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ASTR 1040 ASTR 1040 AccelAccel AstroAstro: : Stars & GalaxiesStars & Galaxies

Prof. Prof. JuriJuri ToomreToomre TA: Kyle TA: Kyle AugustsonAugustsonLecture 4 Lecture 4 ThurThur 24 Jan 0824 Jan 08

zeus.colorado.edu/astr1040zeus.colorado.edu/astr1040--toomretoomre

Topics for Today Topics for Today •• Twinkle and absorption Twinkle and absorption by our atmosphereby our atmosphere•• What light gets through, what does notWhat light gets through, what does not•• Telescopes in spaceTelescopes in space ---- and whyand why•• Radio and xRadio and x--ray telescopes do it differentlyray telescopes do it differently•• ReadRead Chap 14Chap 14 (Our Star .. the Sun)(Our Star .. the Sun)•• Homework Set # 1 due todayHomework Set # 1 due today•• Respond to next discussion Q on Respond to next discussion Q on CULearnCULearn: :

““What are pros and cons of a lunar What are pros and cons of a lunar observatory?observatory?””

Clicker Q: Radio WavesClicker Q: Radio Waves•• You are listening to a radio station You are listening to a radio station

broadcasting at a FM frequency of 97 Mhz. broadcasting at a FM frequency of 97 Mhz. Which is true?Which is true?

•• A.A. The radio waves from the station have a wavelength of 97 million meters.

•• B.B. The “radio waves” received by your radio are not light waves, but rather a special kind of sound wave.

•• C.C. The radio station broadcasts it signal with a power of 97 million watts.

•• D.D. The radio waves are causing electrons in your radio’s antenna to move up and down 97 million times per second.

D.D. RadiosRadios

•• You are listening to a radio station You are listening to a radio station broadcasting at a frequency of 97 Mhz. broadcasting at a frequency of 97 Mhz. Which is true?Which is true?

• D. The radio waves are causing electrons in your radio’s antenna to move up and down 97 million times per second.

fc ⋅= λD.D.

Wavelength = Speed of light / frequencyWavelength = Speed of light / frequency= 3.0 x 10= 3.0 x 1010 10 cm seccm sec--11 / 9.6 10/ 9.6 1077 secsec--11

= 312 cm= 312 cm

Imaging with our EyesImaging with our Eyes•• pupilpupil –– allows light to enter the eyeallows light to enter the eye•• lenslens –– focuses light to create an imagefocuses light to create an image•• retinaretina –– detects the light and generates signals detects the light and generates signals

sent to brainsent to brain

Bending of Light to FocusBending of Light to Focus(Form an Image)(Form an Image)

Telescopes and camerasTelescopes and cameras work much like our eyeswork much like our eyes

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Reflection and refraction (bending of light)Reflection and refraction (bending of light)

slower speedslower speedof light in glassof light in glass

Your digital cameraYour digital camera (CCD detector)(CCD detector)

Optical Telescopes of Two TypesOptical Telescopes of Two Types

REFRACTORREFRACTOR (LENS)(LENS)

REFLECTORREFLECTOR (MIRROR)(MIRROR)

Largest Largest RefractorRefractor

Yerkes 40Yerkes 40””

Modern 8 m Telescope ( Modern 8 m Telescope ( ReflectorReflector ))

GEMINI 8 mGEMINI 8 mMauna Kea, HIMauna Kea, HI

Modern ReflectorsModern Reflectors

6.5 m MMT 6.5 m MMT –– Mt. Hopkins, AZMt. Hopkins, AZ

8.3 m SUBARU 8.3 m SUBARU –– Mauna Kea, HIMauna Kea, HI

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Different FOCUS arrangements (to get image)Different FOCUS arrangements (to get image) Keck 10 m Twins (Segmented Reflectors)Keck 10 m Twins (Segmented Reflectors)

Mauna Kea, HIMauna Kea, HI

Twin Keck 10 m Twin Keck 10 m TELESCOPESTELESCOPES

Mauna Kea, HIMauna Kea, HI

Another viewAnother view

Problems in Looking Through Our Problems in Looking Through Our AtmosphereAtmosphere

•• Many wavelengths are Many wavelengths are absorbedabsorbed (just don(just don’’t make t make it through to surface)it through to surface)

•• Turbulence in atmosphere distorts lightTurbulence in atmosphere distorts light::–– stars appear to stars appear to ““twinkletwinkle””–– angular resolution is degradedangular resolution is degraded

•• ManMan--made light is reflected by air particlesmade light is reflected by air particles, , yielding bright night skyyielding bright night sky–– this is this is light pollutionlight pollution

Light PollutionLight Pollution

90% of Earth90% of Earth’’s population cannot see the Milky Ways population cannot see the Milky Way

How many light bulbs does it take to How many light bulbs does it take to screw up an astronomer?screw up an astronomer?

