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Cliff Lau University of Toronto
AST201 Lecture 11 (February 17, 2011)
Most stars lead very quiet lives, engaged in a protracted but very uneventful
battle with gravity
Loosely, we can group stars into low-mass stars and high-mass stars; for more
purposes, the division occurs at about 8 times the mass of the Sun
Stars of different masses are broken into different layers, according to how
each layer transfers energy (either by radiation or convection)
How a star will die depends strongly on how massive it is
Death of a low-mass star pressure and gravity are battling it out in the Sun
(low-mass star); if the heat source runs out, gravity wins. As solar core fusion
progresses, more and more of the Suns core becomes helium; the Sun is not
currently hot enough to fuse helium nuclei; after 10b years on the main
sequence, most of the Suns core will be helium ash. The Suns core is not
currently hot enough to fuse helium, so once the core is mainly helium, fusion
shuts off and the core starts to cool and contract; this releases a lot of energy
formerly stored as gravitational potential energy. At this stage, hydrogen can
only burn in a shell around the core, but the rate of burning INCREASES
because of pre-heating from below; the Sun becomes a layered onion.
Hydrogen shell burning is so intense that it floods the outer layers of the
Sun with more energy than they used to receive it; they expand
dramatically. As the Sun expands, the temperature of its surface falls, and
the surface changes from yellow to red; the Sun becomes a red giant; at this
point it is dead. When it runs out of hydrogen to burn, the Sun evolves off the
main sequence, climbs the sub-giant branch, and becomes a red giant. On
its way up the sub-giant branch, the Sun is essentially reversing its way along
the path it took to the main sequence as a protostar. About 100m years after
the star leaves the main sequence, the ongoing contraction of the core reaches
a point where helium fusion can begin. Helium fusion turns on very suddenly;
this is called the helium flash. Suddenly, the Suns core is very hot again;
the core expands a bit, energy production falls, and the outer layers cool and
contract. For a brief time, the Sun lives again, fusing helium to make
carbon; it moves to the part of the H-R diagram called the horizontal
branch and stays there for a while. This is the end of the road for fusion in
the Suns core; eventually, it runs out of helium and there is only carbon ash.
The carbon core cannot produce much energy, so it cools and contracts; once it
reaches a density of about 1000kg/cm3, something really weird happens.
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