Luminosity and intensity of starsEdexcel A-Level Physics: Subtopic test

1. 1
   An astronomer measures the intensity of the light received from distant stars using a telescope fitted with a sensitive detector.

   (a)

   What is meant by the luminosity of a star?

   [1 mark]

   • AThe power received per unit area at the Earth
   • BHow bright the star looks to the naked eye
   • CThe energy emitted per unit area of the star's surface
   • DThe total power radiated by the star

   (b)

   Another star has the same luminosity as the first but is three times further from the Earth. How does the intensity received from it compare?

   [1 mark]

   • AOne third as great
   • BOne ninth as great
   • CThree times as great
   • DNine times as great

   (c)

   Explain why the intensity of the light received from a star decreases as the distance from the star increases.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The Sun has a luminosity of 3.85 × 10²⁶ W. The mean distance from the Sun to the Earth is 1.5 × 10¹¹ m. Assume that the Sun radiates equally in all directions and that no radiation is absorbed on its way to the planets.

   (a)

   Calculate the intensity of the sunlight at the Earth.

   [1 mark]

   • A1.4 × 10³ W m⁻²
   • B1.7 × 10⁴ W m⁻²
   • C2.0 × 10¹⁴ W m⁻²
   • D5.4 × 10³ W m⁻²

   (b)

   The planet Jupiter is 5.2 times further from the Sun than the Earth. What is the intensity of sunlight at Jupiter?

   [1 mark]

   • A2.6 × 10² W m⁻²
   • B7.1 × 10³ W m⁻²
   • C5.0 × 10¹ W m⁻²
   • D9.7 W m⁻²

   (c)

   The planet Mars is 2.3 × 10¹¹ m from the Sun. Calculate the intensity of sunlight at Mars.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Astronomers measure the intensity of light from a bright star, S, as 1.2 × 10⁻⁷ W m⁻² at the Earth. Star S is 8.1 × 10¹⁶ m from the Earth.

   (a)

   Calculate the luminosity of star S.

   [3 marks]

   (b)

   A second star, T, has a luminosity four times that of S and produces the same intensity at the Earth. Calculate the distance from the Earth to star T.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The Sun has a luminosity of 3.85 × 10²⁶ W. A distant star, Y, has a luminosity of 1.2 × 10³¹ W, and the intensity of its light measured at the Earth is 3.0 × 10⁻⁹ W m⁻².

   (a)

   Explain how the equation $I = L/4\pi d^2$ arises, and state two assumptions that must be made for it to give the correct intensity from star Y.

   [6 marks]

   (b)

   Calculate the distance from the Earth to star Y. Hence determine how the intensity received from the Sun would compare with that from star Y if the Sun were at the distance of star Y.

   [6 marks]

   Total for question 4: 12 marks

End of questions