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Lecture

AST201H1 Lecture Notes - Triple-Alpha Process, Helium Flash, Planetary Nebula


Department
Astronomy & Astrophysics
Course Code
AST201H1
Professor
Michael Reid

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Chapter 17 summary:
17.1 LIVES IN THE BALANCE
How does a star’s mass affect nuclear fusion:
Stars of greater mass have hotter core temperatures, causing fusion to process
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 stars, with masses less than 2Msun
Intermediate mass stars, with masses between 2Msun and 8Msun
High mass stars, with masses above 8Msun
17.2 LIFE AS A LOW MASS STAR
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, with 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 much come to an end
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
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