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Astronomy 10 Week 9 Summary
Chapters 16 and 17
1. Clusters: There are two types of star clusters. Open Clusters have 100s to 1,000s of stars,
are found in the plane of the galaxy, and contain many blue stars, hence they’re very young.
Globbular clusters have 10,000s to 1,000,000s of stars, are found orbiting around the galaxy
out of the plane, and are full of old, red stars. Ages are found from the main sequence
turnoff since we can assume all the stars in a single cluster are the same age and at the
same distance from us.
2. Stellar Census: Not only do most stars lie on the Main Sequence, but they are also mostly
cooler, smaller, red stars. Hot stars are easiest to see since they are brightest, but they
are far less common in general. On the main sequence, we also find a relation between the
intrinsic luminosity and the mass: L ∝ M 4 , and the main sequence lifetime goes as t ∝ M −3 .
Knowing the color of a main sequence star tells you the temperature, and uminosity, which
in turn tells you the radius of the star and how far away it is.
3. Stellar Birth: Stars form out of giant gas clouds in the galaxy. They first collapse into
a protostar until their core become hot enough to begin nuclear fusion. Once they start
stablely fusing hydrogen in their core, they become a normal, main sequence star. Note that
stars with a mass less than 0.08 times that of the sun never become hot enough for fusion,
and hence are failed stars, also called Brown Dwarves. Since there is no fusion to create
a pressure to counteract the gravity, they are held up by electron degeneracy pressure (see
below).
4. Nuclear Fusion Processes: The main fusion reaction that goes on in low-mass stars like the
sun is the proton-proton chain, which fuses 4 hydrogens in to a helium. Higher-mass stars
are mainly fueled by the CNO-cycle, which also net fuses 4 hydrogens into a helium, but
uses carbon, nitrogen, and oxygen along the way as catlysts. In the later stages of life after
moving off the main sequence, red giant stars also fuse helium into carbon through the triple
alpha process, creating carbon and oxygen. High-mass stars, as they die, can fuse beyond
just those up through neon, magnesium, silicon, and iron.
5. Main Sequence Life: The VAST majority of a star’s life is spent sitting in one single spot on
the main sequence. Stars like the sun have a radiative interrior, with a convective zone near
the outside. For very low-mass stars, this convective zone extends all the way down through
the entire star. High-mass stars, on the other hand, have convective cores and radiative
exteriors. Stars do not evolve down the main sequence. They are born with a mass that
determines a single spot on the main sequence where it will live it’s life. Similarly, it’s mass
also determines it’s post-main sequence life. Once you know the mass, you know everything.
6. Post-Main Sequence Evolution: After a star runs out of hydrogen to fuse in its core, fusion
stops. The loss of energy production means that gravity can cause the core to begin to
collapse. As it does, it radiates away gravitational potential energy, which pushes the outer
layers of the star out, making them expand and cool.
(a) Sub-Giant Phase: Eventually hydrogen begins to fuse in a shell around the inert helium
core. The outer layers cool slightly, but expand much, making the total luminoisity
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rise drastically and become slightly redder. The helium core is held up by degeneracy
pressure (see below) as more is added to it by the fusing hydrogen.
(b) Helium-Flash: Eventually the core hits the point where heium can begin to fuse into
carbon. This happens very rapidly, and makes the star suddenly move back towards
the main sequence (though it never gets there). Very low-mass stars will only form
carbon here, while stars like the sun and higher mass will also form oxygen.
(c) Red Giant Phase: Once the helium is used up, the core again collapses and the outer
layers again expand. Now there is an inert carbon (and maybe also oxygen) core, with
shells around it fusing helium and hydrogen. The surface temperature cools to be a red
star, while the radius increases drastically, again causing a large increase in luminosity
and becoming a red giant. Intense stellar winds in this phase can cause a star to loose
20
(d) Low-Mass Star Death: Stars smaller than about 3 solar masses will end here. Their
cores will shrink down until they are held up by electron degeneracy pressure (see
below), while their upper layers will gently puff away. The large nebulae formed by
the escaping gas is called a planetary nebula, though it has nothing to do with planets.
The exposed core is very hot, but very small, a white dwarf. They slowly cool off,
emitting blackbody radiation, but generating no new energy. Since they are held up by
degeneracy pressure, there is a maximum mass beyond which gravity would overcome
this pressure. Called the Chandrasekhar Limit, no white dwarf can be larger than 1.4
times the mass of the sun.
(e) High-Mass Star Death: Massive stars will be able to fuse beyond carbon and oxygen
all the way up to iron. The different phases going from fusing one thing to the next all
happen very rapidly. Unlike low-mass stars, high-mass stars stay at roughly the same
total luminosity through their post main-sequence life, but their temperature changes
much more drastically as they expand and contract. Once the core becomes completely
iron, no more fusion can occur, and gravity causes the star to collapse onto this very
dense core. Stay-tuned next week to find out what happens after that.
7. Electron Degeneracy Pressure: Electrons obey the Pauli Exclusion Principle, which says
that no two of them can ever be in the same state at the same time. This means that
they are anti-social, and don’t like getting close to each other. Even with no other energy
around, they will create a pressure in order to try and keep away from each other. This is
what holds up brown dwarves, the inert helium cores of sub-giant stars, and white dwarves.
If gravity is strong enough, however, it can win out and overcome this pressure. Stay tuned
next week for what happens when it does.
8. Novae: If a white dwarf is in a binary system, and is close enough to its companion star,
then as the companion becomes a red giant and expands, the white dwarf’s gravity can
actually pull some of the mass off of the other star onto it. This hydrogen builds up on the
surface of the white dwarf and becomes very hot. Eventually, this thin shell of hydrogen
can very quickly fuse into helium in a flash that only lasts a week or so. In that time, the
white dwarf suddenly becomes 10,000 times brighter. This is a nova. Stay tuned next week
for what supernovae are.