Transcript Class 8 (Ch 5b) Feb3

Feb. 3, 2011 Ch 5b
5.1

Outline Ch 5 Light: The Cosmic Messenger
Basic Properties of Light and Matter
Light: electromagnetic waves
1. Velocity (c = speed of light), wavelength and frequency (colors),
energy.
2. Electromagnetic spectrum, visible spectrum, atmospheric windows

5.2
Matter: Atoms. How do light and matter interact?
Learning from Light: Origin of Starlight (some not in book)
1. How photons are produced
2. Relation temperature  motion of atoms
3. Blackbody Radiation (hot iron example). Wien’s Law:
hotter  brighter, cooler  dimmer
hotter  bluer, cooler  redder (max ~1/T)
4. Colors of Stars: redder are cooler, bluer are hotter
5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and
Emission
6. Radial Velocity: Doppler effect
5.3
Telescopes: reflecting and refracting, ground, airborne, space.
Interpreting an Actual Spectrum
• By carefully studying the features in a
spectrum, we can learn a great deal about
the object that created it.
What is this object?
Reflected Sunlight:
Continuous spectrum of
visible light is like the
Sun’s except that some of
the blue light has been
absorbed—object must
look red
What is this object?
Thermal Radiation:
Infrared spectrum peaks
at a wavelength
corresponding to a
temperature of 225 K
What is this object?
Carbon Dioxide:
Absorption lines are the
fingerprint of CO2 in the
atmosphere
What is this object?
Ultraviolet Emission Lines:
Indicate a hot upper
atmosphere
What is this object?
5.2.6 Doppler Effect
Radial Velocity
• Approaching stars: more energy,
• Receding stars: less energy,
Radial Velocity
•
•
•
Approaching stars: more
energy, spectral lines undergo a
blue shift
Receding stars: less energy,
spectral lines undergo a red
shift
/ = v/c
How does light tell us the speed of a distant object?
The Doppler Effect.
Explaining the Doppler Effect
Understanding the Cause of the Doppler Effect
Same for
light
The Doppler Effect for Visible Light
Measuring the Shift
• We generally measure the Doppler effect from shifts in
the wavelengths of spectral lines.
Measuring the Shift
What can you say
about the radial
velocity of these
objects?
• We generally measure the Doppler effect from shifts in
the wavelengths of spectral lines.
The amount of blue or red shift tells
us an object’s speed toward or away
from us:
The Doppler Shift of an Emission-Line Spectrum
Doppler shift tells us ONLY about the part of an
object’s motion toward or away from us.
How a Star's Motion Causes the Doppler Effect
Question
I measure a line in the lab at 500.7 nm. The
same line in a star has wavelength 502.8 nm.
What can I say about this star?
A. It is moving away from me.
B. It is moving toward me.
C. It has unusually long spectral lines.
Measuring
radial
velocity in
emission
spectra
Determining the Velocity of a Gas Cloud
Measuring
radial
velocity in
absorption
spectra
Determining the Velocity of a Cold Cloud of Hydrogen Gas
Doppler Effect Summary
Motion toward or away from an observer causes a shift
in the observed wavelength of light:
• blueshift (shorter wavelength)  motion toward you
• redshift (longer wavelength)  motion away from
you
• greater shift  greater speed
What have we learned?
• What types of light spectra can
we observe?
• Continuous spectrum, emission
line spectrum, absorption line
spectrum
• Continuous– looks like rainbow
of light
• Absorption line spectrum –
specific colors are missing from
the rainbow
• Emission line spectrum– see
bright lines only of specific
colors
•
What have we learned?
• How does light tell us
• How does light tell use the
temperatures of planets and stars?
what things are made of?
• Every kind of atom, ion,
and molecule produces a
unique set of spectral lines.
• We can determine temperature
from the spectrum of thermal
radiation
What have we learned?
• How does light tell us
the speed of a distant
object?
• The Doppler effect tells
us how fast an object is
moving toward or away
from us.
– Blueshift:objects
moving toward us
– Redshift: objects
moving away from us
Outline Ch 5 Light: The Cosmic Messenger
5.1

