I'm trying to draw a tree in the following form

Code:
    *    
   ***
  *****
 *******
*********
as you can see I need both spaces, and "*" stars in every line, but I can't get it quite the way I want
Code:
#!/bin/bash

display_tree() {
	local rows=$1
	local columns=$2

	for ((i=0; i<$rows; i++))
	do
		#spaces loop
		for ((j=0; j<$columns; j++))
		do
			echo -n " "
			#drawing tree loop
			for ((a=0; a<$(($i + 1)); a++))
			do
				echo -n "*"
			done
		done
	echo 
	done
}

if [ $# -eq 2 ]; then
	display_tree $1 $2
else
	echo "Usage: $0 rows columns"
fi