Download Wednesday, Sept. 24 - Department of Physics and Astronomy

Phys 1810: Lecture 9
Recall column
summary
• Next Class
– 21 cm emission lines 18.4
– synchrotron emission
– Doppler Shift 3.5, Box 3-3, 4.5,
– Telescopes 5.2, 5.3, “seeing” in 5.4, 5.55.7
– CCDs, photometry, spectroscopy
– Light gathering power, Resolving Power,
Diffraction Limit, Seeing
Please join us this week,
and the first Thursday of
every month, rain or shine.
Friends and family are
welcome too!
October 2 at 7:30 pm
Meet at Lockhart Planetarium
(University College Room 394)
Also this month:
Oct 1-30: An exhibit of astronomy images in
Degrees Café.
October 8: total lunar eclipse!
October 14: Astronomy in the restaurant – The
Tallest Poppy at 7:30 PM. A panel
presentation & opportunity for the public to ask
questions. Special guest Professor Ken
Freeman (Australian National University).
Topic: dark matter – The stuff that makes up
90% of the matter in the universe.
October 23: partial solar eclipse!
Stars: Their Characteristics
summary
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• Luminosity (L): The total energy
radiated per second, at all wavelengths.
• L = surface area * flux
• Surface area of a sphere is

T== Surface
Temperature
Luminosity is proportional to the radius squared times surface temperature to the 4th power.
Stars: Why Temperature is useful.
summary
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• Notice that if we know the
temperature of a star, then if we
know the radius, we can calculate
the luminosity.
• Alternatively, if we know the
temperature and the luminosity we
can determine the radius.

summary
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The Interaction of light and matter.
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• Photons (γ == gamma)
– Individual packet of EM energy that makes up EM
radiation
• γ & matter interact creating spectra.
• Spectra used to assess
• T (blackbody curve type spectrum)
• processes that produce light or absorb it (i.e. what
is going on)
(Animation)
summary
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Spectra
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Kirchhoff’s Laws
• 3 empirical laws
a) Hot opaque body -> continuous spectrum
b) Cooler transparent gas between source & observer ->
absorption line spectrum
c) Diffuse, transparent gas -> emission line spectrum
Spectra
summary
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• Our sun and other stars have an
atmosphere. Imagine that you are in a
spaceship far above the Earth’s
atmosphere. Which of the following
spectra would you observe when analyzing
sunlight?
a) Continuum rainbow-like spectrum
b) Dark line absorption spectrum
c) Bright line emission spectrum
summary
Spectral Finger Prints
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Solar Spectrum
• Note emission lines for lab spectrum of
iron are at same λs of absorption lines
of iron in 
• Can use line spectra to determine
chemical elements in object.
Interaction of Light and Matter:
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summary
How are line spectra created?
γs of light interact with atoms &
molecules.
• Atoms consist of:
– Electrons (negative charge) == e– Nuclei (balance charge of e-)
• Protons (positive)
• Neutrons (neutral)
• Molecules: group of 2 or more atoms.
Interaction of Light and Matter
summary
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•
•
•
•
•
•
Hydrogen == H: simplest atom.
1 e- & 1 proton.
Classical picture: e- in an orbit.
Contemporary picture: e- as a cloud.
Orbits are really energy levels.
E == energy
Interaction of Light and Matter
summary
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Hydrogen Atom Energy Levels
• Every chemical element has its own specific
set of E levels.
• Each E level is associated with a λ.
Interaction of Light and Matter
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Creating spectral lines at visible wavelengths
• specific (quantized) E levels. Level with
lowest E is ground state.
• How does e- get excited?
– By interactions between γs & matter.
summary
Interaction of Light and Matter:
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Creating spectral lines at visible wavelengths
• The e- can shift between E levels by
absorption & emission of γs.
Interaction of Light and Matter
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summary
Creating spectral lines at visible wavelengths
Absorption:
1. If γ’s E is not matched to any E level
then γ passes by atom. The atom is
unchanged.
Interaction of Light and Matter
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summary
Creating spectral lines at visible wavelengths
Absorption:
2. If γ’s E matches E needed to cause an
e- to jump to a larger E level, then
atom absorbs γ (i.e. absorbs E) & ejumps to that E level. The atom is
now in an excited state.
Interaction of Light and Matter
Recall column
summary
Creating spectral lines at visible wavelengths
Absorption:
3. If γ’s E is larger than any jump within
atom, then atom absorbs E, the γ
disappears, & an e- (or more) are
kicked out of the atom creating an
ion. (In an ion the charge is not
balanced.) The atom is ionized.
Interaction of Light and Matter
summary
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• e-s originally at these E levels in 
atmosphere are kicked into excited or ionized
states.
•  wavelengths of absorption lines are
identical to wavelengths of emission lines.
Interaction of Light and Matter
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summary
Creating spectral lines at visible wavelengths
Emission:
• γs can be emitted spontaneously when
an e- falls back down to lower E levels.
• An atom can be excited or ionized. An
ejected e- can subsequently be
recaptured.
(animation)
summary
Interaction of Light and Matter
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Creating spectral lines at visible wavelengths
Emission:


