All evidence we have about distant stars and galaxies comes from the radiation which astronomers can detect.
Two stars which have the same real brightness (appear as bright as eachother) can have different relative brightness. The star that is further away has a smaller relative brightness.
- Real brightness = appear as bright as each other
- Relative brightness =The star which is further away has a asmaller relative brightness
If you know the distance to one of the stars , the difference in their relative brightness can be used to calculate the other one.
Uncertainties with this method of measuring the distance of stars:
- It is based on the assumption that similar types of stars have the same real brightness.
- Its based on estimating the distance to one of the stars
- Things such as dust, rain, clouds and light pollution make it difficult to make precise observations at night.