Space between multiple columns in align* too large

Question

I want an align* environment with two columns, to display two equality signs per line, both aligned to the corresponding equality signs in the other lines.

For example:

\begin{align*}
    a &= 1 + 2 + 3 &= 6 \\
    b &= 1 + 2     &= 3
\end{align*}

However, when I try this in LaTeX, I get the following output:

I'd like to avoid the large space in front of =6 and =3. Does anybody know what is wrong with my code? I extracted the syntax from wikibooks.org. I also tried to add an additional & in the middle, but the result stays the same for me:

\begin{align*}
    a &= 1 + 2 + 3  &  &= 6 \\
    b &= 1 + 2      &  &= 3
\end{align*}

Answer 1

You can use alignat* for aligning at 2 points.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
  \begin{alignat*}{2}
    a &= 1 + 2 + 3  &&= 6 \\
    b &= 1 + 2      &&= 3
\end{alignat*}
\end{document}