AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and...

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AST 248, Lecture 3 James Lattimer Department of Physics & Astronomy 449 ESS Bldg. Stony Brook University February 9, 2015 The Search for Intelligent Life in the Universe [email protected] James Lattimer AST 248, Lecture 3

Transcript of AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and...

Page 1: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

AST 248, Lecture 3

James Lattimer

Department of Physics & Astronomy449 ESS Bldg.

Stony Brook University

February 9, 2015

The Search for Intelligent Life in the [email protected]

James Lattimer AST 248, Lecture 3

Page 2: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Properties of Light and Radiation

I Speed is c = 3× 1010 cm/s = 186,000 mi/s = 6.7 · 108

mph.

I Distance traversed in 1 year is called a light-year.

I Has properties characteristic of both waves and particles.

I Wave nature: Wavelength × Frequency = c (Speed)λ× ν = c

I Wavelengths from γ−rays (λ ∼ 10−13 cm) toradio (λ ∼ 103 cm).

I Visible light is opticalradiation, 3× 10−5 cm <λ < 7× 10−5 cm.

James Lattimer AST 248, Lecture 3

Page 3: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Why Astronomers Need Space-Based Telescopes

γ rays

X rays

ultraviolet

near–IRfar–IR

mm waves

microwaves

radio waves

Greenhouse Effect

James Lattimer AST 248, Lecture 3

Page 4: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Photons and Inverse Square Law

I Particle nature: Smallest unit of radiation is a photon.

I Photon energy is proportional to frequency E = hν

h is Planck’s constant,6.6 · 10−27 cm2 g s−1

I Brightness (B) or Intensity(I ) is apparent flux from anobject while Luminosity (L) isthe intrinsic or absolutepower output.

I Inverse square law ofbrightness – brightness(intensity) diminishes asdistance squared:B = L/D2 Jose Wudka physics.ucr.edu

James Lattimer AST 248, Lecture 3

Page 5: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Wein’s Law

I Radiation from aluminous object isemitted at virtually allwavelengths, but peakwavelength ofdistribution is inverselyproportional totemperatureλ = 0.29 cm/T ◦ KAKA Wien’s Law

I Therefore the color of astar is a measure of itstemperature.

hyperphysics.phy-astr.gsu.edu/hbase/wien.html

James Lattimer AST 248, Lecture 3

Page 6: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Luminosity

I Total power (luminosity)emitted by a luminousobject (e.g., a star) isdependent upon bothtemperature T andobject size or radius R :L = 4πR2σT 4

σ = 5.567 ·10−5 erg cm−2 K−4 isradiation constant

I This is seen by examingthe total area under theemitted power densitycurves

hyperphysics.phy-astr.gsu.edu/hbase/wien.html

James Lattimer AST 248, Lecture 3

Page 7: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Spectroscopy

I A prism can spreadlight into a spectrum.

I Spectra haveabsorption and/oremission lines, eachcharacteristic ofan electronic transitionin an atomor molecule. Candeduceelemental or chemicalcomposition of stars.

Wikipedia figures

Spectrum of fluorescent lightJames Lattimer AST 248, Lecture 3

Page 8: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Solar SpectrumNigel Sharp NOAO

Dark lines are absorption lines produced by cooler gas abovehot solar surface, and each is due to a specific element (atom)or molecule. Most, but not all, spectral lines have been identified.The chemical composition and temperature of the absorbing gascan therefore be determined.

James Lattimer AST 248, Lecture 3

Page 9: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Doppler Effect

I Speed of light isindependent of source’svelocity (Einstein’s SpecialTheory of Relativity)

I The observed wavelength ofa photon or a spectral linedepends upon the relativevelocities of the source andthe observer.

I The observed change in thewavelength, the Dopplershift, is proportional to thenet relative speed differencebetween the source andobserver: ∆λ

λ = vc

I If the source is movingtowards (away from) you, youobserve a blue-(red-)shift.

I Song to explain it all:www.astrocappella.com/doppler.shtml

James Lattimer AST 248, Lecture 3

Page 10: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Doppler Effect in a Binary

James Lattimer AST 248, Lecture 3

Page 11: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Doppler Effect Applicationsww

w.k

ing

su.a

b.ca

/b

rian

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rse/lectu

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For an interesting demoabout the Doppler effectin binary stars, see

instruct1.cit.cornell.edu/courses/astro101/java/binary/binary.htm

James Lattimer AST 248, Lecture 3

Page 12: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

The Sun and Other Stars

I L� = 4 · 1033 erg/s

I Yellow color means that thepeak wavelength of theSun’s spectrum isλmax ' 5 · 10−5 cm

I Use Wien’s Law to find theSun’s surface temperature:T� = 0.29 cm/λmax

' 6000 K

I Invert the blackbodyluminosity formula to derivethe solar radius:R� =

√L�

4πσT4�

= 7 · 1010 cm

General Properties of StarsI Mass: 0.1–100 M�

I Luminosity: 0.0001–106 L�I Radius: 0.1–1400 R�I Surface Temperature:

2000–40,000 K

I Some ”stars”, white dwarfsand neutron stars, havemore extreme properties.

I Objects between 10 Jupitermasses and 0.02M� beginas stars, then cool off asbrown dwarfs.

James Lattimer AST 248, Lecture 3

Page 13: AST 248, Lecture 3 - Stony Brook University · James Lattimer AST 248, Lecture 3. Properties and Types of Stars The main physical properties of stars are their luminosity L, surface

Properties and Types of Stars

The main physicalproperties of starsare their luminosityL, surfacetemperature T ,radius R and massM . E. Hertzsprungand H. Russeldiscovered thatplotting L vs. T wasa useful way todiscriminate types ofstars.

www.daviddarling.info/encyclopedia/H/HRdiag.html

James Lattimer AST 248, Lecture 3