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Fall, 2005 Astronomy 110 1
Lecture 13+14
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Fall, 2005 Astronomy 110 2
Important Concepts for Understanding Spectra
• Electromagnetic Spectrum• Continuous Spectrum• Absorption Spectrum• Emission Spectrum• Emission line• Wavelength, Frequency
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Fall, 2005 Astronomy 110 3
What can we learn from light?Temperature: What is Black Body Radiation
A “Black body” emits equally well at all wavelengths. An idealization to help model the interaction of light with matter.
Black body or Thermal Radiation• Objects that interact with electromagnetic
energy emit thermal radiation. • This includes stars, planets, you…• An object’s thermal radiation spectrum
depends on only one property: its temperature
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Fall, 2005 Astronomy 110 4
Two Properties of Thermal Radiation:As the temperature increases:• more BB energy per unit area is radiated at all frequencies.• photons with a higher average energy (higher f) are emitted.• the peak of the BB spectrum shifts towards shorter wavelengths.
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Fall, 2005 Astronomy 110 5
SpectraA real object has a spectrum that is a mix of thermal (black body) and discrete (line) features.
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Fall, 2005 Astronomy 110 6
Thought QuestionWhich is hotter?
a) A blue star.b) A red star.c) A planet that emits only infrared light.
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Fall, 2005 Astronomy 110 7
Which is hotter?
a) A blue star.b) A red star.c) A planet that emits only infrared light.
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Fall, 2005 Astronomy 110 8
Thought QuestionWhy 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.
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Fall, 2005 Astronomy 110 9
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.
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Fall, 2005 Astronomy 110 10
What can we learn from light?
Speed: What is the Doppler Effect?
The Doppler Effect.
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Fall, 2005 Astronomy 110 11
The Doppler EffectMotion 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
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Fall, 2005 Astronomy 110 12
The Doppler Effect
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Fall, 2005 Astronomy 110 13
Same for Light
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Fall, 2005 Astronomy 110 14
The amount of blue or red shift tells us an object’s speed toward or away from us:
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Fall, 2005 Astronomy 110 15
Doppler shift tells us ONLY about the part of an object’s motion toward or away from us:
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Fall, 2005 Astronomy 110 16
Thought Question:
A spectral line is measured in the lab at 500.7 nm.The same line in a star has wavelength of 502.8nm. What can we say about this star?
a) It is moving away from me.b) It is moving towards me.c) It has unusually long spectral lines.
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Fall, 2005 Astronomy 110 17
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 towards me.c) It has unusually long spectral lines.
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Fall, 2005 Astronomy 110 18
Measuring Redshift
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Fall, 2005 Astronomy 110 19
Measuring Velocity
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Fall, 2005 Astronomy 110 20
Learning from Light: Types of Spectra
• Thermal radiation spectrum – continuous
• Absorption line spectrum – specific frequencies are missing.
• Emission line spectrum–light only at specific energies.
•
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Fall, 2005 Astronomy 110 21
Learning from Light:• Composition • Temperature
• We can determine temperature from the spectrum of thermal radiationWe determine composition
from spectral lines.
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Fall, 2005 Astronomy 110 22
Learning from Light: Speed
• 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
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Fall, 2005 Astronomy 110 23
Telescopes
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Fall, 2005 Astronomy 110 24
Why do we need Telescopes?
• Telescopes collect more light than our eyes ⇒Sensitivity
• 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)
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Fall, 2005 Astronomy 110 25
Bigger is better
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Fall, 2005 Astronomy 110 26
Angular Resolution
• The minimumangular separation that the telescope can distinguish.
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Fall, 2005 Astronomy 110 27
Angular resolution: smaller is better
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Fall, 2005 Astronomy 110 28
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Fall, 2005 Astronomy 110 29
Basic Telescope Design• Refracting: lenses
Refracting telescope Yerkes 1-m refractor
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Fall, 2005 Astronomy 110 30
Basic Telescope Design• Reflecting: mirrors• Most research telescopes.
Reflecting telescopeGemini North 8-m
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Fall, 2005 Astronomy 110 31
Keck I and Keck II - Mauna Kea, HI
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Fall, 2005 Astronomy 110 32
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Fall, 2005 Astronomy 110 33
Mauna Kea, Hawaii
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Fall, 2005 Astronomy 110 34
Different designs for different wavelengths of light
Radio telescope (Arecibo, Puerto Rico)
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Fall, 2005 Astronomy 110 35
X-ray telescope: “grazing incidence” optics
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Fall, 2005 Astronomy 110 36
Telescopes in Space
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Fall, 2005 Astronomy 110 37
RadioIR
VIS X-Ray
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Fall, 2005 Astronomy 110 38
Observing problems due to Earth’s atmosphere
1. Light Pollution
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Fall, 2005 Astronomy 110 39
Star viewed with ground-based telescope
2. Turbulence causes twinkling ⇒ blurs images.
View from Hubble Space Telescope
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Fall, 2005 Astronomy 110 40
3. Atmosphere absorbs most of EM spectrum, including all UV and X-ray, most infrared
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Fall, 2005 Astronomy 110 41
adaptive optics• Rapid changes in mirror shape compensate for atmospheric turbulence.
How is technology revolutionizing astronomy?
Without adaptive optics With adaptive optics
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Fall, 2005 Astronomy 110 42
Adaptive Optics at Gemini SouthHokupa`a
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Fall, 2005 Astronomy 110 43
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Fall, 2005 Astronomy 110 44
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
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Fall, 2005 Astronomy 110 45
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Fall, 2005 Astronomy 110 46
The Moon might be a great spot for an observatory - but at what price?