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Astronomy 1143 – Spring 2014
Lecture 17:Matter! It matters!
Key Ideas:Temperature of matter is important!• Measures energy of particles
“Normal Matter” in the Universe• hydrogen and helium in atomic/ionized form
Make-up of ordinary matter: • electrons, protons, and neutrons• Stable particles that exist at low temperatures
Atoms • Elements• Isotopes
Key Ideas:Radioactive particles are unstable – decay to other particles•Showed that the atom was divisible
Experiments have shown the existence of other types of particles•Anti-matter
• Same mass, opposite charge• Annihilates when meets its matter counterpart
•Fundamental particles – quarks, leptons, neutrinos, force-carrying bosons•At high temperatures, many more massive fundamental particles present
State of Matter Depends on Conditions
HOT COLD
High Temperatures –•Fast moving particles•Emission of high-energy radiation
Temperature
Temperature is a measurement of the internal energy content of an object.
Solids:• Higher temperature means higher average
vibrational energy per atom or molecule.
Gases:• Higher temperature means more average kinetic
energy (faster speeds) per atom or molecule.
Kelvin Temperature Scale
An absolute temperature system:• Developed by Lord Kelvin (19th century)• Uses the Celsius temperature scale
Absolute Kelvin Scale (K):• 0 K = Absolute Zero (all motion stops)• 273 K = pure water freezes (0º Celsius)• 373 K = pure water boils (100º C)
Advantage: • The total internal energy is directly proportional
to the temperature in Kelvins.
Hot GasFaster Average Speeds
Cool GasSlow Average Speeds
Effects of High TemperatureParticles are moving very fast, so they have high
energy collisions
Energetic photons or high-energy collisions between particles break bonds
Matter becomes dominated by fundamental particles
Energetic photons can create matter-antimatter pairs, so long as energy is greater than the rest mass energy of particle
Einstein’s Famous FormulaEinstein famously unified matter and energy
Particles with lots of mass can be converted into lots of energy – pair annihilation
Energy can turn into mass – photons turning into electron-positron pairs
Most famous example: Hydrogen fusing to He• Mass 1 He nucleus=6.664x10-27kg • Mass of 4 H nuclei =6.690x10-27 kg
4.6x10-29 kg turned into energy
Where do we have high energies?•Supernova•Disks around Black Holes•Early Universe
Experimental Results(Many of) the crazy ideas of particle physics are
accepted because they have been verified by experiment
Experimental techniques• Particle accelerators• Bubble chambers• Energy detectors – large vats of xenon, water, etc,
providing lots of targets• Observations of early Universe
Bubble ChambersParticle beam is sent through a chamber filled with
superheated fluid. Chamber also has magnetic field running through it.
Charged beam particles pass through the liquid • Deposit energy by ionizing atoms• Energy causes liquid to boil along their paths.
Beam particles may also collide with atomic nuclei • Produce new particles• New particles also deposit energy, creating more bubble
Flash photographs taken from several angles
Bubble Chambers
AtomsOrdinary matter is found primarily in the form
of atoms.To make people, rocks (and rocky planets),
plants, animals, etc, nature forms complex structures with atoms called molecules
Molecules are found in interstellar space and in the “cool” atmospheres of stars, but most of the current Universe is in atoms (or ionized atoms)
The Divisible Atom
Ironically, the word “atom” is derived from the Greek word atomos, meaning "indivisible”
Atoms are indivisible chemically, unlike molecules
But radioactivity revealed that the atom is actually divisible and added new particles to the particle zoo
Atomic StructureNucleus of heavy subatomic particles:
• proton: positively charged• neutron: uncharged (neutral)
Electrons orbiting the nucleus:• negatively charged particles• 1/1836th the mass of a proton
Atoms are mostly empty space:• Only 1 part in 1015 of space is occupied• The rest of the volume is threaded by
electromagnetic fields
1H
+
4He
+ 00 +
Elements
Distinguish atoms into Elements by the number of protons in the nucleus.
Atomic Number:1 proton = Hydrogen
2 protons = Helium
3 protons = Lithium ... and so on
Number of electrons = Number of protons
All elements are Chemically Distinct
Top Ten Most Abundant Elements in the Universe10) Sulfur
9) Magnesium
8) Iron
7) Silicon
6) Nitrogen
5) Neon
4) Carbon
3) Oxygen
2) Helium
1) Hydrogen
Known Elements
118 elements are currently known:
• 87 are metals
• 11 are gasses
• 2 occur as liquids (Bromine & Mercury)
• 26 are natural radioactive elements
• 25 are made only in particle accelerators
In addition, each element can have a number of different isotopes.
Isotopes
A given element can have many Isotopes• Same number of protons.• Different number of neutrons.
Example:12C has 6 protons and 6 neutrons13C has 6 protons and 7 neutrons14C has 6 protons and 8 neutrons
Chemically identical, but different masses.
Hydrogen1 proton
Helium2 protons
Lithium3 protons
Proton: Neutron:
1H
3He
2H 3H
4He
6Li 7Li
Deuterium Tritium
Ionization
Electrons absorb enough energy to escape the atom completely
This ionizes the atom.
Example: To ionize from the ground state of hydrogen requires 13.6 eV of energy. This is a photon of 91.2 nm (ultraviolet)
In the center of the Sun, over 99.99 % of the material is ionized – sea of nuclei and free electrons
RecombinationFree electrons (electrons not bound to an atom)
can be captured by an atom, particularly an atom that has been ionized.
This process is called recombination.
Energy is released when the electron recombines
Free electrons can interact with any wavelength of light – not confined to certain quantized levels
Much tougher for a photon to make it through free electrons than atoms
Radioactivity
If a nucleus has too many or too few neutrons, it is unstable to radioactive decay
Examples:3H (1p+2n) 3He (2p+1n) + e + e
14C (6p+8n) 14N (7p+7n) + e + e
(basis of radioactive carbon dating)
Free neutrons are also unstable:
n p + e + e
Who ordered this?
Discovery of NeutrinoProposed in 1930 by Wolfgang Pauli to explain
what happened to the energy in radioactive decay that seemed to “disappear”
Detected in 1956. Found to be extremely weakly interacting, as expected
300 billion neutrinos are passing through my hand per second, mostly thanks to the Sun
A new particle was added to our understanding of the Universe
Neutrino Interaction in Bubble Chamber
Fundamental Particles
Stable and Unstable ParticlesHeavier fundamental particles decay into lighter fundamental particles
So ordinary matter is composed of the “Generation One” particles
Heavier particles continually being created by energetic events
Matter & AntimatterEach particle has an anti-particle
Same mass, but opposite charge (if possible)
If a particle and anti-particle meet, they annihilate each other, leaving behind a burst of energy
It is possible, but unlikely, that there are pockets of anti-matter in the Universe
Why the Universe has a surplus of matter is something that needs to be explained.
Anti-Particles
Quarks
Elementary particles that form hadrons
Free quarks do not exist in normal conditions
Held together by the strong force
The Particle ZooAtoms – not indivisible!
protons, neutrons, electrons
quarks
Hadrons •Baryons (3 quarks)•Mesons (2 quarks)
Particle Physics View of WorldThe modern view of forces is that they are carried by particles
Choice in the way you think about things – what matters is what you can predict
Very successful at predicting the interaction between particles and light
The Four Fundamental Forces
History of the UniverseThe Big Bang theory states that the observable
Universe started out in an extremely hot and dense state
Expanded and cooled since then
Early in the Universe, temperatures are so high that only fundamental particles can exist – e.g., quarks, leptons and bosons
High enough energy that many massive particles being formed
Nature of dominant form of matter has changed over the history of the Universe