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Lecture 20

PHYS 183 Lecture Notes - Lecture 20: Triple-Alpha Process, White Dwarf, Main Sequence

Course Code
PHYS 183
Tracy Webb

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Lecture 20: February 25th, 2013
review from 1 week ago (no class last Wednesday or Friday):
o HR diagrams show every single star we are aware of in the universe & show
the temperature, colour, luminosity, radius, mass & lifetime of the stars
o there are 4 luminosity classes, supergiants, giants, main sequence & white
o the sun will live for about 10 billion years (it has already gone through 5
billion years)
o mass determines everything about a star
o more mass = hotter core = faster burning of H
there is pressure pushing outwards from the core due to energy and light, and
gravity pushing into the core of a star there is an ongoing war between pressure
and gravity
H fusion helps keep this balance, but when H runs out, there is no longer a balance
(less pressure)
when the hydrogen in the core of a star runs out, the core contracts/heats up &
outer layers puff
o the core contracts because gravity has won the war over pressure since
fusion is no longer producing pressure
moving from the main sequence:
o running out of H in the core
o new fusion reactions creates a helium burning shell
o pressure wins once again which creates red giants
o giants/supergiants
o this sequence can continue over and over
entirely runs out of fuel white dwarfs
o don’t create new energy, they get rid of the energy that has been built up
essentially: main sequence star that starts to run out of energy becomes a giant
which burns other elements until it runs out of energy becomes a white dwarf
until it dies
can use HR diagram to measure the age of a star cluster
star cluster: a bound group of stars which formed at the same time, out of the same
gas cloud
o when stars born, they are born in groups together
o globular clusters are very old & very large (~1 million stars)
can tell the age of star clusters based on their size and where they are on the HR
the main sequence turn off if where on the HR diagram the stars are all moving to
the right of the sun
we think globular clusters are some of the oldest objects in the universe
current age estimates are between 11-18 billion years old uncertainty range, not
an age range
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