| Date | May 2013 | Marks available | 6 | Reference code | 13M.3.SL.TZ1.15 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain, Outline, and State | Question number | 15 | Adapted from | N/A |
Question
This question is about variable stars and supernovae.
Cepheid variable stars are used as “standard candles” by astronomers.
(i) State what is meant by a standard candle.
(ii) Outline the properties of a Cepheid star that allow it to be used as a standard candle.
(iii) Explain how astronomers use their observations of a Cepheid star to determine the distance from the star to Earth.
Markscheme
(i) object of known luminosity/power;
(ii) luminosity varies with time in a regular way;
(average) luminosity related to period of variation;
high luminosity so visible from great distances;
(iii) the period of the variation of luminosity/apparent brightness/apparent magnitude is measured;
the luminosity/absolute magnitude is determined from period;
apparent magnitude/brightness is measured (on Earth);
m-M=51g\(\left( {\frac{d}{{10}}} \right)\) or \(b = \frac{L}{{4\pi {d^2}}}\) is used to compute d;
Examiners report
(i) was very poorly done. Some candidates described a standard candle as a star that changes brightness, i.e. a Cepheid variable.
(ii) most candidates knew that Cepheid variables change luminosity in a periodic way, but few mentioned that their luminosity is high and so they are visible from great distances.
Often the general concept seemed known to the candidate in (iii) but the actual practical procedure was not specified.
