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
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
w.k
ing
su.a
b.ca
/b
rian
/a
stro/
cou
rse/lectu
res/fa
ll/a
20
0l1
0g
.htm
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