  Some classes of stars, including supernovae and novae, undergo explosive
outbursts that eject stellar material into space. In 2002, the previously
unknown variable star V838 Monocerotis brightened suddenly by a factor of about
10^4. Unlike a supernova or nova, V838 Mon did not explosively eject its outer
layers; rather, it simply expanded to become a cool supergiant with a
moderate-velocity stellar wind. Superluminal light echoes were discovered as
light from the outburst propagated into surrounding, pre-existing circumstellar
dust. Here we report high-resolution imaging and polarimetry of the light
echoes, which allow us to set direct geometric distance limits to the object.
At a distance of >6 kpc, V838 Mon at its maximum brightness was temporarily the
brightest star in the Milky Way. The presence of the circumstellar dust implies
that previous eruptions have occurred, and spectra show it to be a binary
system. When combined with the high luminosity and unusual outburst behavior,
these characteristics indicate that V838 Mon represents a hitherto unknown type
of stellar outburst, for which we have no completely satisfactory physical
explanation.
