Luminosity and standard candles Edexcel International A Level Physics revision
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In plain words
A star can look faint because it really is dim, or because it is a very long way off. To tell which, astronomers need to know how much light it truly gives out. For certain kinds of object they do know, and those become measuring sticks for the whole Universe.
4 things to know
- The luminosity L of a star is the total power it radiates, in watts.
- The radiation spreads out evenly over a sphere, so the intensity (radiant flux intensity) at a distance d is F = L ÷ 4πd², in W/m². This is an inverse square law.
- A standard candle is an object whose luminosity is known.
- Measure the intensity that reaches the Earth from a standard candle and you can work out how far away it is: d = √(L ÷ 4πF). This is how the distances to galaxies are found.
Worked example
The Sun has a luminosity of 3.83 × 10²⁶ W and is 1.50 × 10¹¹ m from the Earth. Find the intensity of its radiation at the Earth.
- F = L ÷ 4πd².
- = 3.83 × 10²⁶ ÷ (4π × (1.50 × 10¹¹)²) = 3.83 × 10²⁶ ÷ 2.83 × 10²³.
- = 1.4 × 10³ W/m².
Tips and tricks
- Double the distance and the intensity falls to a quarter.
- How bright a star looks depends on its distance as well as its luminosity. Luminosity is the star's own property.
It lands in your notebook with its questions as flashcards.
Luminosity and standard candles: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the luminosity of a star?
It is measured in watts.
What is the unit of luminosity?
It is a power.
The distance from a star is doubled. What happens to the intensity of its radiation?
The same power is spread over four times the area.
What is a standard candle?
Knowing its luminosity lets you find its distance.
Two stars give the same intensity at the Earth, and star X is further away. Which is more luminous?
X must give out more power to look just as bright from further away.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Luminosity and standard candles
Edexcel International A Level Physics WPH · 7 marks · papermunch.org
Name ______________________________ Date ______________
A standard candle of luminosity 4.0 × 10²⁸ W gives an intensity of 2.0 × 10⁻¹⁰ W/m² at the Earth. Find its distance.[3]
Show answerHide answer
4.0 × 10¹⁸ m. d² = 4.0 × 10²⁸ ÷ (4π × 2.0 × 10⁻¹⁰).
State what is meant by a standard candle, and what standard candles are used for.[2]
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An object of known luminosity. Comparing its luminosity with the intensity received on Earth gives its distance, and so the distance to the galaxy it is in.
Two stars have the same luminosity. Star X is three times as far away as star Y. Compare the intensities received from them.[2]
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The intensity from X is one ninth of the intensity from Y.
Answers: Luminosity and standard candles
- 1. 4.0 × 10¹⁸ m. d² = 4.0 × 10²⁸ ÷ (4π × 2.0 × 10⁻¹⁰).
- 2. An object of known luminosity. Comparing its luminosity with the intensity received on Earth gives its distance, and so the distance to the galaxy it is in.
- 3. The intensity from X is one ninth of the intensity from Y.