Physics A2 · Astronomy and cosmology. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The luminosity of a star is the total power of the radiation it emits, in watts. It is a property of the star and does not depend on where the observer is.
What we measure on Earth is the radiant flux intensity: the power arriving per unit area. The radiation spreads out evenly over a sphere, so the intensity falls with the square of the distance. Twice as far away means one quarter of the intensity.
A standard candle is an object whose luminosity is known. If we measure its radiant flux intensity at the Earth, the inverse square law gives its distance. Finding a standard candle inside a distant galaxy gives the distance to that galaxy.
Students forget to square the distance, or forget the square root when finding d. Also do not confuse luminosity (W, at the star) with radiant flux intensity (W m−2, at the observer).
Answer: About 6.3 × 1017 m.
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