In March 2002, the star V838 Monocerotis flared to 10,000 times its normal brightness. Hubble turned its gaze on the surprising star and captured a series of images of a "light echo." As the light of the flash travels away from the star, it illuminates more and more of the usually invisible gas and dust around the star. The cause of this mysterious outburst is still unknown, though scientists have some theories.
Hubble press release:
Hubble Watches Light from Mysterious Erupting Star Reverberate Through Space
Space Phenomenon Imitates Art in Universe's Version of van Gogh Painting
Light Continues to Echo Three Years After Stellar Outburst
Hubble's Latest Views of Light Echo from Star V838 Monocerotis
A light echo is light from a stellar explosion echoing off dust surrounding the star. V838 Monocerotis produced enough energy in a brief flash to illuminate surrounding dust, like a spelunker taking a flash picture of the walls of an undiscovered cavern. The star presumably ejected the illuminated dust shells in previous outbursts. Light from the latest outburst travels to the dust and then is reflected to Earth. Because of this indirect path, the light arrives at Earth months after light from the star that traveled directly toward Earth.
Astronomers do not fully understand the star's outburst. It was somewhat similar to that of a nova, a more common stellar outburst. A typical nova is a normal star that dumps hydrogen onto a compact white-dwarf companion star. The hydrogen piles up until it spontaneously explodes by nuclear fusion - like a titanic hydrogen bomb. This exposes a searing stellar core, which has a temperature of hundreds of thousands of degrees Fahrenheit.
By contrast, V838 Monocerotis did not expel its outer layers. Instead, it grew enormously in size. Its surface temperature dropped to temperatures that were not much hotter than a light bulb. This behavior of ballooning to an immense size, but not losing its outer layers, is very unusual and completely unlike an ordinary nova explosion.
The outburst may represent a transitory stage in a star's evolution that is rarely seen. The star has some similarities to highly unstable aging stars called eruptive variables, which suddenly and unpredictably increase in brightness.
The echoing of light through space is similar to the echoing of sound through air. As light from the stellar explosion continues to propagate outwards, different parts of the surrounding dust are illuminated, just as a sound echo bounces off of objects near the source, and later, objects further from the source. Eventually, when light from the back side of the nebula begins to arrive, the light echo will give the illusion of contracting, and finally it will disappear.
V838 Mon is located about 20,000 light-years away from Earth in the direction of the constellation Monoceros, placing the star at the outer edge of our Milky Way galaxy.