what is the definition of luminosity
the rate at which energy of all types is radiated by an object in all directions
therefore what is the luminosity of a star
the power emitted from the star
what two factors impact luminosity
- size of the object
- its temperature
what is a black body radiator
- a theoretical perfect emitter
- that gives off energy across the entire electromagnetic spectrum
what does the stefan-boltzmann law state
- the output power from a black body is proportional to its surface area
- and the fourth power of its temperature in K
what are the axes for the distribution graph given by the law
- the y axis is the energy output (increasing upwards)
- the x axis is the wavelength
- with the start being the shorter wavelengths (blue)
- and the end being the longer wavelengths (red)
what does the distribution graph show
- there is a distribution for 6000, 4500 and 3000K, coloured red, blue and green
- the 6000K has the highest peak which is closest to the blue end, finishing off by tapering off into the red end
- the 4500K has the second highest peak which is second closest to the blue end, tapering off
- the 3000K has the lowest peak which is closest to the red end, tapering off
what is the equation for stefan-boltzmann’s law
- L = oAT^4
- L = luminosity
- o (sigma) = stefan-boltzmann constant (5.67x10-8 Wm-2K-4)
- A = surface area (m^2)
- T = temperature (K)
what would be the modified equation for a sphere like a star
L = o(4 pi r^2)T^4
what does stefan-boltzmanns distribution show about the relationships between the energy output and wavelengths of light emitted from a black body
- at higher energy outputs (temperatures) the peak is more pronounced and reached sooner
- so the wavelength of the peak output gets shorter as the temperature rises
what is the law that describes this relationship between the output wavelength and temperature
weins law
what is the formula for weins law
- Y(max) * T = 2.898x10-3 mK
- Y(max) = max wavelength (Y = lambda)
- T = temperature (K)
- 2.898x10-3 = weins constant
what are the average temperatures for a red giant, white dwarf and blue supergiant
- red giant = 3,000K
- white dwarf = 10,000K
- blue supergiant = 25,000K
