Worksheet: Luminosity

In this worksheet, we will practice calculating the flux density due to the energy radiated by a star at a given distance away from the star.

Q1:

At what distance from the Sun is the flux density equal to 300 W/m2? Give your answer in astronomical units to 3 significant figures. Use a value of 1 . 5 0 × 1 0 m for the length of 1 AU and 3 . 8 3 × 1 0 W for the luminosity of the Sun.

Q2:

The Peony star, which is located near the galactic center, has a luminosity of 3 . 2 × 1 0 𝐿 . At what distance from this star is the flux density equal to the flux density from the Sun at Earth? Use a value of 1 . 5 0 × 1 0 m for the length of 1 AU and 3 . 8 3 × 1 0 W for the luminosity of the Sun. Give your answer in astronomical units to 3 significant figures.

Q3:

Which of the following is the correct unit of luminous flux density?

  • A W/m2
  • B J/m2
  • C W⋅m
  • D W/m
  • E W⋅m2

Q4:

The flux density at a distance of 15.6 AU from a star is 89.4 W/m2. What is the luminosity of the star? Give your answer in terms of the luminosity of the Sun to 3 significant figures. Use a value of 1 . 5 0 × 1 0 m for the length of 1 AU and 3 . 8 3 × 1 0 W for the luminosity of the Sun.

  • A8.03 𝐿
  • B5.11 𝐿
  • C16.1 𝐿
  • D6.53 𝐿
  • E4.02 𝐿

Q5:

A star has a luminosity of 4 𝐿 . What is the flux density from the star at a distance of 3 AU? Use a value of 3 . 8 3 × 1 0 W for the luminosity of the Sun and 1 . 5 0 × 1 0 m for the length of 1 AU. Give your answer to 3 significant figures.

Q6:

A star has a luminosity of 7 . 6 × 1 0 W. What is the flux density from the star at a distance of 1 . 5 × 1 0 m? Give your answer to 3 significant figures.

Q7:

Star A has a luminosity 𝐿 and star 𝐵 has a luminosity 𝐿 = 4 𝐿 . At a distance 𝑑 away from star 𝐴 and a distance 𝑑 away from star 𝐵 , the flux densities due to each star are equal. What is the ratio 𝑑 𝑑 ?

  • A1
  • B4
  • C 1 2
  • D 1 4
  • E2

Q8:

The flux density at a distance of 0.7 AU from a star is 2,800 W/m2. What is the luminosity of the star? Use a value of 1 . 5 0 × 1 0 m for the length of 1 AU. Give your answer in watts to 3 significant figures.

  • A 9 . 7 0 × 1 0 W
  • B 1 . 9 8 × 1 0 W
  • C 5 . 6 6 × 1 0 W
  • D 7 . 9 2 × 1 0 W
  • E 3 . 8 8 × 1 0 W

Q9:

The Sun has a luminosity of 3 . 8 3 × 1 0 W. What is the flux density from the Sun at Neptune, a distance of 30.1 AU away from the Sun? Use a value of 1 . 5 0 × 1 0 m for the length of 1 AU. Give your answer to 3 significant figures.

Q10:

Sirius A is a nearby star with a luminosity of 9 . 7 3 × 1 0 W. The luminosity of the Sun, 𝐿 , is 3 . 8 3 × 1 0 W. What is the luminosity of Sirius A in terms of 𝐿 ? Give your answer to 3 significant figures.

  • A 3 . 9 4 𝐿
  • B 1 2 . 6 𝐿
  • C 2 5 . 4 𝐿
  • D 0 . 2 5 4 𝐿
  • E 0 . 0 3 9 4 𝐿

Q11:

Which of the following is the correct unit of luminosity?

  • A W⋅m2
  • B W/m2
  • C W
  • D J
  • E W/m

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