Transcript Astro 10-Lecture 5: Heat, Light and the Structure of Atoms

Astro 10-Lecture 5:
Heat, Light and the Structure of Atoms
How do we figure out the properties
of distant objects?
The only things that reach us from distant stars and galaxies:
•Light
•Gravity (virtually impossible to measure).
•Particles (might not get here).
Lets start with another question:
What is matter?
• Normal matter is made up of atoms
• Atoms are made up of electrically charged
particles.
• Protons (positive charge) and Neutrons (neutral)
– Massive
– Stay in the center of the atom (nucleus)
– Held together by strong and weak nuclear force
• Electrons (negative) “orbit” the nucleus
– Usually an atom is neutral (# electrons=# protons)
– Like gravitational potential energy, there is a potential
energy of the electron (called binding energy)
– Only certain specific “orbits” are allowed (specific
energy levels) (Term: Quantized)
• By adding or subtracting energy you can make the electrons
jump between levels.
• By adding enough energy you can make an electron leave
entirely (ionization). Result: a charged atom (an ion)
Hydrogen: The simplest atom.
• Most abundant
element
• 1 proton, 1 electron.
• Simple energy levels
– Higher levels closer
together
Other atoms
• More protons
– chemical properties of an atom are determined by
number of protons and electrons
– Number of protons determine which element it is.
– Number of electrons determine charge (ionization) state
(Neutral when electrons=protons).
• More complicated energy level structure.
– Every element and ionization state is unique in its
energy level structure!
Molecules
• Groups of atoms bound together.
• Interaction of the electrons in the atoms changes the energy level
structure (and rotation and vibration)
• Gets very complicated.
– A molecule can have thousands of energy levels very closely spaced.
Others
• Solids, Metals, and free electrons have essentially infinite numbers of
energy level. I’ll tell you why this is important later.
What is light?
• Electromagnetic Radiation (not nuclear)
– It’s a wave.
• Oscillating electric and magnetic fields
• Generated by moving (accelerating) charged
particles (electrons or protons, usually electrons)
• Self propogating: Doesn’t need a medium to travel
in.
• Can be absorbed by a another charged particle.
– It’s a particle. (Photon)
• Energy per particle is quantized.
Some definitions.
• Wavelength: Distance between wave crests. ()
• Frequency: Number of wave crests that pass per second.
()
• speed=wavelength x frequency ( c= )
• Energy of a photon is proportional to frequency.
– E=h=hc/
– This energy can be transferred to a charged particle
• Photons have momentum.
– Momentum = E/c
– This momentum is also transferred when energy is transferred.
(Light can make things move).
The inverse square law
• Brightness proportional to 1/d2
Demo
The Electromagnetic Spectrum
• From the radio to gamma rays
– low energy to high energy
– low frequency to high frequency
– long wavelength to short wavelength
• Visible light is just a tiny fraction.
– ROY G. BIV (red orange yellow green
blue indigo violet in order low energy to
high)
– Wavelengths from 0.0004 millimeters to
0.0007 millimeters. Usually use nm (10-9
m or Å 10-10 m)
• All travel at speed of light in vacuum.
How do astronomers gather light?
• Eyes: No permanent record, limited wavelength range,
can’t see small details, can’t analyze wavelength.
• Telescopes: Bigger than the eye
–
–
–
–
Gather more light (proportional to D2)
Higher resolving power (proportional to /D)
Can be made at wavelengths from gamma rays to the radio.
Can collect data with electronic detectors for analysis (CCDs, film,
etc.)
Problems that affect telescopes
• Atmosphere is opaque at many wavelengths.
– UV-Gamma rays
– Infrared
– Space telescopes or telescopes on balloons
Problems that affect telescopes
• Atmosphere causes twinkling which limits resolution at
optical and infrared wavelengths.
– Solutions:
• Space Telescopes
• Interferometry (combine light from separate telescopes into a
single image)
• Adaptive optics: Change the shape to the telescope mirror to
correct for atmospheric effects.
• Light pollution from major cities interferes with
observations.
– Solutions: Convince cities to direct light downward and use lamps that
only contaminate at a few wavelengths.
