Standard Candles (Edexcel A Level Physics): Revision Note

Determining distance using standard candles

Standard Candles

• A standard candle is defined as:

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

• Examples of standard candles are:

• Cepheid variable stars

  • 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

• Type 1a supernovae

  • A supernova explosion involving a white dwarf

  • The luminosity at the time of the explosion is always the same

Determining distances using standard candles

• Measuring astronomical distances accurately is an extremely difficult task

• A direct distance measurement is only possible if the object is close enough to the Earth

• For more distant objects, indirect methods must be used - this is where standard candles come in useful

• 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

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