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