Cottonwood tree twigs with star-shaped pattern

[Thos]

Can a botanist please explain to this layman how that tiny star(five pointed) winds up in some twigs from cottonwood trees? When twigs about 1/4" diameter are cut in cross section at a "joint" (where the twig is slightly swollen and growth rings appear to circle the twig's bark), I sometimes find a perfect little star--other times I don't. And in twigs that have dried to the point of brittleness that feature is not visible. Thanks for your time and patience.