Sept 14 and 16, 2010 Ch 5b and c. 5.2Learning from Light: Origin of Starlight 1. How photons are...

Sept 14 and 16, 2010 Ch 5b and c

5.2 Learning from Light: Origin of Starlight1. 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

Review from last class: 5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission (page 117-119 of book)

a. Model of atoms: energy levelsb. Continuous spectrumc. Emission lines and absorption lines

What types of light spectra can we observe?

This process produces an emission spectrum

This process produces an absorption spectrum

Continuous Spectrum

Emission Spectrum

Absorption Spectrum

Solar Spectrum

How does light tell us what things are made of?

• Electrons in atoms have distinct energy levels.

• Each chemical element, ion, molecule, has a unique set of energy levels.•We can identify the chemicals in gas by their fingerprints in the spectrum.

Distinct energy levels lead to

distinct emission or absorption

lines.

If the temperature of a star goes from 6000 K to 5000 K, what happens to its light?

A. 1. It becomes brighterB. 2. It becomes bluerC. 3. It becomes fainterD. 4. It becomes redderE. 5. It remains constant

The correct answer is: A. A. 3 onlyB. B. 4 onlyC. C. 5 onlyD. D. 1 and 2E. E. 3 and 4

Question 1

Can one use the visible color of the Moon to determine its temperature?

A. Yes, because the Moon is similar to stars

B. Yes, because the Moon does not reflect light

C. Yes, because the Moon orbits Earth

D. None of the above are correct

Question 2

Which is hotter?

a) A blue star.

b) A red star.

c) A planet that emits only infrared light.

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 (infrared light).c) People are too small to emit enough light for us

to see. d) People do not contain enough radioactive

material.

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


Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K


Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere


Ultraviolet Emission Lines: Indicate a hot upper atmosphere


Mars!

Radial Velocity• Approaching stars: more

energy, • Receding stars: less energy,

5.2.6 Doppler Effect

• Approaching stars: more energy, spectral lines undergo a blue shift

• Receding stars: less energy, spectral lines undergo a red shift

/ = v/c

Radial Velocity

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

• We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.

Stationary

Moving Away

Away Faster

Moving Toward

Toward Faster

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

A. It is moving away from me.

B. It is moving toward 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?

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

what things are made of?• Every kind of atom, ion,

and molecule produces a unique set of spectral lines.

• How does light tell use the temperatures of planets and stars?

• 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

5.1 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

Matter: Atoms. How do light and matter interact? 5.2 Learning from Light: Origin of Starlight

1. How photons are produced2. 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 hotter5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission6. Radial Velocity: Doppler effect

5.3 Telescopes: reflecting and refracting, ground, airborne, space. Remember atmospheric windows

Outline Ch 5 Light: The Cosmic Messenger

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 telescopeGemini North 8-m

Mauna Kea, Hawaii

Keck I and Keck IIMauna Kea, HI

NASA’s IRTFMauna 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

Star viewed with ground-based telescope

2. Turbulence causes twinkling blurs images.

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.

Sept 14 and 16, 2010 Ch 5b and c. 5.2Learning from Light: Origin of Starlight 1. How photons are...

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