We have discussed various methods to measure the distance to a star, classified into two groups:
Current detailed knowledge of these distances is the result of a combination of various techniques and a development over centuries. Measurements outside the atmosphere in space, both parallactic and spectrographic observations, have greatly enhanced this knowledge, in terms of quantity as well as quality. This development will continue on the basis of future space missions.
But there is more
There are many more techniques employed in astronomy to find distance, especially for objects much further away than we have discussed. And, to put the ultimate question, how do astronomers know the size of the entire visible Universe?
The graph shows the whole raft of techniques that are used in astronomy today, and here we have discussed only part of the bottom half, because first we need to discuss several other topics, such as how astronomers measure velocity, and about stellar spectra, and stellar evolution.
What is called the Cosmic Distance Ladder is the succession of methods by which astronomers determine the distances to celestial objects.
A real direct distance measurement (e.g. with parallax) to an astronomical object is only possible on a relatively small scale (in astronomical terms).
The ladder analogy arises because no one technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung.
Because the more distant steps of the cosmic distance ladder depend upon the nearer ones, the more distant steps include the effects of errors in the nearer steps, both systematic and random ones. The result of these propagating errors means that distances in astronomy are generally quite imprecise, and that the precision necessarily is poorer for more distant objects.
Even worse, the overall distance scale used in astronomy, is prone to systematic effects in any of these individual measurement techniques and these affect our knowledge about the scale of the entire Universe. Much discussion is still going on among astronomers today.
Read more about measuring distances in astronomy and these problems in our module "Hubble's Law".