CIE A Level Physics (9702) exams from 2022

Revision Notes

25.1.2 Standard Candles & Stellar Distances

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

Using Standard Candles as a Distance Indicator

  • 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

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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