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The Cosmological Distance Ladder

Supernovae -- Reaching Further

As useful as cepheids are in extending the distance ladder beyond our own galaxy, they cannot take us to the more distant galaxies.  Although cepheids are are bright, they are not bright enough for us to see them in very remote galaxies.  Fortunately another even brighter object with a known luminosity comes to the rescue.  These are the type Ia supernovae.

Type Ia supernavae appear to always have the same peak luminosity (about 1010 times the luminosity of the sun), so they can be used as another type of standard candle, (similar to cepheids).  Since supernovae are billions of times brighter than cepheids, they can be observed at much greater distances and therefore provide the next step along the cosmological distance ladder.

To determine distances using type Ia supernovae, we have to catch them as they increase in brightness to their peak and then decrease again.  From the apparent magnitude observed at their peak, we can apply the distance equation to find their distance: