Stars as Black Bodies Flashcards
What is a black body?
A perfect absorber and emitter
A body that absorbs and emits all wavelengths of EM radiation
Appears perfectly black when cold
When heated above absolute zero, it emits light across the whole EM spectrum
Objects hotter than absolute zero emit electromagnetic radiation due to what?
Their temperature.
What radiation is everyday temperature?
Infrared (which we can’t see)
Why do things glow (and emit visible light) if we heat them up?
The wavelengths of radiation emitted depend on the temperature of the object.
(High energy means shorter wavelength - visible shorter than infrared)
In what way does a pure black surface emit radiation?
strongly and well defined
The graph of intensity against wavelength varies with what?
TEMPERATURE
What does black body radiation produce?
A Continuous spectrum of EM radiation
What are the main two characteristics of a black body curve, with power output against wavelength?
peak on short wavelengths, so steeper on the curves left side
What does the peak in a black body curve give?
Temperature
What things are considered approximately black bodies?
Stars (and other astronomical sources)
The power output is what?
The luminosity
Total energy it emits per second
What is power output related to?
Temperature and surface area
What does Wein’s law say?
For a hot object, the wavelength of peak emission intensity is inversely proportional to the absolute temperature of the object.
(Higher surface temperature of a star = shorter peak wavelength)
What does a hotter black body mean?
Emit more radiation
Peak wavelength is a shorter wavelength
Higher total energy output (assuming surface area is the same)
Why may hotter stars not be as bright?
May mostly emit radiation that isn’t visible light.
If a cooler star emits more visible light radiation, it will appear brighter.
P = σ = A = T = What is each one? What is it measured in? What is the equation?
Stefan’s Law:
Power output in watts
Stefan’s Constant
Surface area in meters squared
Temperature in Kelvin
What does the intensity equation show?
As radiation spreads out and becomes more diluted. Intensity decreases.
