Standard Candles & Distance
To find a galaxy's distance, astronomers need an object inside it whose luminosity is already known. That object provides the missing reference.
A standard candle supplies the known luminosity
Definition
1 markWhat is meant by a standard candle?
Model answer: An astronomical object of known luminosity.
Cepheid variables are one example. Their brightness changes, but the pulsation period can be used to infer their luminosity. “Standard” therefore means calibrated, not constant.
Each distance method calibrates the next one
A method that works for nearby objects is used to calibrate another method that can be seen farther away. This linked chain is called the cosmic distance ladder.
Near → far
The cosmic distance ladder
Nearby stars
Up to thousands of light-years
NASA, ESA and A. Riess; processing by Gladys Kober
Parallax gives a direct distance and calibrates Cepheids.
Nearby galaxies
Up to about 100 million light-years
ESA/Hubble & NASA, D. Sand and R. J. Foley
Cepheid period → luminosity → galaxy distance; this calibrates Type Ia.
Distant galaxies
About 100 million–10 billion light-years
NASA / STScI Hubble Deep Field Team
Type Ia distance + redshift speed → H₀; Hubble's law extends the scale.
Known luminosity plus measured flux gives distance
- Identify a standard candle in the galaxy.
- Use its calibrated luminosity.
- Measure its radiant flux intensity at Earth.
- Rearrange the inverse-square relation to calculate distance.
These are the three mark-scheme moves in the 2024 and 2025 questions (9702/42/F/M/25 Q10(a)(ii)).
Worked example
Distance to a standard candle
A calibrated object has a luminosity of 1.90 × 1036 W. Its measured flux is 8.42 × 10−16 W m−2.
State what is meant by a standard candle. [1]
Show worked answer
An astronomical object of known luminosity.
Common mistake