Elements and Isotopes

1
Elements and Isotopes
We define an element by the number of protons
in its nucleus. There can be isotopes with
different numbers of neutrons. The number of
protons and neutrons must be similar.
2
Thermonuclear Fusion
In order to get fusion, you must overcome the
electric repulsion. You can do this by having
high density (lots of particles) and high
temperature (particles moving very quickly). But
actually, you must also have both a proton and a
neutron.
3
The Importance of Neutrons

• 1) Neutrons can't build the elements without
  them
•     the strong nuclear force holds nuclei
  together even though protons repel each other
•     it works like velcro only unlike particles
  can stick
• Result the elements have almost equal numbers
  of protons neutrons
•  
• 2) Neutrons fall apart by themselves after about
  10 minutes!
•     you can make them by squeezing electrons
  into protons
• at high density
•     they are held by the weak nuclear force,
  which makes them pretty stable in nuclei
•     sometimes they decay even in nuclei (beta
  decay)

4
The Proton-Proton Cycle
This is the primary energy source for the Sun and
most stars. The slowness of the first reaction
makes the Sun last billions of years.
5
Inside the Sun
The temperature and pressure in the core are
extreme enough for fusion (and the Suns gravity
keeps them that way). Most energy is produced in
the inner 20. Convection carries the energy in
the outer 30. Most of the mass is in the inner
50 because the density is much higher.
6
The Next Fusion Cycles
The most common elements are H (90) , He
(10), C,N,O (0.1) They are made by stars
Hydrogen burning in high mass stars
The CNO cycle
Start here
Burning Helium in Red Giants (core temperature 1
billion degrees)
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What Happens when Hydrogen runs out?
A Red Giant
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Something beautiful
This looked like round blurs to early
astronomers, who called them planetary nebulae
(inappropriately)
9
Final Throes Planetary Nebulae
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Space art
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The Lifecycle of the Sun
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Brown Dwarfs
If the mass is too low, the object is a failed
star, and can never stabilize its luminosity
with fusion. Brown dwarfs fuse at least
deuterium, but then find their pressure support
without heat, and fade slowly away.
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Astro Quiz
Which of the following is the weakest reason to
call something a planet instead of a brown dwarf?

1. The object never has any fusion in it.
2. The object is not in orbit around anything.
3. The object is smaller than Jupiter.

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Planets, Brown Dwarfs, Stars
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High Mass Stars
Massive stars have such extreme core pressures
that they can continue the process of getting
energy from fusion all the way up to iron.
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Life of High Mass Star
10 million years
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Stellar Lifetimes
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Open Star Clusters
Hundreds to thousands of stars (and brown
dwarfs). Relatively young (still containing
massive stars). Found in galactic plane. May not
even stay together if really young (might be
dispersing). Light dominated by hot stars.
19
Globular Star Clusters
Hundreds of thousands of old stars. Often not in
plane of Milky Way. Have fewer heavy elements.
Pictures dominated by red giants.
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HR Diagrams and cluster evolution
All stars are born at the same time (and
distance).
You can get the cluster age from the most massive
MS star left.
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Cluster HR Diagrams
Good tests of stellar evolution theory.