Standard Candles & Stellar Distances (CIE A Level Physics)

• 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 include:
  • cepheid variable stars
  • type 1a supernovae

Cepheid variable stars

• 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

Type 1a supernovae

• A type 1a supernova is an explosion involving a white dwarf
• The peak luminosity of the explosion is always the same

• 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

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