An immediately curable pollution: simply turn the lights off!

Stop “uplight”, glare: wastes billions of $$ in energy, use “low pressure sodium”

Several famous observatories are now useless…

LA Basin View from Mt. Wilson LA Basin View from Mt. Wilson Observatory, 1908 and 1998Observatory, 1908 and 1998

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Quest for Good Weather and SeeingQuest for Good Weather and Seeing

•• Mauna Kea, Big Mauna Kea, Big Island of HawaiiIsland of Hawaii, , 14,00014,000’’ elevation, elevation, middle of the middle of the PacificPacific

•• Dry, high, dark and Dry, high, dark and isolatedisolated. Best on . Best on the planet?the planet?

.

Sites in Arizona, Chile,Sites in Arizona, Chile,Canary Islands, EuropeCanary Islands, Europe….

Concept of Concept of ““Angular ResolutionAngular Resolution””•• Ability to separate two objectsAbility to separate two objects•• Angle between two objects Angle between two objects

decreases as your distance to decreases as your distance to them increasesthem increases

•• Smallest angleSmallest angle at which you can at which you can distinguish two objects is your distinguish two objects is your angular resolutionangular resolution

Two Properties of Two Properties of AnyAny TelescopeTelescope1. 1. ResolutionResolution

• smallest angle which can be seen:•• θθ = 1.22 = 1.22 λλ / 2 R/ 2 R

2. Light2. Light--Collecting AreaCollecting Area•• think of telescope as a think of telescope as a ““photon bucketphoton bucket””• its area: A = A = ππ RR22

( ( λλ is light is light wavelengthwavelength, R is mirror or lens , R is mirror or lens radiusradius ))

Kyle will discuss RESOLUTION in recitationKyle will discuss RESOLUTION in recitation

Adaptive Optics (AO) Adaptive Optics (AO) –– ““dede--twinkletwinkle”” starsstars• Wavefronts of star light are deformed by atmosphere• Can distort shape of mirror (very fast) to correct for

distortions by atmosphere – hot new technology

AO mirror OFFAO mirror OFF AO mirror ONAO mirror ON

Two stars!Two stars! JUST WHY DO BIG OPTICAL TELESCOPES JUST WHY DO BIG OPTICAL TELESCOPES USE USE MIRRORSMIRRORS (REFLECTORS) AND (REFLECTORS) AND NOTNOTLENSES (REFRACTORS) ?LENSES (REFRACTORS) ?

DISCUSSION TOPICDISCUSSION TOPIC

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Why big aperture telescopes are reflectors Why big aperture telescopes are reflectors

•• Can Can support mirror from backsupport mirror from back, not just at edges as , not just at edges as with lenses (biggest: 1 m lens, 10 m with lenses (biggest: 1 m lens, 10 m diamdiam mirror)mirror)

•• Mirror needs only Mirror needs only one good optical surfaceone good optical surface to be to be ground, not four as with achromatic (2 ground, not four as with achromatic (2 elemelem) lens) lens

•• Can Can recoat mirror surfacerecoat mirror surface easily with highly easily with highly reflective aluminum (even silver)reflective aluminum (even silver)

•• Lens has to be Lens has to be optically pure and uniformoptically pure and uniform, but , but mirror can be made of anything that holds its mirror can be made of anything that holds its shape (shape (fuzedfuzed quartz, zero quartz, zero expanexpan pyroceramicspyroceramics, , even beryllium)even beryllium)

Instruments in the Focal PlaneInstruments in the Focal Plane

1. ImagingImaging• use camera to take pictures (images)• photometry → measure amount and color

(with filters) of light from object2.2. SpectroscopySpectroscopy

• use spectrograph to separate light in detail into its different wavelengths (colors)

3. TimingTiming• measure how amount of light changes with

time (sometimes in a fraction of a second)

How astronomers use light collected by a telescope:How astronomers use light collected by a telescope:

Imaging (Digital with Imaging (Digital with CCDsCCDs))

• Filters are placed in front of camera to allow only certain colors to be imaged

• Single color images are superimposed to form “true color” images.

Spectroscopy Spectroscopy –– analyzing the light analyzing the light

•• Spectrograph reflects Spectrograph reflects light off a light off a gratinggrating: finely : finely ruled, smooth surfaceruled, smooth surface

•• Light (by interference) Light (by interference) disperses into colorsdisperses into colors

•• This This spectrumspectrum is is recorded by digital CCD recorded by digital CCD detectordetector

Which RADIATIONWhich RADIATIONgets through ourgets through ourATMOSPHERE ?ATMOSPHERE ?

