25.1.2 Standard Candles & Stellar Distances

• 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

• 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