Multiple align in equation with single equation number

Question

I'm trying to get the following equation aligned two times per line and also I want to assign a single number to the equation. This is the code I have:

 \begin{alignat*}{2}
  [M/H] &\cong\; 0.917 [Fe/H] &(1)\\
   &\cong\; 0.977 [Fe/H] &(2)\\
   &\cong\; [Fe/H] + log(0.638f_{\alpha}+0.362) \;\;\;&(3)
 \end{alignat*}

This produces the properly aligned equations but no numbering. If I remove the asterisks I get one number for each line, which I don't want.

I've tried wrapping the above statement inside a \begin{equation} \end{equation} command, but it won't work. Ideas?

Answer 1

Use the alignedat environment:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{alignedat}{2}
  \textup{[M/H]} &\cong 0.917 \textup{[Fe/H]} &&(1)\\
   &\cong 0.977 \textup{[Fe/H]} &&(2)\\
   &\cong \textup{[Fe/H]} + \log(0.638f_{\alpha}+0.362) \quad&&(3)
 \end{alignedat}
\end{equation}
\end{document}

The \; commands you're using are wrong, as they introduce an asymmetric space. Also the chemical symbols should be upright.