Starshine

Question 1

what is the definition of luminosity

Answer 1

the rate at which energy of all types is radiated by an object in all directions

Question 2

therefore what is the luminosity of a star

Answer 2

the power emitted from the star

Question 3

what two factors impact luminosity

Answer 3

• size of the object

- its temperature

Question 4

what is a black body radiator

Answer 4

• a theoretical perfect emitter

- that gives off energy across the entire electromagnetic spectrum

Question 5

what does the stefan-boltzmann law state

Answer 5

• the output power from a black body is proportional to its surface area
• and the fourth power of its temperature in K

Question 6

what are the axes for the distribution graph given by the law

Answer 6

• 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)

Question 7

what does the distribution graph show

Answer 7

• 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

Question 8

what is the equation for stefan-boltzmann’s law

Answer 8

• L = oAT^4
• L = luminosity
• o (sigma) = stefan-boltzmann constant (5.67x10-8 Wm-2K-4)
• A = surface area (m^2)
• T = temperature (K)

Question 9

what would be the modified equation for a sphere like a star

Answer 9

L = o(4 pi r^2)T^4

Question 10

what does stefan-boltzmanns distribution show about the relationships between the energy output and wavelengths of light emitted from a black body

Answer 10

• 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

Question 11

what is the law that describes this relationship between the output wavelength and temperature

Answer 11

weins law

Question 12

what is the formula for weins law

Answer 12

• Y(max) * T = 2.898x10-3 mK
• Y(max) = max wavelength (Y = lambda)
• T = temperature (K)
• 2.898x10-3 = weins constant

Question 13

what are the average temperatures for a red giant, white dwarf and blue supergiant

Answer 13

• red giant = 3,000K
• white dwarf = 10,000K
• blue supergiant = 25,000K