r - How to write an asterisk (*) symbol in superscript of a Greek letter in ggplot2? -

with reference following figure, want write Δ* instead of Δ in xlab. using following line in r code.

g=ggplot(ss4, aes(x = delta, y = sre, group = estimators, colour = estimators)) + geom_line(size=1.1)+xlab(expression(delta))+ theme(legend.position="none",axis.text=element_text(size=12)) direct.label(g, list(first.points, hjust = 1.1,vjust = 1)). 

how should modify xlab(expression(delta)), able write Δ*? have tried xlab(expression(delta^{*})), not working.

use paste() within expression()

xlab(expression(paste(delta, "*")))