black body radiation physics 113 goderya chapter(s): 7 learning outcomes:
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Black Body RadiationPhysics 113 Goderya
Chapter(s): 7Learning Outcomes:
The Amazing Power of Starlight
Just by analyzing the light received from a star, astronomers can retrieve information about a star’s
1. Total energy output
2. Mass
3. Surface temperature
4. Radius
5. Chemical composition
6. Velocity relative to Earth
7. Rotation period
Temperature Scales
• o F = 9/5(o K) -459.4
• o F = 9/5(o C) + 32
• o K = 273 + o C
o Co K o F
0 32273
100 212373
-100173 -148
Brightness and Luminosity
• Luminosity (L): The total amount of energy a star radiates in 1 second per square meter.
• Brightness (B): Energy received from the source at different distances.
• Inverse square Law:B ≈ 1/(distance)2
• B = L / 4πR2
• Sun 5 x 1026 watts
4 /12 = 4
4 / 22 = 1
Light rays
Star as a Black Body
• Black Body Radiator. A hypothetical object that emits Electromagnetic radiation and whose spectrum is continuous with a peak in the wavelength that corresponds to the temperature of the object.
Wavelength
Ene
rgy
Peak wavelength
Black Body Radiation (1)The light from a star is usually concentrated in a rather narrow range of wavelengths.
The spectrum of a star’s light is approximately a thermal spectrum called a black body spectrum.
A perfect black body emitter would not reflect any radiation. Thus the name “black body”.
Two Laws of Black Body Radiation
2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases. Wien’s displacement law:
max ≈ 3,000,000 nm / TK
(where TK is the temperature in Kelvin).
1. The hotter an object is, the more luminous it is:
L = A**T4
where = Stefan-Boltzmann constant A = surface area;
Sun’s Temperature• The sun =500
nm
• T = 3 x 10 6/500 = 6000 K
• 10,000 F
• Wein’s Law gives the surface temperature
Sun’s Luminosity• The sun: T=
6000 K , R=7 x 108 meters. What is its Luminosity?
• L = 4x 3.14 x (7 x 10 8)2 x 6 x 10-8 (6000)4 = 5 x 1026 Watts
• Compare with 40 watts light bulb
Color and Temperature
Orion
Betelgeuse
Rigel
Stars appear in different colors,
from blue (like Rigel)
via green / yellow (like our sun)
to red (like Betelgeuse).
These colors tell us about the star’s
temperature.
The Color Index (1)
B bandV band
The color of a star is measured by comparing its brightness in two different wavelength bands:
The blue (B) band and the visual (V) band.
We define B-band and V-band magnitudes just as we did before for total magnitudes (remember: a larger number indicates a fainter star).
The Color Index (2)
We define the Color Index
B – V(i.e., B magnitude – V magnitude).
The bluer a star appears, the smaller the color index B – V.
The hotter a star is, the smaller its color index B – V.