have you ever wondered how do scientists measure distances far away in space? Maybe you have heard about supernova which was found about 11 billions of light years away. The problem is that when something is far away you don`t actually know if what you see is bright or close or dim or far away or the combination of both.
So here comes the standard candle. The standard candle is object in Universe which has always about the same luminosity.
The example of thing that is not standard candle is for example any star since when you are looking at it you don`t know its luminosity since it could be in spectral class O or K and it could be next to you or far away.
Standard candles are supernovas of type Ia which are the brightest of all supernovas.
First of all I will try to explain what this supernova type Ia is.
How this happens? This white dwarf has to be in binary star system so he can feed on his companion`s gases and get the mass again to trigger fusion. When white dwarf exceeds the Chandrasekhar limit which is 1.4 of Sun`s mass as mentioned above then it explodes in violent explosion called supernova Ia.
Actually it is not so simple because the supernova explodes in what is called “double detonation” which is caused by first explosion happening even before Chandrasekhar limit because of the hydrogen which is fused on the surface on helium (I am going to find out more about this on astronomy stack exchange so I will update it.)
Then the second and main explosion is triggered and the donor, the first star is thrown away from the system by the energy released (1-2*10^44 J).
So the important thing is that supernova works as standard candle because it will be always about the same brightness which is very important because than astronomers can calculate how far away it is because they know how bright this thing is.
Supenovas are used for distances greater than 1,000,000 light years because closer there is not enough of them.