makes sense but is it easy to figure out how old the supernova is? or what state it is in?
Therefore, when observing a type Ia SN, if it is possible to determine what its peak magnitude was, then its distance can be calculated.
this is what i mean. how is it possible to figure out the peak magnitude?
You could just read the Wiki link
It is not intrinsically necessary to capture the SN directly at its peak magnitude; using the multicolor light curve method (MCLS), the shape of the light curve (taken at any reasonable time after the initial explosion) is compared to a family of parameterized curves that will determine the absolute magnitude at the maximum brightness. This method also takes into effect interstellar extinction/dimming from dust and gas.
Similarly, the stretch method fits the particular SN magnitude light curves to a template light curve. This template, as opposed to being several light curves at different wavelengths (MCLS) is just a single light curve that has been stretched (or compressed) in time. By using this Stretch Factor, the peak magnitude can be determined.
