A105
Stars and Galaxies
Today’s APOD
Homework 4 due Sept. 21
Telescopes
Read units 26, 27
News Quiz Tuesday
OBSERVING OPPORTUNITIES
• Rooftop session TONIGHT at 8:30 PM
– SWAIN WEST roof – take the elevator to
the 3rd floor and follow the signs
• Remote Observing Friday and Saturday
evenings at 10 PM
– Swain West 403 – take the elevator to the
3rd floor and follow the signs.
REVIEW THE E.M. SPECTRUM
Where does light come from?
THERMAL
EMISSION
ATOMIC
EMISSION
Thermal radiators emit light at
all wavelengths
Atomic emission occurs only at
particular wavelengths
Understanding
Atomic Emission
What is the structure of matter?
Atom
Electron
Cloud
Nucleus
The
Nucleus
Electrons
Atomic Terminology
• Atomic Number = # of protons in nucleus
• Atomic Mass Number = # of protons +
neutrons
Atomic
Structure
Electrons may have only certain, discrete
energies
We can think of those energies as only
certain allowed distances from the
nucleus
Atomic Emission (and Absorption)
Light is
emitted or
absorbed
when
electrons
jump
between
energy
levels in
an atom
How is energy stored in atoms?
Excited States
Ground State
• Electrons in atoms are restricted
to particular energy levels
Energy Level Transitions
• The only
allowed
changes in
energy are
those
corresponding
to a transition
between
energy levels
Not Allowed
Allowed
Atoms and Light
Hands-On Atom
Three
types of
spectra
Continuous – Thermal Radiator
Emission – from hot gas
Absorption – continuous spectrum passes
through cooler gas
Continuous Spectrum
• The spectrum of a common
(incandescent) light bulb spans all visible
wavelengths, without interruption
Emission Line Spectrum
• A thin or low-density cloud of gas emits
light only at specific wavelengths that
depend on its composition and
temperature, producing a spectrum with
bright emission lines
Absorption Line Spectrum
• A cloud of gas between us and a light
bulb can absorb light of specific
wavelengths, leaving dark absorption lines
in the spectrum
Chemical Fingerprints
• Each type of
atom has a
unique set of
energy levels
• Each transition
corresponds to
a unique photon
energy,
Energy levels of Hydrogen
frequency, and
wavelength
Chemical Fingerprints
• Each type of atom has a
unique spectral fingerprint
Is It Hydrogen or Helium?
Hydrogen
Helium
Reviewing Spectra
Which letter(s) labels absorption lines?
A
B
C
D E
Reviewing Spectra
Which letter(s) labels emission lines?
A
B
C
D E
How do
light and
matter
interact?
Emission
Absorption
Transmission
Transparent objects transmit light
Opaque objects block (absorb) light
Reflection or Scattering
Reflection and
Scattering in
Nature
The moon
reflects sunlight.
Moonlight
scatters off the
fog and reflects
off the lake
Dust near the stars of the
Pleiades scatters starlight
Checking Understanding….
Why is a rose red?
a) The rose
b) The rose
light.
c) The rose
d) The rose
light.
absorbs red light.
transmits red
emits red light.
reflects red
The Doppler Shift
Johan Christian Doppler
Doppler Pumpkin
Police Siren
The Doppler Effect
• How does light tell us the speed of
a distant object?
• How does light tell us the rotation
rate of an object?
Doppler Principles (Police Traffic
Radar Handbook)
The Doppler Shift of Sound Waves
What's Happening?
The Doppler
Shift in
Astrophysical
Situations
• Emission from a moving cloud of gas
• Absorption by a moving cloud of gas
• Identifying shifted spectral lines
The Doppler Shift
for Light
• Astronomers us the
Doppler effect to
measure the “radial”
velocities of
astronomical objects
• Radial velocities are
motions toward or
away from us
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster
Measuring the Shift
Doppler shift tells us ONLY about the part
of an object’s motion toward or away from us
The amount of blue or red shift tells us an
object’s speed toward or away from us:
The Doppler Shift in YOUR Life
 Read units 26, 27
News Quiz on Tuesday
 Telescope homework (#4)
worksheet due Thursday