A Sense of Scale
Reliably calculating distances is one of the most difficult tasks in astronomy beyond our galaxy. The distances to objects within our own galaxy can be calculated using parallax - a technique that measures the amount by which the object shifts compared to background stars as the Earth orbits around the Sun and uses trigonometry to work out the object's distance. For more distant objects, however, the shift is far too small to be measured. A range of other techniques are used to estimate the distances of extragalactic objects.
Cepheid and RR Lyrae variable stars can be used as distance markers, often referred to as 'standard candles', because astronomers can reliably judge the distances of these stars from Earth. The amount of light that is given off by Cepheid or RR Lyrae stars varies periodically, and the relationship between the period of the variation and the star's intrinsic brightness, or absolute magnitude, is well known to astronomers. By calculating the absolute magnitude of the star and comparing it with the star's apparent magnitude, astronomers can work out how far away the star is.
Similarly, type 1a supernovae are used as standard candles because a relationship exists between the shape of the supernova's light curve and its absolute magnitude. Supernovae are visible at much greater distances than Cepheid variables, which makes them extremely useful when studying distant regions.
At the largest scales, astronomers must rely on Hubble's Law to work out the distance of objects such as quasars or distant galaxies. This relationship, discovered by Edwin Hubble in 1929, states that the redshift of an object, which indicates how fast it is moving away from us, is proportional to its distance. The more distant an object is, the more quickly it is speeding away from us. This effect is due to the expansion of the Universe.
The most distant known objects in the observable Universe are galaxies up to 13 billion light years away. Light that is currently arriving at Earth left these galaxies 13 billion years ago. When astronomers focus their telescopes onto these distant galaxies, they are taking a glimpse into the Universe's past. In this way, extragalactic astronomy allows us to investigate the Universe's history.