Latex中的Wrapfigure不换行（文档类假想）

Question

论文中的数字并没有环绕文本，而是一直到结尾。请帮忙？

下面是代码示例：

\documentclass[12pt,twoside,bind,ams,a4paper]{hepthesis}

\usepackage{graphicx}   % Including figure files
\usepackage{wrapfig}

\begin{document}
%....
\begin{mainmatter}
\newpage 
\subsection{Stellar classification}

\begin{wrapfigure}{r}{0.5\textwidth} 
  \vspace{-20pt}
  \begin{center}
    \includegraphics[width=0.4\textwidth]{img/hydrostatic-equilibrium}%
    \caption{Hydrostatic equilibrium inside stars}
    \label{fig:hydrostatic-equilibrium}
  \end{center}
  \vspace{-20pt}
  \vspace{1pt}
\end{wrapfigure}

Stars are massive gaseos bodies which generate energy through nuclear fusion in the core, and release it as radiation. The mass of the star determines the star's proerties (such as temperature, luminosity and size), but most importantly its evolution track. The pressure of the energy emitted from the core would cause the star to explode, were it not counteracted by the gravity of the surrounding material. When the two forces are in equilibrium (Figure \ref{fig:hydrostatic-equilibrium}), also called hydrostatic equilibrium, the star is stable. When the balance changes, however, so does the state of the star. The Sun is an example of a main-sequence star, which has been fusing hydrogen into Helium for 5 billions years, and is predicted to do so for 5 billion more. 

The Sun is an example of a main-sequence star, which has been fusing hydrogen into Helium for 5 billions years, and is predicted to do so for 5 billion more. The Sun is an example of a main-sequence star, which has been fusing hydrogen into Helium for 5 billions years, and is predicted to do so for 5 billion more. The Sun is an example of a main-sequence star, which has been fusing hydrogen into Helium for 5 billions years, and is predicted to do so for 5 billion more.  
\end{mainmatter}
\end{document}