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Lecture 24:
Life as a High-Mass Star
Review from Last Time:
life for low-mass stars
molecular cloud to proto-star
main sequence star (core Hydrogen
burning)
core Hydrogen exhausted (sub-giant)
shell Hydrogen burning (red giant)
core Helium burning (Helium Flash)
shell Helium burning (double-shell burning
red giant)
planetary nebula
white dwarf
Life as a High Mass Star
basic story from proto-star to main
sequence is the same as for low-mass stars
– everything just happens faster.
the high temperature inside a high mass
star make additional kinds of fusion
reactions possible
for example, the CNO cycle is a different
way that Hydrogen can be fused into
Helium – it is much faster than the protonproton chain.
Quick review of the p-p chain…
The CNO cycle
remember low mass stars can never get hot
enough to fuse Carbon…
High mass stars can continue the fusion
process: Carbon Neon, Neon Oxygen,
OxygenSilicon, Silicon Iron
Helium flash
High Mass stars:
No Helium Flash
Mass-loss from a supergiant star
Mass per nucleon
fusion of elements lighter than Iron
releases energy because the mass of
the product is less than the mass of
the components
another way of saying this is that the
mass per nucleon is smaller for
Helium than for Hydrogen, smaller
for Carbon than for Helium, etc…
Iron: the end of the road
The death of a high mass star
for a while, the star
is supported by
degeneracy pressure
in the Iron core
when the
temperature gets
high enough, the
electrons combine
with the protons to
form neutrons and
neutrinos…
Supernova!
the collapse releases a tremendous amount
of energy, about 1046 J!
Most of this energy is carried out of the
star by neutrinos.
the neutrinos produce a shock wave that
blows the outer envelope of the star away
at huge speeds.
the star becomes as bright as 10 billion
Suns, as bright as a whole galaxy!
Supernova Remnants
if the degeneracy pressure of the
neutrons is enough to balance the
force of gravity, the core of the star
is left behind as a giant ball of
neutrons called a neutron star
otherwise, the core of the star
collapses into a black hole…
Star Clusters
Many stars are born in clusters
Open clusters are loose, irregular
groups of young stars, found mainly in
the disk of the Galaxy.
Globular clusters are round, regular
balls of old stars, found mainly in the
halo of our Galaxy.
Star Clusters
Open Cluster
Globular Cluster
Star Clusters are useful
laboratories for studying stars:
All the stars in the cluster are at
about the same distance from us
All the stars in the cluster formed at
about the same time (so they are
about the same age)
the H-R diagram of a cluster
represents stars at all stages of
their evolution
Pleiades
Luminosity
main sequence
turnoff
Temperature
What is the
age of this
cluster?
Palomar 3
The Age of the Universe
Stars in the oldest clusters have ages
of 10-15 billion years
From the expansion rate of the
universe, we can estimate the time
since the Big Bang. Current values are
around 13 billion years.
Are there stars older than the
Universe???