SPACE - Galaxy growth examined like rings of a tree

(Phys.org) —Watching a tree grow might be more frustrating than waiting for a pot to boil, but luckily for biologists, there are tree rings. Beginning at a tree trunk's dense core and moving out to the soft bark, the passage of time is marked by concentric rings, revealing chapters of the tree's history ...

http://phys.org/news/2013-11-galaxy-growth-tree.htm