Lecture 12 Notes

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7 Mar 2011
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Cliff Lau University of Toronto
AST201 Lecture 12 (March 1, 2011)
Death of a high-mass star the main differences (between high and low-mass
stars) are that they die faster and they can fuse elements heavier than helium
before they run out of things to burn. Due to the higher compression its extra
gravity provides, massive stars can fuse elements all the way up to iron. They
cannot fuse iron. Once a massive stars core stops burning, its own gravity
wins out over pressure and starts crushing it. Low-mass stars stop collapsing
because their electrons refuse to get close together; massive stars get around
this by simply destroying their electrons. In seconds, most of the stars core is
turned into neutrons. Further collapse is prevented by neutron degeneracy
pressure. Meanwhile, the rest of the star is rushing inward at about 20% of
the speed of light. It suddenly hits a stationary ball of neutrons the size of
Manhattan. The inrushing material rebounds explosively and the star tears
itself apart. This is called a supernova. When supernovae occur in our part of
the Galaxy, people notice. In the past, they have been so bright that they were
said to appear in the sky like a second Sun giving people a second shadow.
When a star goes supernova, a rapid burst of a super-heated fusion allows it
to create elements heavier than iron. This is where all such elements come
from. Nearly all of the heavy elements in the Universe, including the ones
making up the Earth and your body, were created and scattered through
space by supernovae. You are literally made of atoms from the core of a dead
star.
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