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STARS AS BLACKBODIES3RF Sciences, LLC
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Blackbody defined…
A blackbody is an object that absorbs all light that hits it
Also emits light provided that its temperature is above absolute zero
http://www.handprint.com/HP/WCL/IMG/bbody.gif
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A Blackbody…
Perfect “black body” – something which absorbs all the radiation that falls on it Good absorber of radiant heat is also a good
emitter Main scientist - 1859, G. Kirchhoff
Foundation of blackbody radiation lies in the idea that radiation is released from blackbodies in the form of "quanta" or discrete packets of light called photons Main scientist – 1900, Max Planck
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More about a Blackbody…
Is the best possible emitter of radiant energy
Must both radiate and absorb energy at the same rate in order to maintain a constant temperature
Total radiation from a black body depends only on temperature of the body, not on chemical or physical characteristics
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Plotting Curves
A curve can be generated plotting the temperature, intensity, or brightness of the black body versus the wavelength coming from it.
These curves are sometimes called Planck curves.
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Blackbody curves, 4 objects
a) Cool, invisible galactic gas cloud called Rho Ophiuchi. Temperature of 60 K Emits mostly low-
frequency radio radiation
http://www.daf.on.br/jlkm/astron2e/AT_MEDIA/CH03/CHAP03AT/AT03FG13.JPG
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Blackbody curves, 4 objects
b) A dim, young star (shown here in red) near the center of the Orion Nebula. Temperature of star's
atmosphere ~ 600 K Radiates primarily in
infrared (IR) http://www.daf.on.br/
jlkm/astron2e/AT_MEDIA/CH03/CHAP03AT/AT03FG13.JPG
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c) The Sun Surface ~ 6000 K Brightest in the
visible (v) region of the electromagnetic spectrum
http://www.daf.on.br/jlkm/astron2e/AT_MEDIA/CH03/CHAP03AT/AT03FG13.JPG
Blackbody curves, 4 objects
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Blackbody curves, 4 objects
d) A cluster of very bright stars, called Omega Centauri, as observed by a telescope aboard the space shuttle Temperature ~ 60,000
K Radiate strongly in
ultraviolet (UV) http://www.daf.on.br/jlkm/
astron2e/AT_MEDIA/CH03/CHAP03AT/AT03FG13.JPG
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How is a star a blackbody?
Because blackbody radiation is solely dependent on temperature (simple)
And to maintain a constant temperature, a blackbody must emit radiation in the same amount as it absorbs
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Wein’s Law
The hotter a blackbody becomes, the shorter its wavelength of peak emission becomes
The wavelength of peak emission is simply the wavelength at which a blackbody emits most of its radiation
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Wein’s Law
1893, German physicist Wilhelm Wien Quantified relationship between
blackbody temperature and wavelength of spectral peak
λmax = 2.9 x 10-3 (microns)/T λmax (lambda max) = wavelength of Peak
emission 2898 microns T = temperature of Blackbody in Kelvin (K)
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Wein’s Law in action…
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Plank Curves - 1
1900 , Max Planck Electromagnetic radiation absorbed or
emitted only in “chunks” of energy, quanta, E Quanta are proportional to the frequency of
the radiation E = h. (Constant of proportionality “h” is Planck's constant.)
Wanted to understand the shape of Wien's radiative energy distribution as a function of frequency.
http://abyss.uoregon.edu/~js/glossary/planck_curve.html
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http://www.oglethorpe.edu/faculty/~m_rulison/Astronomy/Dictionary/Laws%20of%20Radiation_files/radiation_curve.gif
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Plank Curves - 2
Postulated that radiators or oscillators can only emit electromagnetic radiation in finite amounts of energy of size.
At a given temperature T, there is not enough thermal energy available to create and emit many large radiation quanta.
More large energy quanta can be emitted when temperature is raised.
http://abyss.uoregon.edu/~js/glossary/planck_curve.html
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Plank’s Law
The amount of blackbody radiative flux emitted by a blackbody for a given wavelength is given by Planck's Law:
Where T is object temperature (in degrees Kelvin); l is wavelength in microns; units are (W/m2) per micron
The wavelength of peak emission is:
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Stefan–Boltzmann Law
Independently formulated by Josef Stefan (1879) and Ludwig Boltzmann (1884, 1889)
Relationship between radiant energy and temperature for a black body radiator
Relates total radiant flux (F) (in W/m2), from surface of black body to its temperature (T)
F= σ T4
σ = 5.6703 x 10-8 watt / m2 K4
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Stefan–Boltzmann Law 2
How much power a blackbody radiates per unit area of its surface
For a blackbody of temperature T, the power radiated per unit area is: P = constant x T4
http://zebu.uoregon.edu/~imamura/122/images/stefanboltzmanlaw.jpg
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Stefan–Boltzmann Law
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Why use Stefan-Boltzmann(S-B) Law?
Using the Stefan-Boltzmann law in conjunction with other known quantities, it can be used to infer properties of a star
For example, if a star radiates like a blackbody, then the luminosity of the star can be written as L = (Surface Area of the Star) x (power per unit
area produced by the star)= 12.6 x R2 x constant x T4 So, if we know certain information (obtained through independent means) about a star, we can infer other properties. For example,
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What can we learn from S-B law?
If we know the luminosity and temperature, we can infer the radius of the star;
If we know the luminosity and radius of a star, we can infer its temperature;
If we know the radius and temperature of a star, we can infer its luminosity
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Blackbody Review
Stefan-Boltzmann Law - Area under the curve increases as the temperature is increased
Wien's Law – Peak of the curve in emitted energy changes wavelength
Planck’s Law – Peak of the curve or the peak emission wavelength of a blackbody is related to the temperature of the object – hotter objects emit in higher wavelengths.