Sept 14 and 16, 2010 Ch 5b and c

5.2Learning 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 Review from last class: 5. Types of spectra ( Kirchhoff’s 3 laws ): Continuous, Absorption and Emission (page of book) a.Model of atoms: energy levels b.Continuous spectrum c.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 brighter B.2. It becomes bluer C.3. It becomes fainter D.4. It becomes redder E.5. It remains constant The correct answer is: A.A. 3 only B.B. 4 only C.C. 5 only D.D. 1 and 2 E.E. 3 and 4 Question 1

If the temperature of a star goes from 6000 K to 5000 K, what happens to its light? A.1. It becomes brighter B.2. It becomes bluer C.3. It becomes fainter D.4. It becomes redder E.5. It remains constant The correct answer is: A.A. 3 only B.B. 4 only C.C. 5 only D.D. 1 and 2 E.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

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.

Which is hotter? a)A blue star. b)A red star. c)A planet that emits only infrared light.

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 (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

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 CO 2 in the atmosphere

What is this object? Ultraviolet Emission Lines: Indicate a hot upper atmosphere

What is this object? Mars!

Radial Velocity Approaching stars: more energy, Receding stars: less energy, 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 nm. The same line in a star has wavelength nm. What can I say about this star?

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 nm. The same line in a star has wavelength 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? 5.2Learning 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 Outline Ch 5 Light: The Cosmic Messenger

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 telescopeYerkes 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

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

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.