Centering a tag on an align environment

Make the inner construction inside an array, which boxes it into a single entry:

\documentclass{article}
\usepackage[leqno]{amsmath}
\begin{document}

\begin{align*}
  \tag{$\star$}
  \renewcommand{\arraystretch}{1.2}% Spread out array rows
  \begin{array}{c @{\quad} l}
    (1) & \text{stuff here} \\
    (2) & \text{more stuff here}
  \end{array}
\end{align*}

\end{document}

Some adjustments, if needed:

• \quad is the space between the entries;
• c l is the column specification for the left (numbered) and right (textual) columns;
• \arraystretch greater than 1 gives a little air between the array rows.

Caveat: array's interior is not set in \displaystyle, so you may have to force this explicitly if your construction contains something that is set differently under that style.

---

You would obtain a similar output if you'd used aligned (and there's also alignedat for multiple alignment points):

\begin{equation}
  \tag{$\star$}
  \begin{aligned}
    (1) &\quad \text{stuff here} \\
    (2) &\quad \text{more stuff here}
  \end{aligned}
\end{equation}