lecture 13+14 - institute for astronomyftaclas/default/astro110 web files/lecture 13+14.pdflecture...

Fall, 2005 Astronomy 110 1

Lecture 13+14


Important Concepts for Understanding Spectra

• Electromagnetic Spectrum• Continuous Spectrum• Absorption Spectrum• Emission Spectrum• Emission line• Wavelength, Frequency


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


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.


SpectraA real object has a spectrum that is a mix of thermal (black body) and discrete (line) features.


Thought QuestionWhich 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 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.


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.


What can we learn from light?

Speed: What is the Doppler Effect?

The Doppler Effect.


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


The Doppler Effect


Same for Light


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:


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.


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.


Measuring Redshift


Measuring Velocity


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.

•


Learning from Light:• Composition • Temperature

• We can determine temperature from the spectrum of thermal radiationWe determine composition

from spectral lines.


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


Telescopes


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)


Bigger is better


Angular Resolution

• The minimumangular 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.

Reflecting telescopeGemini 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


Telescopes in Space


RadioIR

VIS X-Ray


Observing problems due to Earth’s atmosphere

1. Light Pollution


Star viewed with ground-based telescope

2. Turbulence causes twinkling ⇒ blurs images.

View from Hubble Space Telescope


3. Atmosphere absorbs most of EM spectrum, including all UV and X-ray, most infrared


adaptive optics• Rapid changes in mirror shape compensate for atmospheric turbulence.

How is technology revolutionizing astronomy?

Without adaptive optics With adaptive optics


Adaptive Optics at Gemini SouthHokupa`a


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 might be a great spot for an observatory - but at what price?

lecture 13+14 - institute for astronomyftaclas/default/astro110 web files/lecture 13+14.pdflecture...

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