A type Ia supernova happens in a binary system containing a white dwarf and a companion star. The white dwarf pulls matter from its companion. When its mass exceeds about 1.4 solar masses (the Chandrasekhar limit), runaway fusion begins and the white dwarf explodes completely.Because every type Ia supernova explodes at the same mass, they all reach the same peak absolute magnitude, about M = −19.3. An object of known absolute magnitude is called a standard candle. They are so bright that they can be seen in very distant galaxies.Light curve: the absolute magnitude rises rapidly to its peak over about 20 days, then declines more slowly over the following months.
m − M = 15.7 − (−19.3) = 35
log (d/10) = 35 / 5 = 7, so d/10 = 107
d = 108 pc = 100 MpcIn the late 1990s very distant type Ia supernovae were found to be dimmer than expected, so they were further away than Hubble's law predicted. This suggested that the expansion of the universe is accelerating, driven by an unknown dark energy. This is still controversial because the nature of dark energy is unknown and the result relies on the supernovae being perfect standard candles.
m − M = 15.7 − (−19.3) = 35
log (d/10) = 35 / 5 = 7, so d/10 = 107
d = 108 pc = 100 MpcIn the late 1990s very distant type Ia supernovae were found to be dimmer than expected, so they were further away than Hubble's law predicted. This suggested that the expansion of the universe is accelerating, driven by an unknown dark energy. This is still controversial because the nature of dark energy is unknown and the result relies on the supernovae being perfect standard candles.