How do we determine the distance to an object?
For nearby objects, astronomers use parallax, a visual effect produced by a shift in the relative position of the observer with respect to the object.
For objects further away, astronomers make use of ’standard candles’. These are objects with known brightness, such as Type Ia supernovae, which have normalisable peak brightnesses. Astronomers also use variable stars that pulsate with a period that is directly related to their brightness. By measuring how bright these objects appear to be, you can work out how far away they are.
For objects outside our galaxy to the most distant objects in the universe, astronomers use spectroscopy, which splits the light from an object into its various wavelengths to identify features such as emission or absorption lines. Due to the expansion of the universe, objects appear to be moving away from us at velocities proportional to their distance. The further away an object is from us, the faster it appears to be moving. By measuring the ‘redshift’ of a spectral line, astronomers can accurately determine the distance of the object.