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Lecture

PHYS 183 Lecture Notes - Planetary Nebula, Energy Star, Helix Nebula


Department
Physics
Course Code
PHYS 183
Professor
Tracy Webb

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PHYS 183 The Milky Way Inside and Out Tracy Webb Winter 2013
Lecture 23: March 11th, 2013
review
o the mass of a star determines everything about it
o stars of higher mass have higher core temperature & pressure higher rates of fusion
they are more luminous & short-lived
o stars of lower mass have lower temperature & pressure lower rates of fusion
they are less luminous & long-lived
o high mass stars are in the top left of the HR diagram; low mass stars are in the bottom right
of the HR diagram
o the sun is expected to live for 10 billion years has already been through 5 billion ears
o magnitude is another way of measuring luminosity
o stars go from main sequence giants super giants white dwarfs
main sequence stars burn H slow & steady
there is H everywhere in the star but in the core it is hot enough to fuse
once all the H in the core is burned, you have inert He in the core no longer has pressure to
support against gravity & star begins to collapse
then, a H shell forms around the He core and it begins to burn but it starts to become too efficient
& pressure wins so outer layers of star expands so it gets bigger
the star appears red on the outside because it is colder since it has released a lot of energy
star moves off the main sequence & becomes larger/redder
we have 5 million years until we will want to move off of earth
He fusion to C (+ O)
o He fusion doesn’t begin right away because it requires even higher temperatures than H
o positive charge of nucleus is higher & so repulsive force is greater
o need more & more energy as you go higher in elements so overcome this force
o 3 He C + energy
o once the core gets hot enough to fuse He, fusion takes off again in a He flash & the core
becomes stable
o the star settles down to the horizontal branch, supported by He burning in the core
fusing elements releases gamma photons
once He runs out
o star collapses again
o now have He & H burning shells surrounding inert C & O core
o causes the blow-off of outer layers planetary nebula
in the centre of the planetary nebula is the leftover star; surrounding it is bubbles of gas from the
outer layers different colours/emission lines since different elements/temperatures
the helix nebula is a sun-like star there are streams of material being pushed outwards
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