Nobody even knew the star V838 Monocerotis existed before it suddenly became the brightest star in the entire Milky Way in January 2002. But we don't actually know why this star suddenly became 600,000 times brighter than the Sun.
The star's brief reign as our galaxy's brightest star didn't last long, and it faded back into obscurity almost as quickly as it had appeared in the first place. Almost a decade later, astronomers still aren't exactly sure what caused this tremendously bright outburst, but there are some theories, one of which is the so-called light echo. A NASA astronomer explains:
It's true that supernovae and novae expel matter out into space. But while the V838 Mon 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. V838 Mon lies about 20,000 light years away toward the constellation of Monoceros the unicorn. In this Hubble Space Telescope image from February 2004, the light echo is about six light years in diameter.
Via NASA.