink while the star as a whole expands to become a red giant, with hydrogen shell burning around an inert helium core. When the core becomes hot enough, a helium flash initiates helium fusion in the core, which fuses helium fusion in the core, which fuses helium into carbon. This phase lasts until core helium is exhausted. Low-mass stars never become hot enough for carbon fusion, so at this point their lives must come to an end.

What does a Type II Supernova create

Neutron star or black hole, all elements above Iron

How does a star's mass affect nuclear fusion?

Stars of greater mass have hotter core temperatures, causing fusion to proceed more quickly and enabling fusion of heavier elements to take place. A star's mass at birth therefore determines almost every aspect of its life and death. To understand the general characteristics of stellar lives, we divide stars into three groups by mass: low-mass (less than 2M Sun), intermediate-mass star (2M Sun and 8M Sun), and high-mass stars (higher than 8M Sun).