Splitting up light into a spectrum
• Spectroscopy
How do we make light?
• Move charged particles!
– In an atom or molecule, make an electron jump
from a higher energy level to a lower energy
level.
• Have to get the electron into the higher
level in the first place.
– Collision with another atom or electron
– Absorption of a photon.
Which brings us to heat...
• Heat is energy in the form of moving atoms or molecules.
– More energy = faster moving molecules = hotter
– Transferred by collisions. (One object to another, one atom to
another).
– Some of the energy can be transferred into changing the energy
level of an electron. Later fall to lower level releases a photon.
– Hotter atoms have more energy, can cause jumps between farther
apart levels. This results in higher energy photons. Also results in
more photons at all energies.
– Which leads us to black body radiation….
• Blackbody=something that emit and absorbs at every wavelength
perfectly (infinite energy levels: most solids, liquids, dense gasses,
free electrons, ionized gasses can be approximately a blackbody)
Blackbody Radiation
• Hotter = Bluer (max=constant/T)
• Hotter = Brighter at all wavelengths (LtotT4)
Blackbody radiation
• Just about every object you can imagine emits blackbody
radiation. Even you!
Concept Test:
If a 3000 K blackbody emits with a peak wavelength
of 1000 nm, what is the peak wavelength of the
radiation emitted by your body (300 K)?
A) 100 nm
B) 10,000 nm
C) 100,000 nm
D) 2,000 nm
How much more energy (per square meter) does it the
3000 K blackbody emit?
A) 10 times as much B) 1000 times as much
C) 1/10th as much
D) 10,000 times as much
Atomic Spectra
• Atoms have few discrete energy levels.
– Collisions can cause photons to jump to a higher state.
Again, the hotter it is, the bigger a jump the electron
can make and the more photons can be made.
– Only photons of energy corresponding to the difference
in levels can be emitted.
Each element has a unique signature because of its
energy level structure
Absorption of Photons
• Atoms can absorb photons that correspond to differences between their
energy levels.
• Photon energy must be very exact. Atoms absorb at the same
wavlength that they emit.
Absorption of Photons
• If they jump more than 1 level, they may emit more than 1 photon (but
the total energy emitted is the same as the energy absorbed)
Kirchoff’s Laws
• The brightness of lines from a gas depend upon its
temperature.
Hotter = Brighter
• A cool cloud in front of a hot blackbody.
– Lines from the cloud are darker than the blackbody.
• Dark lines in appear in the blackbody spectrum
• A hot cloud in front of a cool blackbody
– Lines from the cloud are brighter than the blackbody
• Bright lines appear in the blackbody spectrum
• By identifying the lines, you can tell what the cloud is
made of and what temperature it is!
Kirchoff’s Laws
Absorption Spectrum
Emission Spectrum
Demo
Kirchoff’s Law Exercise
Applied to
Stars
• Outer layers of a
star are cooler
than the dense
inner part.
• The differences
between these
spectra are
primarily due to
temperature.
This is how stars
are classified.
Emission Nebulae
• Nebula is hotter than the background (empty space)
• Different colors from different elements at different
temperatures
Scattering from particles (dust)
• Short wavelengths (blue) more strongly
scattered than long (red)
Why is the sky blue?
Why is the sunset red?
• Both for the same reason, dust particles in
the air.
Reflection Nebulae
• Clouds of dust
surrounding young
stars scatter some of
the blue light
• Stars embedded in
thick dust clouds
appear redder
because of scattering
Emission or Reflection Nebula?
Emission or Reflection Nebula?
Views of the Galaxy
Visible Light
Views of the Galaxy
Infrared
Views of the Galaxy
Radio
Views of the Galaxy
Xray
The Doppler Effect
How is light affected by the velocity of a source?
Click Here
Motion toward the observer shortens wavelength
(Blue Shift)
Motion away from the observer lengthens
wavelength (Red Shift)
Change in  proportional to velocity v/c
The speed of the light doesn’t change. It’s always c
Demo
Spectroscopic Binary
• Two stars in orbit around one another, as
they circle their velocities change.
Click Here