In January 2002, a dull star in an obscure constellation suddenly became
600,000 times more luminous than our Sun, temporarily making it the
brightest star in our Milky Way galaxy.

The mysterious star has long since faded back to obscurity, but
observations by NASA’s Hubble Space Telescope of a phenomenon called a
“light echo” have uncovered remarkable new features. These details
promise to provide astronomers with a CAT-scan-like probe of the
three-dimensional structure of shells of dust surrounding an aging star.
The results appear tomorrow in the journal Nature.

“Like some past celebrities, this star had its 15 minutes of fame,” says
Anne Kinney, director of NASA’s Astronomy and Physics program,
Headquarters, Washington. “But its legacy continues as it unveils an
eerie light show in space. Thankfully, NASA’s Hubble has a front row
seat to this unique event in our galaxy.”

Light from a stellar explosion echoing off circumstellar dust in our
Milky Way galaxy was last seen in 1936, long before Hubble was available
to study the tidal wave of light and reveal the netherworld of dusty
black interstellar space.

“As light from the outburst continues to reflect off the dust
surrounding the star, we view continuously changing cross-sections of
the dust envelope. Hubble’s view is so sharp that we can do an
‘astronomical cat-scan’ of the space around the star,” says the lead
observer, astronomer Howard Bond of the Space Telescope Science
Institute in Baltimore.

Bond and his team used the Hubble images to determine that the petulant
star, called V838 Monocerotis (V838 Mon) is about 20,000 light-years
from Earth. The star put out 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 coming
directly toward Earth from the star itself.

The outburst of V838 Mon 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, however, V838 Mon did not expel its outer layers. Instead,
it grew enormously in size, with its surface temperature dropping to
temperatures 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.

“We are having a hard time understanding this outburst, which has shown
a behavior that is not predicted by present theories of nova outbursts,”
says Bond. “It may represent a rare combination of stellar properties
that we have not seen before.”

The star is so unique it 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 circular light-echo feature has now expanded to twice the angular
size of Jupiter on the sky. Astronomers expect it to continue expanding
as reflected light from farther out in the dust envelope finally arrives
at Earth. Bond predicts that the echo will be observable for the rest of
this decade.

The research team included investigators from the Space Telescope
Institute in Baltimore; the Universities Space Research Association at
the U.S. Naval Observatory in Flagstaff, Ariz.; the European Space
Agency; Arizona State University; the Large Binocular Telescope
Observatory at the University of Arizona at Tucson; the Isaac Newton
Group of Telescopes in Spain’s Canary Islands; and the INAF-Osservatorio
Astronomico di Padova in Asiago, Italy.

Electronic image files and additional information are available at
http://hubblesite.org/news/2003/10