How to input a empty star by LaTeX

There are plenty of empty stars, both in text and math mode.

In the following some examples:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mnsymbol}

\begin{document}
From \texttt{mnsymbol}: $\largestar \medstar \smallstar$ 
\end{document}

Some others:

\documentclass{article}
\usepackage{amsmath}
\usepackage{fdsymbol}
\usepackage{bbding}
\usepackage{fontawesome}
\begin{document}
From \texttt{fdsymbol}: $\largewhitestar \medwhitestar \smallwhitestar \smwhitestar$ 

From \texttt{bbding}: \FiveStarOpen

From \texttt{fontawesome}: \faStarO

\end{document}

And some others again:

\documentclass{article}
\usepackage{amsmath}
\usepackage{stix}

\begin{document}
From \texttt{stix}; $\bigwhitestar \medwhitestar \smwhitestar$
\end{document}

Directly from: The Comprehensive LaTeX Symbol List.

All the previous examples can be compiled with pdflatex.

With pdflatex and tikz package:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes}
\begin{document}

\newcommand\STAR{\raisebox{-.7em}{\tikz{\node[draw,star,star point height=.7em,minimum size=1em]{};} }}

Put a star \STAR here ..

\end{document}