Download Rainbows and Atoms

Visible Spectrum
•  To understand Stars & Planets we need spectra
•  Prism splits Visible light into Visible Spectrum
•  Newton discovered that white light is made up of
many colors in 1666
•  Newton thought light was corpuscles/particles
•  Young etc. showed light is a continuous wave
Electric Field
•  Electrons & Protons are the
negative & positively charged
elementary particles
•  They are surrounded by an
electric field
•  Which changes as they move
Electromagnetism
•  Changing electric fields (currents) make magnetic fields
•  Changing magnetic fields make electric currents
•  Changes in one induce changes in the other Maxwell 1860
Waves Have a Frequency f, Period P,
Wavelength λ (lambda) & Amplitude
•  Frequency f =ν=nu
counts waves per sec.
•  Frequency = 3 cycles/
sec or 3 Hertz
•  Period = 1/3sec = 1/f
•  Period of 1 second
•  Frequency of 1 cycle/
sec= 1 Hertz
•  Has three times
wavelength λ
•  Both have same height
or Amplitude
All Electromagnetic Waves
Move at the Speed of Light c
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λ= The distance between waves
multiplied by
frequency f = the number of waves per second
Equals the speed of the waves C=λ f Speed of light =300,000km/sec
If ‘The Q’ has frequency of 100Megahertz, what it’s wavelength λ?
Doppler Effect 1842
•  Stationary Source
•  Speed of waves equals
the wavelength λ
times the frequency f
•  C=λf
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
•  Vr / C = (λ - λo ) / λo = Δ λ/λo
•  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 II sees wavelength decreased due to
his relative motion
Transverse & Radial Velocities
•  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 (true space motion)
of the nearby stars
Electromagnetic Spectrum
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Radio
Waves
Hertz discovered radio waves in 1888
Marconi and Tesla made first radio
First radio telescope built by Karl Jansky 1932
Cassini radar of Titan
Microwaves
•  WIFI & Microwave ovens use
wavelengths of ~10cm - 2.4GHz
•  15meter JCM Telescope studies
interstellar dust gas, solar system
& distant galaxies
•  BICEP2 may tell us about
inflation of early universe or
maybe not??
Infrared Region
•  Bag is transparent to IR light
(=heat) - Glasses are not
Atmospheric Windows
•  Opacity is how much light is blocked
•  Atmosphere opaque except: Visible Light & Radio
Ultraviolet Region
•  Need to go into space
•  MAVEN pictures of Mars in UV
•  SOHO studies the sun
X-ray Region
•  First discovered by Roentgen in 1895
•  Wife’s hand with wedding ring
•  WASP-18 not emitting the expected X-rays implying
planet 10Jupiters and period 23hours suppresses sunspots
Gamma γ-Rays
•  Produced by very hot gas falling
onto a neutron stars & black holes
•  Produced by radioactive elements
Cosmic Rays
•  Very high energy subatomic particles
•  Raining down on us from all directions –
no big worry
•  Unknown production mechanism
•  AMS-02 on ISS 2010, excess positrons
but no Dark Matter yet
The regions of the electromagnetic
spectrum in order of increasing
frequency are:
a)  Radio, microwave, infrared, visible, UV, Xray, γray
b)  γray, Xray, UV, visible, infrared, microwave, radio
c)  Radio, infrared, visible, Xray, UV
d)  γray, infrared, microwave, radio, UV, visible, Xray
e)  Xray, UV, radio, microwave
A-toms = Not Cutable
•  Continuous or Quantized?
•  To understand light we need Atoms
•  Democritus 420BC popularized the
theory matter was made of Atoms:
•  Too small to be seen
•  Indivisible - Solid
•  Surrounded by a void
Pictures of Atoms
•  48 iron atoms on copper
•  Made with a scanning tunnelling
microscope
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 20C & high pressure.
•  Speed of 1500 km/hr = 0.5km/sec slowed down a trillion
fold with some colored red to make them easy to see
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
Heat = 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
Blackbody/Thermal Radiation
•  When charged particles are accelerated they emit photons
•  A blackbody is opaque = stars, incandescent light bulbs
Blackbody
Curve
•  Because the particles are going
different speeds,
the photons
have a range of energies
•  Similar to bell curve (Gaussian
or normal distribution)
Wien’s Law
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As temperature increases there’s more collisions & more violent ones
So more photons AND more energetic photons
Higher temperature, shorter wavelength of peak emission = λmax
So higher temperature = bluer color
Stefan-Boltzmann Law
•  E= σ T4 E=energy per unit area, T=temperature
•  Total amount of energy radiated per square area:
proportional to temperature to the fourth power
•  Twice the temperature give 2X2X2x2=16 times the energy
Blackbody 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
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Star temperatures vary from ~100,000K to 1500K
Maximum of the curve tells us star’s temperature
Luminosity of star depends on its temperature & size
Albireo composed of hot blue star & cool red one
Planet Impact?
