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Stellar Evolution
In post-Main-Sequence evolution, what you see
on the surface is not a good indicator of what is
happening deep in the interior
Core contraction occurs, core
smaller and hotter. Star “ascends
the Giant Branch”
Question: Why don’t
The Helium nuclei
Begin to fuse?
In low mass PMS stars, the
“Helium Flash” occurs. Nuclear
reactions involving Helium –
Carbon. Also called the “Triple
Alpha Process”
3He
C
At time of the Helium Flash, a star
evolves to the right on the HR
diagram, at near constant luminosity
Helium Flash converts Helium
core to Carbon, Oxygen
Stellar evolution
making chemical
structure of the star
more and more
complicated
As star evolves further, it
develops a Helium-burning shell
All the action is deep in the tiny core of the star
As Helium shell burning stars,
the star moves onto the
“Asymptotic Giant Branch”
The Helium shell-burning is
unstable and sends thermal pulses
through the star, throwing off the
outer layers of the star into space.
As the outer layers are peeled
back, it reveals the extremely hot,
ultraviolet-emitting carbon and
oxygen core which ionizes the
stellar wind
Hot core photoionizes the
surrounding stellar wind or ejecta.
Resulting
object is a
Planetary
Nebula,
like M57
An incredible yarn, but all the
pieces are there in the night sky
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Red Giants
Horizontal Branch Giants
Asymptotic Giant Branch Stars
Mira Variables
Planetary Nebulae
White Dwarf Stars