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Transcript
How does light tell us the temperatures of planets
and stars?
Thermal Radiation
• Nearly all large or dense objects emit thermal
radiation, including stars, planets, you…
• An object’s thermal radiation spectrum depends
on only one property: its temperature
Two Properties of Thermal Radiation:
1. Hotter objects emit more light at all frequencies per unit area.
2. Hotter objects emit photons with a higher average energy.
Spectra
Thought Question
Which is hotter?
a) A blue star.
b) A red star.
c) A planet that emits only infrared light.
Which is hotter?
a) A blue star.
b) A red star.
c) A planet that emits only infrared light.
Thought Question
Why don’t we glow in the dark?
a) People do not emit any kind of light.
b) People only emit light that is invisible to our
eyes.
c) People are too small to emit enough light for us
to see.
d) People do not contain enough radioactive
material.
Why don’t we glow in the dark?
a) People do not emit any kind of light.
b) People only emit light that is invisible to our
eyes.
c) People are too small to emit enough light for us
to see.
d) People do not contain enough radioactive
material.
How does light tell us the speed of a distant object?
The Doppler Effect.
The amount of blue or red shift tells us an object’s
speed toward or away from us:
Doppler shift tells us ONLY about the part of an
object’s motion toward or away from us:
The Doppler Effect
Explaining the Doppler Effect
Same for
Light
Measuring
Redshift
Measuring
Redshift
Measuring
Velocity
Measuring
Velocity
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 toward you
• greater shift ⇒ greater speed
What have we learned?
• What types of light spectra
•
can we observe?
• Thermal radiation spectrum
– looks like rainbow of light
• Absorption line spectrum –
specific colors are missing
from the rainbow
• Emission line spectrum– see
light only of a specific color
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
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 is technology revolutionizing
astronomy?
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
Angular Resolution
• The minimum
angular separation
that the telescope
can distinguish.
Angular resolution: smaller is better
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
Keck I and Keck II
Mauna Kea, HI
Mauna Kea, Hawaii
Different designs for different wavelengths of light
Radio telescope (Arecibo, Puerto Rico)
X-ray telescope: “grazing incidence” optics
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 ball point,
15 m from Sun
• Nearest stars 4,000 km away
• Hubble orbit microscopically
above ball- point 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
3. Atmosphere absorbs most of EM spectrum, including
all UV and X-ray, most infrared
Telescopes in space solve all 3 problems.
• Location/technology can help overcome
light pollution and turbulence.
• Nothing short of going to space can solve
problem of atmospheric absorption of light.
Chandra X-ray
Observatory
How is technology revolutionizing astronomy?
adaptive optics
• Rapid changes in mirror shape compensate for
atmospheric turbulence.
Without adaptive optics
With adaptive optics
interferometry
• Allows two or more small telescopes to work together to
obtain the angular resolution of a larger telescope.
Very Large Array (VLA), New Mexico
The Moon would be a great spot for an observatory (but at
what price?)
What have we learned?
• How do telescopes help • Why do we put
telescopes in space?
us learn about the
universe?
• They are above earth’s
atmosphere and
therefore not subject to
• We can see fainter
light pollution,
objects and more detail
than we can see by eye. atmospheric distortion,
or atmospheric
Specialized telescopes
absorption of light.
allow us to learn more
than we could from
visible light alone.
What have we learned?
• How is technology
revolutionizing
astronomy?
It makes possible more
powerful and more
capable telescopes
•
Adaptive optics
•
Interferometry