This Hubble Space Telescope image of the star V838 Monocerotis reveals dramatic changes in the illumination of surrounding dusty cloud structures. The effect, called a light echo, unveiled never-before-seen dust patterns when the star suddenly brightened for several weeks in early 2002.
A light echo is light from a stellar explosion echoing off dust surrounding the star that produces enough energy in a brief flash to illuminate surrounding dust. 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.
The phenomena is similar to that of a nova. 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 -- exposing a searing stellar core with 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.
V838 Monocerotis 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.
Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
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