What caused this outburst of V838 Mon?
For reasons unknown, star 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 --
and novas expel matter out into space.
Although the V838 Mon flash appears to expel material into space, what is seen in the
above image from the Hubble Space Telescope 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 complex array of ambient
interstellar dust that already surrounded the star.
V838 Mon lies about 20,000
light years away toward the constellation of the unicorn
(Monoceros), while the
light echo above spans about six light years in diameter.
H. E. Bond (STScI)