Calculating Relative Brightness of Type Ia Supernovae

[shadishacker]

Dear all,

I am trying to understand the plot below:
http://supernova.lbl.gov/PDFs/expansionhistoryphystoday.pdf
by S.Perlmutter.
However, I don't get the meaning of the "relative brightness" of SN.
How is it calculated?
Is the brightness of each SN compared to a special one?

 

[Orodruin]

The type of supernovae studied are thought to be good so-called "standard candles", i.e., they are all expected to have the same brightness. This is what you can compare to.

 

[shadishacker]

The type of supernovae studied are thought to be good so-called "standard candles", i.e., they are all expected to have the same brightness. This is what you can compare to.

So they suppose what value each SN 's brightness should have and then reduce the value from the observed one?!

 

[shadishacker]

As I can see in the plot, it should be some logarithmic relation. right?

 

[Chalnoth]

The way it's done is to compare distances as estimated by supernovae to distances estimated using other methods. So if we know how far away some supernovae are, we can calculate how bright they are. Then we can use the brightness of other supernovae to estimate how far away they are.

 

[newjerseyrunner]

The relative brightness is how bright it appears to you. Imagine you are in a dark room, with a single light bulb at some distance away from you. You know that the bulb is putting out 1000 lumens, but your light meter reads 10 lumens, you can use the inverse square law to determine exactly how far away the bulb is.

We can do the same with that very particular type of supernova because of how it works, it slowly accretes mass until it reaches a very specific critical mass and detonates. Astrophysicists have already calculated the critical number and expanding on that using the same laws of physics, you can calculate precisely how bright it much be.