Editing Cosmic distance ladder (section)

==== Type Ia light curves ====

[[Type Ia supernovae]] are some of the best ways to determine extragalactic distances, as introduced by Stirling A. Colgate.<ref>{{Cite journal |last=Colgate |first=S. A. |date=September 1979 |title=Supernovae as a standard candle for cosmology |url=http://adsabs.harvard.edu/doi/10.1086/157300 |journal=The Astrophysical Journal |language=en |volume=232 |page=404 |doi=10.1086/157300 |bibcode=1979ApJ...232..404C |issn=0004-637X}}</ref> Ia's occur when a binary white dwarf star begins to accrete matter from its companion star. As the white dwarf gains matter, eventually it reaches its [[Chandrasekhar limit]] of <math> 1.4 M_{\odot} </math>.

Once reached, the star becomes unstable and undergoes a runaway nuclear fusion reaction. Because all Type Ia supernovae explode at about the same mass, their absolute magnitudes are all the same. This makes them very useful as standard candles. All Type Ia supernovae have a standard blue and visual magnitude of

<math display="block">\ M_B \approx M_V \approx -19.3 \pm 0.3 \,.</math>
Therefore, when observing a Type Ia supernova, if it is possible to determine what its peak magnitude was, then its distance can be calculated. It is not intrinsically necessary to capture the supernova directly at its peak magnitude; using the '''multicolor light curve shape''' method ('''MLCS'''), 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 supernovae magnitude light curves to a template light curve. This template, as opposed to being several light curves at different wavelengths (MLCS) 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.<ref>{{cite journal |last1=Coelho |first1=R. |last2=Calv˜ao |first2=M. |last3=Ribamar |first3=R. |last4=Siffert |first4=B. |title=Standardization of type Ia supernovae |year=2015 |journal=European Journal of Physics |volume=36 |issue=1 |page=015007 |doi=10.1088/0143-0807/36/1/015007 |display-authors=1 |arxiv=1411.3596 |bibcode=2015EJPh...36a5007C |s2cid=119096479}}</ref>

Using Type Ia supernovae is one of the most accurate methods, particularly since supernova explosions can be visible at great distances (their luminosities rival that of the galaxy in which they are situated), much farther than Cepheid Variables (500 times farther). Much time has been devoted to the refining of this method. The current uncertainty approaches a mere 5%, corresponding to an uncertainty of just 0.1 magnitudes.