How can astronomers tell how ‘old’ light is? In other words, how can they tell if they are looking at an object 10 light years away or 1009 light years?

Question

How can astronomers tell how ‘old’ light is? In other words, how can they tell if they are looking at an object 10 light years away or 1009 light years?

In: Physics

Answer 1

To add to the methods mentioned:

They also use a certain type of stars called Cepheids. These are stars that pulsate perdiodically. Lucky for us, the period of the star and its absolute brightness are linearly connected. So by measuring the light we can get the period, from which we can derive the absolute brightness (that is the brightness of the star when it would be at 10 parsec away from us.) As we know how light diminishes with distance, we can calculate the distance if we know the measured and the absolute brightness.

This follows the following formula:

Measured brightness-absoluye brightness=5*log(d)-5

A next method is using a nearby supernova/ the supernova of the star. Of type Ia to be exact. Because supernovae of that type always emit practically the same amount of light, we again know the absolute brightness. As these things are WAY brighter than regular stars, we can use them across greater distances. (You can look up the ranges of all of the mentioned methods in this thread, I can’t recall them correctly)