Looking at a snapshot
Teaching Guidance for 11-14
Forests are similar to galaxies
On walking into a mature forest you see a snapshot of tree development. There are seedlings, saplings, young trees, mature trees, dying trees, and decaying trees. From this collection, visible all at once, you can infer an ordered development, the life cycle of a tree. The auxiliary data that you need are a means of ordering the specimens, putting them in a time order: this is provided by the tree rings.
Looking out at the stars is just like walking into a forest. We can see a huge variety of luminous sources. Here the time-ordering principle is rather simple. As it takes time for light to travel from the stars to us, in any one snapshot the oldest stars at that moment are those which are furthest away.
The parallel reasoning goes like this:
Those trees with the greatest number of rings are oldest: the photons that have been in transit longest, and so have travelled furthest, come from stars that are oldest.
The conversion rate for trees is one ring every year. The conversion rate for light is one light-year of travel per year.
That makes getting the scale of distance, and the particular distance to any star, important, as our construction of the life-cycle of stars depends on it.