CAIE A-Level Physics 25.1 Standard Candles

Practise calculating luminosity and radiant flux, applying the inverse-square relationship and using standard-candle observations to determine astronomical distance.

Exam points

define and calculate stellar luminosity and radiant flux intensity
use the inverse-square relationship F = L/(4?d?) to determine luminosity, flux or distance
define a standard candle and use its known luminosity with measured flux
determine the distance of a galaxy from standard-candle observations

Covered syllabus points

25.1.1—The term luminosity as the total power of radiation emitted by a star 25.1.2—The inverse square law for radiant flux intensity F 25.1.3—An object of known luminosity is called a standard candle 25.1.4—The use of standard candles to determine distances to galaxies

[Maximum number: 3]

(a)

State what is meant by luminosity of a star.

[ 1 ]

(b)

The luminosity of the Sun is $3.83 \times 10^{26} \mathrm{~W}$ . The distance between the Earth and the Sun is $1.51 \times 10^{11} \mathrm{~m}$ .

Calculate the radiant flux intensity F of the Sun at the Earth. Give a unit with your answer.

unit

[ 2 ]

$\begin{aligned} F=\frac{L}{4 \pi d^{2}} =\frac{3.83 \times 10^{26}}{4 \times \pi \times 1.51 \times 10^{112}} \end{aligned}$

C1

$=1340 \mathrm{~W} \mathrm{~m}^{-2}$

A1

[Maximum number: 3]

(a)

A student observes different stars from the Earth.
Give two reasons why some stars appear brighter than others.
1
2

[ 2 ]

(b)

State what is meant by a standard candle.

[ 1 ]

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Covered syllabus points

25.1 Standard candles question 1

Question (a)

Question (b)

25.1 Standard candles question 2

Question (a)

Question (b)