| Date | November 2016 | Marks available | 2 | Reference code | 16N.3.SL.TZ0.16 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 0 |
| 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 × 1026 W. The average apparent brightness of the Cepheid star is 1.1 × 10–9 W m–2.
Explain why Cephids are used as standard candles.
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
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.
