Render text between asterisks as emphasized only if there is no space between the asterisk and the contained text, and only for pairs of asterisks. If there is any space, or for unmatched asterisks, assume the author really meant to place a star there. This is to prevent things like the following

http://news.ycombinator.com/item?id=47137

where the author meant to use the asterisk as a multiplication symbol.