a. Define the luminosity of a star.

b. A red giant is a star bigger than our Sun. Explain how the surface of a red giant star can be cooler than the Sun, yet have a luminosity much greater than the Sun.

c. An astronomer has determined the surface temperature of a white dwarf star to be 7800 K and its radius as 8.5 × 10⁶ m. Calculate the luminosity of this star.

d. The surface temperature T of a star depends on the wavelength λ_max at the peak intensity of the emitted radiation from the star. The T against λ_max graph for a cluster of stars in our galaxy is shown.

i. Use the graph to show Wien’s displacement law is obeyed.
ii. Estimate the surface temperature of a star with λ_max = 400 ± 10 nm. In your answer, include the absolute uncertainty.

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5000- 4000 - 3000- 2000 500 600 700 800 900 1000 2/nm Figure 31.11 T/K