V838 Mon is one of the best examples of the light echo phenomenon. It was believed to be a nova at first -- Nova Mon 2002 -- but its subsequent photometric behavior shown in this light curve is unlike any typical nova. The subsequent light echo was observed propagating outwards from the star for several years afterwards. The light echo is caused by the light of the outburst propagating outwards from the star and being reflected by dust clouds emitted by V838 Mon hundreds or thousands of years ago. The cause of the outburst is not known, but the prevailing theory is some kind of merger of two old stars not quite massive enough to generate a supernova.