Basic Properties of Light and Matter
Light: electromagnetic waves
1. Velocity (c = speed of light), wavelength and frequency (colors), energy.
2. Electromagnetic spectrum, visible spectrum, atmospheric windows

5.2
Matter: Atoms. How do light and matter interact?
Learning from Light: Origin of Starlight
1. How photons are produced
2. Relation temperature  motion of atoms
3. Blackbody Radiation (hot iron example). Wien’s Law:
hotter  brighter, cooler  dimmer
hotter  bluer,
cooler  redder (max ~1/T)
4. Colors of Stars: redder are cooler, bluer are hotter
5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission
6. Radial Velocity: Doppler effect
5.3
Telescopes: reflecting and refracting, ground,
airborne, space. Remember atmospheric windows
5.3 Collecting Light with Telescopes
5.3 Collecting Light with Telescopes
Our goals for learning:
• How do telescopes help us learn about the
universe?
• Why do we put telescopes into space?
How do telescopes help us learn
about the universe?
• Telescopes collect more light than our eyes 
light-collecting area
• Telescopes can see more detail than our eyes 
angular resolution
• Telescopes/instruments can detect light that is
invisible to our eyes (e.g., infrared, ultraviolet)
Bigger is better
1. Larger light-collecting area
2. Better angular resolution
Bigger is better
Light Collecting Area of a Reflector
Angular Resolution
• The minimum
angular separation
that the telescope
can distinguish
Angular Resolution Explained using Approaching Car Lights
Angular resolution: smaller is better
Effect of Mirror Size on Angular Resolution
Basic Telescope Design
• Refracting: lenses
Refracting telescope
Yerkes 1-m refractor
Basic Telescope Design
• Reflecting: mirrors
• Most research telescopes
today are reflecting
Reflecting telescope
Gemini North 8-m
Mauna Kea, Hawaii
Keck I and Keck II
Mauna Kea, HI (were world’s largest
until 2009)
Gran Telescopio Canarias:
World’s Largest Telescope
NASA’s IRTF
Mauna Kea, HI
Different designs for different wavelengths of light
Radio telescope (Arecibo, Puerto Rico)
Why do we put telescopes into
space?
It is NOT because they are closer
to the stars!
Recall our 1-to-10 billion scale:
• Sun size of grapefruit
• Earth size of a tip of a ball
point pen,15 m from Sun
• Nearest stars 4,000 km
away
• Hubble orbit
microscopically above tip of
a ball-point-pen-size Earth
Observing problems due to Earth’s atmosphere
1. Light Pollution
2. Turbulence causes twinkling  blurs images.
Star viewed with
ground-based telescope
View from Hubble
Space Telescope
Remember: Atmosphere absorbs most of EM spectrum,
including
all UV and X-ray, most infrared
NASA’s Stratospheric Observatory For Infrared
Astronomy (SOFIA)
SOFIA Airborne!
26 April 2007, L-3 Communications, Waco Texas: SOFIA takes to the
air for its first test flight after completion of modifications
Kuiper Airborne Observatory
It began operation in 1974 and was retired in 1995.
The Moon would be a great spot for an observatory
What have we learned?
• How do telescopes help us learn about the
universe?
—We can see fainter objects and more detail
than we can see by eye. Specialized
telescopes allow us to learn more than we
could from visible light alone.
• Why do we put telescopes in space?
—They are above Earth’s atmosphere and
therefore not subject to light pollution,
atmospheric distortion, or atmospheric
absorption of light.
Light Pollution
Want to buy your own telescope?
• Buy binoculars first (e.g., 7  35) — you get
much more for the same money.
• Ignore magnification (sales pitch!)
• Notice: aperture size, optical quality,
portability
• Consumer research: Astronomy, Sky &
Telescope, Mercury magazines; Astronomy
clubs.