A-toms = Not Cutable Democritus 420BC popularized the theory matter was made of Atoms: Too small to be seen Indivisible Surrounded by a void Solid No internal structure
Pictures of Atoms 48 iron atoms on copper Made with a scanning tunnelling microscope
Relative Radii of Atoms Atomic radii are all about 0.1-0.2 nanometer Smaller than the wavelength of visible light
What s an Atom?- Nucleus&Electron(s) Rutherford (1911) fired helium nuclei at gold foil and a very few bounced straight back Nucleus is 100,000 times smaller than the atom Nucleus of atom is like pinhead in a Stadium
Periodic Table Chemical properties determined by number of electrons Which match number of protons Increasing by 1 proton/electron for each element
Neutrons & Isotopes
Radioactive Decay of Carbon 14 Isotopes have same chemical properties In 10 13 carbon atoms 1 Carbon 14 = 14 C Half the Carbon 14 will decay in 5700 years
Clocks in Rocks Clocks start at time of solidifying and reset by melting 238 U decays to 234 Th 206 Pb with half-life of 4.5billion years We have rock samples of the Earth, Moon, Mars, meteorites All date to a maximum of ~4.6 Billion years Time of formation of Solar System
Time to Solar System Formation Meteorites with chondrules (spherical) formed before Earth White Calcium Aluminum inclusions were formed 4.57 billion years Short lived radioactive elements indicate that ~million years elapsed from supernova/solar system formation and rock crystallization
Temperature Temperature is a measure of the average kinetic energy of the atoms in gas, liquid or solid = speed of atoms Animation is of Helium atoms at 1950 atmospheres; speed slowed down two trillion fold with some colored red to make them easy to see
Heat or Thermal Energy The hotter it is; the faster its atoms move The amount of heat or thermal energy depends on temperature AND mass/number of atoms The atoms cease moving at absolute zero = zero Kelvin (record=0.45nk) Bowtie nebula -272C=1K
Pressure Gas is made of tiny atoms and molecules which are in constant motion The higher the temperature the faster the particles move and The more often the gas atoms hit the wall the larger the pressure Bubble nebula star wind exerts pressure to expand nebula Cool Hot
Brownian Motion Shows Atoms Exist Microscopic view of milk fat droplets moving randomly due to impacts from molecules of water higher temperature faster motion Einstein published mathematical explanation in 1905
Primary Atmosphere First atmosphere of Hydrogen and Helium Is lost to space because the temperature / velocity is above the escape velocity
Blackbody Radiation A blackbody is a perfect emitter/absorber = stars, light bulbs When charged particles are accelerated they emit photons
Blackbody Curves As the temperature goes up there are more collisions and they are more violent more photons and more energetic photons Higher temperature more light Higher temperature shorter wavelength of maximum intensity Higher temperature bluer color
Wien s Law Wavelength of maximum intensity depends on temperature = 3,000,000/Temp in nanometers and degrees Kelvin
E= T 4 Stefan-Boltzmann Law The amount of energy radiated is proportional to the temperature to the fourth power Twice the temperature give 2X2X2x2=16 times the energy
Planck Curve After attempts by Wien, Stephan-Boltzmann, Rayleigh Max Planck finds equation (1900) which matches - BUT energy must be quantized => Quantum Mechanics
Stellar Energy Distributions Star temperatures vary from 100,000K to 1500K Maximum of the curve tells us star s temperature Albireo composed of hot blue star & cool red one
Star s With Dust Disks Star radiates light like a hot Blackbody Some starlight warms the disk Disk radiates light (Infrared) like a cooler Blackbody
Planet Impact Star HD172555 has a very bright disk Disk has spectral signatures of vaporized rock, lava, gravel and dust Observations consistent with planetary impact
Electrons JJ Thompson discovers the electron (1897) How does a CRT work?
