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Edexcel | Physics WPH0 | W 11 5 1 | 1 hour 30 minutes
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A Cepheid variable star contracts and expands repeatedly and as it does, so its

luminosity varies. By measuring the period of this variation, astronomers can determine the star’s average luminosity.

13
(a)
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4 Mark s

A Cepheid variable star is a type of standard candle. Discuss the use of standard

candles in astronomy.

Object must have a standard/known luminosity OR luminous properties

independent of its position 

It can be used to calculate distances 

Reference to any two of the following:

♦ Radiation/energy flux measured 

♦ Observed brightness compared with luminosity 

♦ Use of inverse square law [accept if equation quoted] 

♦ Object must be commonly found in the universe

13
(b)
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2 Mark s

As well as the variation in luminosity of the Cepheid, changes in the frequency of

the detected radiation are also observed.

Suggest how the Doppler effect may account for these changes.

When star contracts (front of) star is moving away from observer OR

explanation in terms of a rotating/binary star 

Movement away from observer results in a decrease in the frequency of

the radiation/red shift 

Accept converse argument for an expanding star

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