Aligning equations in a cases environment

Question

In my work, I am trying to typeset the following system of equations.

\begin{equation*}
\begin{cases}
\pi_n  (i) = {n^2}/{\vol(G_n)} \cr
\pi_n (j) =  {n}/{\vol(G_n)} \cr
q^{(n)}_{ij} = {1}/{n^2 } \cr
q^{(n)}_{ji} = {1}/{n}
\end{cases}
\end{equation*}

However, I would like to align the equal signs. If I do this by adding ampersands (see below), too much space is added before the equal signs, which makes the system of equations look worse than before. Is there any other non-manual way to align the equations?

\begin{equation*}
\begin{cases}
\pi_n  (i) &= {n^2}/{\vol(G_n)} \cr
\pi_n (j) &=  {n}/{\vol(G_n)} \cr
q^{(n)}_{ij} &= {1}/{n^2 } \cr
q^{(n)}_{ji} &= {1}/{n}
\end{cases}
\end{equation*}

Answer 1

I think that you want to use \left\{...\right. around your equation. To be able to align your equations inside these you can use the split environment from the amsmath package.

EDIT

In the comments both above and below the consensus is that the aligned environment is superior to split in this situation. Perhaps I am missing it but I can't see the difference in this example, however, as people more knowledgeable than I are advocating using the aligned environment there must be cases where it does matter.

Here is a comparison of the two environments for this example:

This produces:

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator\vol{\text{vol}}
\begin{document}
\[\left\{
  \begin{aligned}
    \pi_n  (i) &= {n^2}/{\vol(G_n)} \cr
    \pi_n (j) &=  {n}/{\vol(G_n)} \cr
    q^{(n)}_{ij} &= {1}/{n^2 } \cr
    q^{(n)}_{ji} &= {1}/{n}
  \end{aligned}\right.\tag{\textbf{aligned}}
\]

\[\left\{
  \begin{split}
    \pi_n  (i) &= {n^2}/{\vol(G_n)} \cr
    \pi_n (j) &=  {n}/{\vol(G_n)} \cr
    q^{(n)}_{ij} &= {1}/{n^2 } \cr
    q^{(n)}_{ji} &= {1}/{n}
  \end{split}\right.\tag{\textbf{split}}
\]

\end{document}

Answer 2

Stab with empheq:

\documentclass{article}
\usepackage{empheq}
\DeclareMathOperator\vol{\text{vol}}
\begin{document}
\begin{empheq}[left=\empheqlbrace]{align*}
\pi_n  (i)   &= {n^2}/{\vol(G_n)} \cr
\pi_n (j)    &=  {n}/{\vol(G_n)} \cr
q^{(n)}_{ij} &= {1}/{n^2 } \cr
q^{(n)}_{ji} &= {1}/{n}
\end{empheq}
\end{document}