Here we must extrapolate again, since open clusters can be difficult
to find against rich star fields in the plane of the Milky Way, and
since richer clusters may be identified farther away than poor
ones. The electronic version of the catalogue of open cluster data
compiled by Gosta Lynga, Lund Observatory, Box 43, S-221 00 Lund,
Sweden, 1987 version, lists 1111 identified open clusters in our
galaxy. There are certainly at least ten times this number, since we
have trouble seeing even rich open clusters more than about 7000
light-years away in most directions through the obscuring dust in the
plane of our Galaxy. This effect is especially acute since young star
clusters are strongly concentrated to this plane (no coincidence since
the gas from which new clusters are formed is associated with dust).