(Source of text: Astrodidact)
V838 Monocerotis is a red variable star in the constellation Monoceros located about 20,000 light years from the sun. For reasons not quite understood, V838 Mon’s outer surface suddenly greatly expanded with the result that it became the brightest star in the entire Milky Way Galaxy in January 2002. Then, just as suddenly, it faded. A stellar flash like this had never been seen before. It’s true that supernovae and novae expel matter out into space, but while the flash appears to expel material into space, what is seen here is actually an outwardly moving light echo of the bright flash. In a light echo, light from the flash is reflected by successively more distant rings in the ambient interstellar dust that already surrounded the star.