title: How to write an asterisk (*) symbol in superscript of a Greek letter in ggplot2?
body: <p>With reference to following figure, I want to write Δ* instead of Δ in the <code>Xlab</code>. I am using following line in my <code>R</code> code.</p>
  
  <pre><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)).
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/nZRnq.png" alt="enter image description here"></p>
  
  <p>How should I modify <code>xlab(expression(Delta))</code>, so that I am able to write Δ*? I have tried <code>xlab(expression(Delta^{*}))</code>, but its not working.</p>