Transcript Elements and Isotopes - University of California, Berkeley

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.
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.
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!
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.
Inside the Sun
The temperature and pressure
in the core are extreme
enough for fusion (and the
Sun’s 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.
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)
What Happens when Hydrogen runs out?
A Red Giant
Something beautiful…
This looked like round blurs to early astronomers,
who called them “planetary nebulae” (inappropriately)
Final Throes : Planetary Nebulae
Space art…
The Lifecycle of the Sun
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.
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.
Planets, Brown Dwarfs, Stars
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.
Life of High Mass Star
10 million years
Stellar Lifetimes
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.
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.
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.
Cluster HR Diagrams
Good tests of stellar evolution theory.