Standard Candles & Stellar Distances (Cambridge (CIE) A Level Physics)

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

• A standard candle is an object of known luminosity

Step 2: Identify the quantities required to calculate distance

• The inverse square law of flux is:

• 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