Question 1

Paper 2Medium11 marks

An observer on Earth measures the apparent brightness of stars A and B shown in Figure 1. Assume that both stars emit uniformly in all directions and that absorption between each star and Earth is negligible.

Earth (observer)dA=2.0×1017 mdB=4.0×1017 mAbA=3.0×10−8 W m−2BbB=1.0×10−8 W m−2
(a)

State which star has the greater apparent brightness.

[1]
(b)

Determine the ratio LB/LAL_B/L_A

[3]
(c)

Calculate the luminosity of star A.

[2]
(d)

A third star, C, has twice the luminosity of star A and an apparent brightness of 1.2×10−8 W m−21.2\times10^{-8}\,\mathrm{W\,m^{-2}} Determine its distance from Earth.

[3]
(e)

Explain why the apparent brightness of a star decreases as its distance from Earth increases, if its luminosity is unchanged.

[2]

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