Hubble Watches Light From Mysterious Erupting Star Reverberate Through Space
NASA Headquarters, Washington
Lars Lindberg Christensen
Hubble European Space Agency Information Center, Garching,
Space Telescope Science Institute, Baltimore
|March 26, 2003
Light echo from the star known as
V838 Monocerotis or V 838 Mon.
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's Space Telescope of a phenomenon
called a "light echo" has 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 also
do an 'astronomical catscan' 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
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 the Earth. Bond predicts that the echo will be observable
for the rest of this decade.
Electronic image files and additional information are available
The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center, Greenbelt,
Md. The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).
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