Type Ia Supernovae as Standard Candles

Type Ia supernovae are particularly useful because they provide one of the best ways to measure distances to other galaxies in the range from 10 - 3300 Mly (for which other methods in the cosmic distance ladder may not work as well).

To be able to use Type Ia supernovae to determine distance, we must first determine their actual, or intrinsic, peak brightness from the data we receive in our observations.

Type Ia supernovae are called standard candles because they all reach a peak intrinsic brightness that is nearly the same. Because there are small variations from one Type Ia Supernova to the next, an adjustment needs to be applied to calculate the correct peak intrinsic brightness for each.

To do this, astronomers fit a model light curve with an emphasis on aligning the curve to the decrease in a supernova’s observed brightness (its apparent magnitude, m) from the peak to 15 days following the peak. This decrease in observed brightness is called the “delta m 15” (𝚫m₁₅). There is a relationship between the 𝚫m₁₅ and the actual peak brightness (peak absolute magnitude) of a Type Ia supernova. Supernovae that are more intrinsically bright have smaller values of 𝚫m_15, indicating that they decrease in brightness at a slower rate.