Standard Candles & Stellar Distances | CIE A Level Physics Revision Notes 2025

Standard candles

An astronomical object which has a known luminosity due to a characteristic quality possessed by that class of object

Examples of standard candles include:
- cepheid variable stars
- type 1a supernovae

A cepheid variable is a type of pulsating star which increases and decreases in brightness over a set time period
This variation has a well-defined relationship to the luminosity

Measuring astronomical distances accurately is an extremely difficult task
- A direct distance measurement, e.g. using parallax measurements, is only possible if the object is close enough to the Earth
- For more distant objects, indirect methods must be used e.g. using standard candles

If the luminosity of a source is known, then the distance can be estimated based on how bright it appears from Earth
- Astronomers measure the radiant flux intensity, of the electromagnetic radiation arriving at the Earth
- Since the luminosity is known (as the object is a standard candle), the distance can be calculated using the inverse square law of flux

Each standard candle method can measure distances within a certain range
Collating the data and measurements from each method allows astronomers to build up a larger picture of the scale of the universe
- This is known as the cosmic distance ladder

Cosmic Distance Ladder Image (1), downloadable AS & A Level Physics revision notes

A combination of methods involving standard candles allows astronomers to build up a cosmic distance ladder from nearby stars to distant galaxies

Cepheid variable stars and type 1a supernovae are both used by astronomers as standard candles.

(a)

Explain how standard candles are used to determine distances to galaxies.

(b)

Suggest why type 1a supernovae are more suitable than Cepheid variable stars to determine distances to the most distant galaxies.

Answer:

Part (a)

Step 1: State what is meant by a standard candle

Step 2: Identify the quantities required to calculate distance

$F = \frac{L}{4 π d^{2}}$

Where:
- F = radiant flux intensity of an object in the galaxy
- L = luminosity of an object in the galaxy
- d = distance to the object in the galaxy

Step 3: Explain how the distance to a galaxy is determined

A standard candle in the galaxy of interest is identified
The standard candle's radiant flux intensity (i.e. its observed brightness on Earth) is measured
Using this and its known luminosity, the distance to the standard candle (and, therefore, galaxy) is calculated using the inverse square law of flux

Part (b)

Step 1: Identify how each object is used as a standard candle

A Cepheid variable star is an object whose radius varies periodically, and this period of variation is related to its luminosity
A type 1a supernova is an explosive outburst of a binary pair containing a white dwarf and another star. The peak luminosity of the explosion reaches the same value each time

Step 2: Suggest why the type 1a supernova can be used to measure greater distances

A type 1a supernova has a much greater luminosity than a Cepheid variable star
As a result, the radiant flux intensity (observed brightness) of a type 1a supernova is much greater
Therefore, type 1a supernovae are more likely to be observed and measured in the most distant galaxies