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 Universejames.lattimer@stonybrook.edu

James Lattimer AST 248, Lecture 3

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

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

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

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

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

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

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

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

Doppler Effect in a Binary

James Lattimer AST 248, Lecture 3

Doppler Effect Applicationsww

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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

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

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