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Date	November 2016	Marks available	2	Reference code	16N.3.SL.TZ0.16
Level	Standard level	Paper	Paper 3	Time zone	0 - no time zone
Command term	Explain	Question number	16	Adapted from	N/A

Question

The first graph shows the variation of apparent brightness of a Cepheid star with time.

The second graph shows the average luminosity with period for Cepheid stars.

Determine the distance from Earth to the Cepheid star in parsecs. The luminosity of the Sun is 3.8 × 10²⁶W. The average apparent brightness of the Cepheid star is 1.1 × 10^–9W m^–2.

[3]

a.

Explain why Cephids are used as standard candles.

[2]

b.

Markscheme

from first graph period=5.7 «days» ±0.3 «days»

from second graph $\frac{L}{{{L_{{\text{SUN}}}}}} = 2300$ « $\pm {\text{200}}$ »

d = « $\sqrt {\frac{{2500 \times 3.8 \times {{10}^{26}}}}{{4\pi \times 1.1 \times {{10}^{ - 9}}}}} = 8.3 \times {10^{18}}{\text{m}}$ » =250 «pc»

Accept answer from interval 240 to 270 pc If unit omitted, assume pc.
Watch for ECF from mp1

a.

Cepheids have a definite/known «average» luminosity

which is determined from «measurement of» period
OR
determined from period-luminosity graph

Cepheids can be used to estimate the distance of galaxies

Do not accept brightness for luminosity.

b.

Examiners report

[N/A]

a.

[N/A]

b.

Syllabus sections

Option D: Astrophysics » Option D: Astrophysics (Core topics) » D.2 – Stellar characteristics and stellar evolution

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16N.3.SL.TZ0.15e:
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Option D: Astrophysics » Option D: Astrophysics (Core topics)

Option D: Astrophysics