In March, astronomers working with the Hubble Space Telescope released a breathtaking series of images that illustrate a stellar process never before seen. In January 2002, an amateur discovered an eerie dust cloud surrounding the binary system V838 Monocerotis; several months later professional astronomers decided to focus Hubble's gigantic eye on the system at regular intervals. The astronomers now think the luminous cloud is a result of brief but powerful light flashes that emanate from one of two stars in the system. The reason for the light flashes is unknown, but astronomers believe they cause the star to slough off layers of dust, which under normal conditions are too dark to see. However, as captured in the January 2002 observation, one of the stars again underwent a period of intense luminosity, temporarily making the star 600,000 times brighter than the sun. As this light flash moved outward from the star, it illuminated increasingly distant layers of dust surrounding the star system. The astronomers have dubbed this phenomenon a light echo, since it is the light from V838 Monocerotis, not the dust itself, that created this spectacular view.
Dust surrounding the binary star system V828 Monocerotis is lit up by an enormous flash of light bursting out from one of the system's two stars. The Hubble Space Telescope captured the light moving outward from the system as it illuminated dust believed to have originated from an earlier outburst of the same type. Astronomers still don't know what causes the star to swell and shed light this way.
Howard Bond, an astronomer at the Space Telescope Science Institute in Baltimore, says the outburst does not conform to any known stellar explosion, such as a supernova, which usually leaves behind a beautiful gas cloud called a planetary nebula. "In the case of a planetary nebula, the entire nebula glows through fluorescence from a hot central star and changes its appearance very slowly," says Bond. "With the light echo, the relatively cool star in the center rose suddenly in brightness, sending a flash out into the dust and briefly making it shine." Bond and his colleagues still do not know exactly what causes this particular type of stellar outburst, but they believe its effects will most likely be visible for the rest of this decade.
