NASA - National Aeronautics and Space Administration

Series of images over time of the light echo from the star known as V838 Monocerotis or V 838 Mon. Photo credit: NASA.


On the next hot summer day, imagine what would happen if the Sun suddenly became one million times brighter. Ice cream would quickly melt, sunscreen lotion wouldn't work very well — and that's just the beginning. Thankfully, our Sun doesn't misbehave this way.

Yet, in early 2002, we witnessed strange behavior by a star in the dim winter constellation Monceros (Moh-NO-ser-os) the Unicorn. The star, named V838 Monocerotis (Moh-NO-ser-u-tis) or V 838 Mon for short, grew very bright then cooled and faded away. The glow was bright enough to light up layers of dust surrounding the star, like a flashlight shining through smoke in a dark room.

What caused this star to become the brightest star in our Milky Way Galaxy for a short time?

Astronomers think V838 Mon suddenly grew hotter and swelled to nearly 1,000 times larger than our Sun. At that size, we could see much more of the star, which explains why V838 Mon appeared so much brighter. Just think...at that size, our Sun would swallow Earth and stretch all the way to Jupiter's orbit!

NASA's Hubble Space Telescope watched the dust shell around V838 Mon over many months after the outburst. Every time Hubble astronomers take a picture, the dust ring looks larger than before. But, in the Hubble pictures, the dust really isn't growing. Instead, light from the flash is sweeping through the dust, lighting up different parts. This illusion is called a "light echo."

How can light "echo?"

Think of how sound echoes. If you stand in a cave and shout, you will hear the echo of your own voice. Some of the sound waves bounce off a wall before they come back to you. You hear them even after you stop shouting. Light travels in waves too, like ripples from a stone dropped in a pond. We only see light "echoing" when it bounces off an object in space much larger than a canyon on Earth.

The light echo produced by V838 Mon occurred when light from the star bounced off rings of dust surrounding the star. After bouncing off the dust, the light then traveled to Earth. The bouncing light echo arrived at Earth months after the direct "starlight."

+ View press release

+ View animation of star V838 Mon expanding to a red super giant, Jan 2002.
(12.5 Mb 640x480)
+ View animation of star V838 Mon expanding to a red super giant, Jan 2002.
(2.9 Mb 320x240)

+ View dissolve sequence of the 4 epochs of the light echo of star V838 Mon from
May 20, Sept. 2, Oct. 28 and Dec.17. (5 Mb)
+ View dissolve sequence of the 4 epochs of the light echo of star V838 Mon from
May 20, Sept. 2, Oct. 28 and Dec.17. (918 Kb)