• These e-s cascade through different E
levels, generating γs
• Bottom path  E level called “H α ” &
glows red
• (α== “alpha”)
Interaction of Light and Matter
Recall column
summary
Creating spectral lines at visible wavelengths
Emission:
The Orion Nebula
David Malin
• Clouds of gas that glow due to this process have a
few names:
– Emission nebulae
– H II regions
– H α regions
• If very bright, then pinker.
• Ionizing γs come from hot stars.
summary
Spectral Finger Print
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Hydrogen Atom Energy Levels
• Each chemical element has its own
“finger print” of lines.
• The # of lines for one element
depends only on the # of E levels in
its atom.
• The more elements in a star, the
more lines in the star’s spectrum.
Spectral Finger Print
summary
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• The strength of absorption lines gives
the # of atoms of that element in the
gas.
• Comparison of strengths of absorption
lines of different elements in the gas
gives
– Density
– Temperature
• Can get these characteristics for outer
layers of star from its absorption line
spectrum.
What can we do with spectral information?
summary
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Study activity on the sun!
• the Sun in extreme ultraviolet light (Solar
Dynamics Observatory.)
• false-color image shows emission from highly
ionized iron atoms.
• Loops and arcs trace the glowing plasma
suspended in magnetic fields above solar
active regions.
What can we do with this information?
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• Consider stars...
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Spectral Finger Print
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• What can we do with this information?
• If 2 stars have the same elements,
same density, and same temperature
then they have the same intrinsic
luminosity.
• If they have the same intrinsic
luminosity we can use their apparent
brightnesses to derive their relative
distances using the Inverse Square
Brightness Law!
Instrumentation for observing spectra:
summary
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• spectrograph on optical
telescope.
• use diffraction grating to
disperse light.
Spectra at other λ:
e.g. ν of radio receiver
Neutral Atomic Hydrogen emission
H I: 21 cm emission
Spin Flip Transition
4. Radio Continuum Emission.
summary
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Nick Strobel
a) Synchrotron radiation: an e- spirals
around a magnetic field line.
4. Radio Continuum Emission.
summary
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Nick Strobel
b) Thermal radiation generated when eaccelerates near p+. (Define accel. includes change in
direction.)
 "free-free" emission, "bremsstrahlung” or
“braking”.
Visible Phenomena
Our Milky Way Galaxy: ionized hydrogen around hot stars (thermal).
Optical image courtesy of Charles Dyer
W3/4/5 Region
Imaging the Invisible – Radio Radiation
W3/4/5 Region
Our Milky Way Galaxy in Radio Radiation: The Interstellar Medium ISM
21 cm radio data: Canadian Galactic Plane Survey atomic hydrogen gas
(spin flip transition).
Following images by Jayanne English, Russ Taylor and Tom Landecker using the Dominion Radio Astrophysical
Observatory.
Imaging the Invisible – Radio and IR Radiation
Our Milky Way Galaxy in Radio Radiation: The Interstellar Medium
Include infrared data (thermal) from the IRAS satellite.
W3/4/5 Region
Imaging the Invisible – Radio and IR Radiation
Our Milky Way Galaxy in Radio Continuum Radiation:
Heated dust infrared data (thermal) plus e-s moving in B fields
 synchrotron in supernova remnants, galaxy cores, ISM.
 free-free in ionized shells.
W3/4/5 Region
Review of Processes Producing Radiation
1. Black body radiation (continuum
emission)
2. Spectral line emission and
absorption
3. Spin-flip transition emission
4. Radio continuum emission
Dopper Shift
• How can we use spectral lines?
• What properties of objects can we measure?
• Recall continuum didn’t move – only lines
moved.
• Continuum when plotted is a b.b. curve.
• For peak of the black body curve to change
colour, star would need to travel at least
10,000 km/s. Within our Milky Way Galaxy
most stars orbit at a speed of 220 km/s. Even
nearby galaxies – which I am showing detailed
images of - are moving at only a few thousand
km/s.
What your eye sees for colour.
• mathematically defined colour space (CIE) of
colour perception.
• λ associated for colours perceived by humans
(on the outer edge of the shape).
Change in colour in 50 nm.
Say we observe at 500 nm rest wavelength.
Doppler Shift:
summary
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• If an object is moving towards you,
you will observe its spectral lines are
shorter in wavelength.
• Analyzing a star’s spectral lines will tell
us about its density, surface
temperature, rotation, chemical
composition but NOT its transverse
(side-to-side) motion.
• We can use the velocity from the
Doppler shift of an object in orbit to
measure the mass of the object it is
orbiting.
Stars in our Milky Way Galaxy