•  Star HD172555 has a very bright disk
•  Disk has spectral signatures of
vaporized rock, lava, gravel & dust
•  Observations consistent with planetary
impact
A hotter opaque object:
a)  Like a hotter star will appear bluer than
cooler redder star (Wien’s Law)
b)  Is composed of faster moving atoms than a
cooler object
c)  Will look brighter at all wavelengths
according to Plank’s curve
d)  Will be called a blackbody emitting thermal
radiation
e)  All of these
What’s in 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
Electrons
•  JJ Thompson discovers the electron (1897)
•  Cathode Ray Tube = TV
Number of Protons in the Nucleus
determines the Element
•  Isotopes of an element contain the same number
Isotopes of protons but a different number of neutrons
•  Neutron: Just like proton but not charged
•  Isotopes: same chemistry BUT different mass
•  Many are radioactive – decay into something
else
Bohr
Atom
1913
•  Tiny positive nucleus
contains protons &
neutrons and most of mass
•  “Orbited” by negatively
charged electron(s)
•  Held in orbit by Coulomb
=Electromagnetic Force
Quantum Mechanics
•  Because electron orbits; accelerates; it should radiate photons
•  But energy is quantized so only certain orbitals/energies
allowed (just like stairs, piano keys, bookshelf)
•  Electron can be in the lowest energy(=ground state) or in a
higher energy level (=excited 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”
Photon Emission/Absorption
•  When a photon is absorbed,
the electron jumps to a higher
energy level = absorption
•  Excited state
•  When a photon is emitted, the
electron jumps to a lower
energy = emission
•  Ground state = lowest energy
•  Difference in energy of the two
states equals energy of photon
Transitions of Hydrogen
•  Difference in energy
levels determines
photon energy=
wavelength = color
•  More energy =
Bluer color
•  Balmer series in
visible region ;
Hα-Alpha, Hβ-Beta,
Hγ-Gamma
Absorption
Lines
Match
Emission
Lines
èèKirchhoff’s Spectra Lawsçç
•  Continuous – solid, liquid or dense gas will radiate at all wavelengths
•  Emission - a low density gas will emit light at specific wavelengths
•  Absorption spectrum - results from a continuous spectrum passing
through a low density gas resulting in dark spectral lines
Photon Absorption
and Emission
•  Usually electron can stay
in excited state for
~nanosecond
•  If electron is boosted to 2nd
excited state it may fall
first to 1st excited state and
then to ground state
•  Emitting 2 lower energy
photons
•  If electron gains enough
energy to leave atom it is
called an Ionized atom or
Ion
Each Element Has Its Own Spectrum
•  More protons = more electrons & different energy levels
•  Like a fingerprint or a bar code
Solar Spectrum
•  Fraunhofer discovered lines
in solar spectrum 1817
•  D=Sodium,
C&F&h=Hydrogen,
H&K=Calcium
Cecilia Payne-Gaposchkin
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Spectroscopy is the study of spectral lines
Found the abundances of different elements in stars
First PhD in astronomy from Harvard/Radcliffe 1925
First woman full professor at Harvard & chair in 1956
Composition of Stars
Line strength & 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%
Almost all stars SAME!
Which of the following is NOT one
of Kirchhoff’s Spectra Laws?
a.  A luminous solid, liquid or dense gas emits light
of all wavelengths producing continuous spectrum
b.  A low density hot gas emits light whose spectrum
consists of bright lines = emission spectrum
c.  A cool thin gas absorbs certain wavelengths from
a continuous spectrum leaving dark absorption
lines superimposed on the continuous spectrum
d.  All of the above are Kirchhoff’s Laws
e.  None of these are Kirchhoff’s Laws
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
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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
Electromagnetic Spectrum
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All electromagnetic radiation travels at the speed of light
Energy of a photon depends on frequency/wavelength
Light has both wave and particle properties
Electrons and protons have both wave & particle properties
Everything has wave properties so: Heisenberg uncertainty
principle “You can not know anything exactly”
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Light is a Particle: A Photon
Einstein showed light is quantized (1905).
Each quantum of light is called a photon
Planck`s constant relates the energy of a photon to frequency. E=hf
Photons are like money; Radio photons are pennies, X-rays-dollars
Spectra of
Molecules
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Carbon Monoxide has
A) Electronic transitions
B) Rotational transitions
C) Vibrational transitions
Spectrum of Hydrogen molecule in (a)
Spectrum of Hydrogen atoms in (b)
Thermal Broadening
Rotational
Broadening
Energy Level
Diagram of a
Hydrogen Atom
-If an electron gains more than the
binding energy then it will
escape from the atom
-This process is called ionization
-  The atom is a positive ion
-  Denoted by H+
-  Called a negative ion if it gains
an electron H-
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
Turbulent Line Broadening
Pressure
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Gas is made of tiny atoms & 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
Pressure/Collisional Broadening
•  Spectra of Vega & Deneb
Relative Radii of Atoms
•  Atomic radii are all about 0.1-0.2 nanometer
•  Smaller than the wavelength of visible light
Radioactive Decay of Carbon 14
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Isotopes have same chemical properties
1 Carbon 14 = 14C in 1013 carbon atoms
Half the Carbon 14 will decay in 5700 years
Your body is radioactive (thousands of
disintegrations/second)
Clocks in Rocks
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Clocks start at time of solidifying and reset by melting
238U decays to 234Th → 206Pb 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
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Atom
Number of Protons determines the Element
Neutrons hold the nucleus together
Number of electrons determined by protons in nucleus
Chemistry determined by electrons
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
Spectral Sequence = Temperature Sequence
Kirchhoff’s Laws=Kinds of Spectra
Types of Spectra
We usually look at graph of Intensity
versus Wavelength
•  No lines = Continuous
•  Absorption lines = Dark lines
•  Emission lines = Bright Lines
Hydrogen Energy Level Diagram
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Lyman Series
Balmer Series
Paschen Series
Ion, Ionized, Ionization and Binding energy
Recombination
(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
Image/Line Spectra
•  We rarely look at bands of color
•  Usually we graph intensity versus wavelength
Loss of
Atmosphere
•  First atmosphere of Hydrogen
and Helium
•  Is lost to space because the
temperature / velocity is above
the escape velocity
Neutrons & Isotopes
Waves
•  Wavelength λ is distance between
crests
•  Frequency f is number of waves
per second which pass a point