A "light echo" is one of the rarest and most beautiful phenomena in astronomy. The most spectacular light echo in history is occurring around the previously unknown star V838 Monocerotis, and is being imaged regularly by the Hubble Space Telescope.

A light echo is created when light from a sudden stellar brightening spreads out into space and illuminates nearby interstellar dust. Because of the detour that the light takes in going out to the dust, scattering off it, and then traveling to the Earth, the light arrives months or years after the light from the star itself.

The light echo around V838 Mon, which had a sudden outburst in early 2002, leads to a direct geometric determination of the distance to the star through a novel technique based on polarimetric imaging with the Hubble telescope. The resulting distance of 20,000 light-years shows that V838 Mon was temporarily one of the brightest stars in the Milky Way. Its unusual outburst--the star expanded to become a red supergiant within a few weeks--poses a severe puzzle to stellar theoreticians. Transients of similar behavior have been discovered in the galaxies M31 and M85. The mystery is deepened by our recent discovery that V838 Mon belongs to a young stellar cluster, lying at the outer edge of our Galaxy.

Perhaps more important to the non-specialist, however, the images of the light echo are among the most stunning obtained to date by the Hubble Space Telescope.