GOODGOOD

LOUSYLOUSY

Atmospheric Absorption of Atmospheric Absorption of ““LightLight””• Earth’s atmosphere absorbs most types of light

(not entirely bad, or we would be dead!)• Only visible, radio, and some IR and UV light get

through to the ground

To observe other wavelengths, must put telescopes in space!To observe other wavelengths, must put telescopes in space!

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So what gets through our atmosphere?So what gets through our atmosphere?

•• RADIO WAVESRADIO WAVES:mostly get throughmostly get through

•• Thus radio telescopes are Thus radio telescopes are built on the groundbuilt on the ground

•• Weather is not an issue Weather is not an issue --radio waves come right radio waves come right through the cloudsthrough the clouds

Infrared TelescopesInfrared Telescopes•• INFRAREDINFRARED can be can be

absorbed by molecules absorbed by molecules like Hlike H220, CO0, CO22, CO, etc. , CO, etc.

•• Absorption is in specific Absorption is in specific wavebands, leaving wavebands, leaving ““windowswindows”” where we can where we can see above the see above the atmosphereatmosphere

•• Combination of groundCombination of ground--based, airplane, balloon, based, airplane, balloon, rockets, satelliterockets, satellite……

UV, XUV, X--rays and Gammarays and Gamma--raysrays•• These all have These all have enough energy to ionize electronsenough energy to ionize electrons out of out of

atoms or atoms or break apartbreak apart moleculesmolecules

Heavily absorbed by the atmosphere Heavily absorbed by the atmosphere

•• Space or high altitude (balloon, rocket) observatories are Space or high altitude (balloon, rocket) observatories are necessarynecessary

Clicker Q Clicker Q -- galaxygalaxy•• In observing a distant galaxy, the In observing a distant galaxy, the H alpha H alpha

spectral line of hydrogenspectral line of hydrogen (usually in the (usually in the visible) is now in the visible) is now in the IR portion of the IR portion of the spectrumspectrum. What can you conclude?. What can you conclude?

•• A. Galaxy is made purely of hydrogenA. Galaxy is made purely of hydrogen•• B. Galaxy is moving away from usB. Galaxy is moving away from us•• C. Galaxy is moving towards usC. Galaxy is moving towards us•• D. Galaxy has very weak gravityD. Galaxy has very weak gravity

B.B.REDSHIFTEDREDSHIFTED

Space Based TelescopesSpace Based Telescopes

Chandra XChandra X--ray Observatoryray ObservatoryHubble Space Telescope (HST)Hubble Space Telescope (HST)

optical, UVoptical, UV

SIRTF: Space SIRTF: Space InfraredInfraredTelescope FacilityTelescope Facility

now now SPITZERSPITZER•• Launched August 03Launched August 03•• Trails behind Earth to get Trails behind Earth to get

away from Earthaway from Earth’’s thermal s thermal spectrumspectrum

•• 0.85m aperture , T ~ 5.5 K0.85m aperture , T ~ 5.5 K•• Cooled with liquid heliumCooled with liquid helium, ,

22--5 years worth5 years worth

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Hubble Space Hubble Space Telescope (HST)Telescope (HST)

•• VISIBLE and UVVISIBLE and UVvisible: atmosphere is atmosphere is transparent but turbulent transparent but turbulent (seeing)(seeing)

•• HST: Small (2.5 meters), HST: Small (2.5 meters), diffractiondiffraction--limitedlimited

•• Low orbit accessible by Low orbit accessible by Shuttle, refurbishing means Shuttle, refurbishing means long lifetime (1990long lifetime (1990--2010+?) 2010+?)

•• Costs: $5 billion over 20 Costs: $5 billion over 20 years, or 10 years, or 10 -- 100 times 100 times more than ground scopesmore than ground scopes

How do you point a space How do you point a space telescope in orbit ?telescope in orbit ?

ANGULAR MOMENTUM ANGULAR MOMENTUM DEMONSTRATIONDEMONSTRATION

1.1. Squirt from jetsSquirt from jets to change direction to change direction (hydrazine)(hydrazine)

2.2. TorqueTorque by electric currents in big coilsby electric currents in big coilswhile flying through Earthwhile flying through Earth’’s magnetic fields magnetic field

3.3. TorqueTorque by electric motors spinning up by electric motors spinning up or down or down ““reaction wheelsreaction wheels””

Next to Our Nearest Star Next to Our Nearest Star Chap 14Chap 14