object-variable in selector syntax?

object-variable in selector syntax?

I want to select all children of an element.
so I want to use:
$("#main > *").css("border", "3px double red");

as suggested on:
http://docs.jquery.com/Selectors/child#parentchild

however, I'm within a loop and have defined the parent object as such:
tr_element = $(td_element).parent();

now I want to loop through the td's in the tr. So I tried:
$(tr_element+" > td").each(function (i) {
}

but that does not seem to work.

how to use an object in a selector when it is a variable?