9.2.13 Supernovae as Standard Candles

• A standard candle is defined as:

An astronomical object of known brightness that can be used to calculate galactic distances

• The most common examples of standard candles are:
  • Cepheid variable stars
  • Type 1a supernovae

• Type 1a supernovae can be used as standard candles because they reach the same peak value of absolute magnitude each time
  • This type of supernova involves an exploding white dwarf in a binary star system
  • The white dwarf increases in mass as it attracts material from its binary pair
  • Eventually the white dwarf reaches a critical mass, known as the Chandrasekhar Limit 
  • This critical mass means the explosion is the same each time, hence it produces a very consistent light curve

Typical Type 1a Supernova Light Curve

The light curve of a type 1a supernova always has the same characteristic shape and reaches the same peak absolute magnitude

• Another advantage of using Type 1a supernovae as standard candles is that they are extremely bright
  • This means they can be used to measure the distance to the furthest galaxies