Atom Number of Protons determines the Element Isotope determined by number of neutrons Number of electrons determined by protons in nucleus Chemistry determined by electrons
Bohr Atom Tiny positive nucleus contains most of mass Orbited by negatively charged electron(s) Held in orbit by Coulomb Force
Electron in Ground & Excited State Because electron orbits; it accelerates; it should radiate photons But only certain (permitted) orbits/energies allowed (just like stairs, piano keys, bookshelf) Electron can be in lowest energy (=ground state) can raised to higher energy (=excited state) by collision or absorption of a photon
Electron Transitions The electron jumps to a higher energy level when a photon is absorbed = absorption Excited state The electron jumps to a lower energy when it emits a photon = emission Ground state
Electron Cloud Because an electron is a wave as well as a particle we can not say exactly where it is due to its quantum mechanical nature Heisenberg s uncertainty principle you can t know everything about anything
Transitions of Hydrogen Difference in energy levels determines photon energy Photon energy depends on wavelength equals photon s color Balmer lines in visible Lyman lines in UV Paschen in infrared
Hydrogen Emission Lines Difference in energy level determines photon energy Visible lines are Balmer series; Ultraviolet are Lyman and infrared are Paschen series
Ionizing a Hydrogen Atom -If an electron gains more than the binding energy then it will escape from the atom -The atom is then ionized - The atom is an ion - Denoted by H + - Also called an ion if it gains an electron H -
Photon Absorption and Emission Electron can usually only stay in excited state for a nanosecond Electron transitions can take place due to collisions
Electron Shells More protons requires more electrons More populated energy levels More possible transitions More complicated spectra
Molecular Spectra Carbon Monoxide has A) Electronic transitions B) Rotational transitions C) Vibrational transitions Spectrum of Hydrogen molecule in (a) Spectrum of Hydrogen atoms in (b)
Types of Spectra We usually look at graph of Intensity versus Wavelength No lines = Continuous Absorption lines = Dark lines Emission lines = Bright Lines
Kirchoff s Spectrum Laws Continuous - a solid liquid or dense gas will radiate at all wavelengths Emission - a low density gas will emit light at specific wavelengths Absorption - results from a continuous spectrum passing through a low density gas
Solar Spectrum Fraunhofer discovered lines in solar spectrum 1817 D=Sodium, C&F&h=Hydrogen, H&K=Calcium Different elements have a different set of lines like a bar code
Photographic/Line Spectra We rarely look at bands of color Usually we graph intensity versus wavelength
Annie Jump Cannon Originated the modern stellar classification scheme in 1901 Based on strength of H lines Found to be temperature sequence Classified 400,000 stars for the Henry Draper Catalogs
Balmer Thermometer Balmer lines originate at the n=2 energy level If it is too cool then all atoms in n=1 level If it is too hot then all hydrogen atoms are ionized Use Calcium, Helium and molecules as well
Spectral Classes / Spectral Sequence Oh Be A Fine Girl/Guy Kiss Me Spectral Types have subdivisions of 0-9 Pick a star to be representative/standard star
Cecilia Payne-Gaposchkin Found the abundances of the different elements of the stars First PhD in astronomy from Harvard/Radcliffe 1925 First woman full professor at Harvard and chair in 1956
Composition of Stars Line strength and line profile depend on abundance of element And temperature (which energy levels are populated) Sun is composed of: Element Mass Hydrogen 71% Helium 27% All the Rest 2% Most other stars SAME as sun!
Waves Wavelength is distance between crests Frequency f is number of waves per second which pass a point
Stationary Source Speed of waves equals the wavelength times the frequency f C= f
Moving Source Wavelengths in direction of motion are compressed Wavelengths when source is moving away from observer are stretched Speed of source determines how much stretching
Car Horn
Doppler Effect V r / C = ( - o ) / o = / o If the source is receding (moving away) then it is a redshift If the source is approaching then the light is blueshifted
Relative Motion Stationary observer sees wave with emitted wavelength Moving observer sees wavelength decreased due to his relative motion
Transverse/Radial Velocity We can measure the radial velocity of objects on the other side of the universe but We can measure the transverse velocity of only nearby stars Thus we can only measure the true velocity of the nearby stars
Discovering Extrasolar Planets Spectra from largest telescopes have enough precision to see radial velocity variations of star moving around center of mass First was 51 Peg in 1995 Big close planets easiest Called Hot Jupiters
Spectral Sequence = Temperature Sequence
Spectrum of a Nebulae
Solar Spectrum Millions of absorption lines of different elements and ions Width of lines depends on temperature and abundance
Kirchhoff s Laws=Kinds of Spectra
Hydrogen Energy Level Diagram Lyman Series Balmer Series Paschen Series Ion, Ionized, Ionization and Binding energy Recombination
Blackbody Radiation max T = 3,000,000 K nm color depends on temperature= Wein s Law E = T 4 brightness depends on temperature =Stefan-Boltzmann law
(B-V) Color Index Counting the number of photons which pass through a filter and comparing to another filter will tell us the temperature of a star A hot blue star has a (B-V) = -0.1 and (B-V)=1.5 for a cool red star
Carbon Atom in Ground State Six electrons populating many energy levels gives much more complex spectrum