Floating figures and tables ordinarily come out the same width as the
page, but in two-column documents they're restricted to the width of
the column. This is sometimes not good enough; so there are alternative
versions of the float environments - in two-column documents,
figure* provides a floating page-wide figure (and table* a
page-wide table) which will do the necessary.
The "*"ed float environments can only appear at the top of a page,
or on a whole page - h or b float placement directives are
simply ignored.
Unfortunately, page-wide equations can only be accommodated inside
float environments. You should include them in figure environments,
or use the float or ccaptionpackage to define a
new float type.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=widefigs