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 1.50×10 m for the length of 1 AU and 3.83×10 W for the luminosity of the Sun.

Q2:

The Peony star, which is located near the galactic center, has a luminosity of 3.2×10𝐿. At what distance from this star is the flux density equal to the flux density from the Sun at Earth? Use a value of 1.50×10 m for the length of 1 AU and 3.83×10 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?

  • AW⋅m
  • BW/m
  • CW/m2
  • DW⋅m2
  • EJ/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 1.50×10 m for the length of 1 AU and 3.83×10 W for the luminosity of the Sun.

  • A4.02 𝐿
  • B6.53 𝐿
  • C5.11 𝐿
  • D16.1 𝐿
  • E8.03 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 3.83×10 W for the luminosity of the Sun and 1.50×10 m for the length of 1 AU. Give your answer to 3 significant figures.

Q6:

A star has a luminosity of 7.6×10 W. What is the flux density from the star at a distance of 1.5×10 m? Give your answer to 3 significant figures.

Q7:

Star A has a luminosity 𝐿 and star 𝐵 has a luminosity 𝐿=4𝐿. 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 𝑑𝑑?

  • A1
  • B2
  • C12
  • D4
  • E14

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 1.50×10 m for the length of 1 AU. Give your answer in watts to 3 significant figures.

  • A1.98×10 W
  • B3.88×10 W
  • C9.70×10 W
  • D7.92×10 W
  • E5.66×10 W

Q9:

The Sun has a luminosity of 3.83×10 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 1.50×10 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 9.73×10 W. The luminosity of the Sun, 𝐿, is 3.83×10 W. What is the luminosity of Sirius A in terms of 𝐿? Give your answer to 3 significant figures.

  • A12.6𝐿
  • B0.0394𝐿
  • C25.4𝐿
  • D0.254𝐿
  • E3.94𝐿

Q11:

Which of the following is the correct unit of luminosity?

  • AW/m2
  • BW/m
  • CW
  • DJ
  • EW⋅m2

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