1843 stellar eruption new type of star explosion (Forwarded)

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10 September 2008

1843 stellar eruption new type of star explosion

BERKELEY -- Eta Carinae, the galaxy's biggest, brightest and perhaps most
studied star after the sun, has been keeping a secret: Its giant outbursts
appear to be driven by an entirely new type of stellar explosion that is
fainter than a typical supernova and does not destroy the star.

Reporting in the Sept. 11 issue of Nature, University of California,
Berkeley, astronomer Nathan Smith proposes that Eta Carinae's historic 1843
outburst was, in fact, an explosion that produced a fast blast wave similar
to, but less energetic than, a real supernova. This well-documented event in
our own Milky Way Galaxy is probably related to a class of faint stellar
explosions in other galaxies recognized in recent years by telescopes
searching for extragalactic supernovae.

"There is a class of stellar explosions going off in other galaxies for
which we still don't know the cause, but Eta Carinae is the prototype," said
Smith, a UC Berkeley postdoctoral fellow.

Eta Carinae (Eta Car) is a massive, hot, variable star visible only from the
Southern Hemisphere, and is located about 7,500 light years from Earth in a
young region of star birth called the Carina Nebula. It was observed to
brighten immensely in 1843, and astronomers now see the resulting cloud of
gas and dust, known as the Homunculus nebula, wafting away from the star. A
faint shell of debris from an earlier explosion is also visible, probably
dating from around 1,000 years ago.

Presumably blown off by the star's fierce wind, the shells of gas and dust
are moving slowly -- at speeds of 650 kilometers per second (1.5 million
miles per hour) or less -- compared to the blast shell of a supernova.

Smith's recent observations using the international Gemini South 8-meter
telescope and the Blanco 4-meter telescope at Cerro Tololo Inter-American
Observatory in Chile reveal something new: Extremely fast filaments of gas
moving five times faster than the debris in the Homunculus nebula were
propelled away from Eta Carinae in the same event. The amount of mass in the
relatively slow-moving Homunculus was already at the edge of plausibility in
terms of what an extreme stellar wind could do physically, Smith said. The
much faster and more energetic material he discovered poses even harsher
difficulties for current theories.

Instead, the speeds and energies involved are reminiscent of material
accelerated by the fast blast wave of a supernova explosion.

The fast speeds in this blast wave could roughly double earlier estimates of
the energy released in the 1843 eruption of Eta Carinae, an event that Smith
argues was not just a gentle surface eruption driven by the stellar wind,
but an actual explosion deep in the star that sent debris hurtling into
interstellar space. In fact, the fast-moving blast wave is now colliding
with the slow-moving cloud from the 1,000-year-old eruption and generating
X-rays that have been observed by the orbiting Chandra Observatory.

"These observations force us to modify our interpretation of what happened
in the 1843 eruption," he said. "Rather than a steady wind blowing off the
outer layers, it seems to have been an explosion that started deep inside
the star and blasted off its outer layers. It takes a new mechanism to cause
explosions like this."

If Smith's interpretation is correct, supermassive stars like Eta Carinae
may blow off large amounts of mass in periodic explosions as they approach
the end of their lives before a final, cataclysmic supernova blows the star
to smithereens and leaves behind a black hole.

Much fainter than a supernova, the explosion that generated the fast-moving
blast wave around Eta Carinae would have been similar to faint stellar
explosions, sometimes called "supernova imposters," now being discovered in
other galaxies by Earth-based robotic telescopes and other supernova
searches. Such searches have been looking primarily for Type Ia supernovae
that could help astronomers understand the accelerating expansion of the
universe, but they also find other gems along the way, Smith said.

"Looking at other galaxies, astronomers have seen stars like Eta Carinae
that get brighter, but not quite as bright as a real supernova," he said.
"We don't know what they are. It's an enduring mystery as to what can
brighten a star that much without destroying it completely."

Eta Carinae is a rare supermassive star in our galaxy, probably once having
had a mass 150 times that of the sun. Such large stars burn brightly for
only a few million years, all the while shedding mass as the intense light
pushes the outer layers of the star away in a stellar wind. After 2 to 3
million years of this, Eta Carinae now weighs about 90 to 100 solar masses,
having shed about 10 solar masses in its most recent 1843 eruption alone.

"These explosions may be the primary way by which massive stars can shed
their outer hydrogen layers before they die," Smith said. "If Eta Carinae is
able to shed 10 solar masses every thousand years or so, that's an efficient
mechanism for peeling off a large fraction of the star."

Astronomers now believe that Eta Carinae and other luminous blue variable
stars are nearing the end of their lives, having burned hydrogen in their
cores into helium. If they explode at the stage where they still have an
envelope of hydrogen shrouding the helium core, the resulting supernova will
look vastly different from one that results from a star that sloughs off all
its hydrogen before exploding.

Smith suggests that it is still unclear if supernova impostors are
scaled-down versions of supernovae, failed supernovae, precursor events or
entirely different kinds of explosions.

"This could be an important clue for understanding the last violent phases
in the lives of massive stars," he said, noting that astronomers still
cannot accurately predict the fate of stars that are 30 or more times the
mass of the sun.

The observations reported in the Nature paper included visible spectra from
the Blanco telescope, which is part of the U.S. National Optical Astronomy
Observatory (NOAO), and near-infrared spectra taken with the Gemini South
telescope. Both telescopes are in Chile's Andes mountains near an elevation
of 9,000 feet. NOAO and the Gemini Observatory are operated by the
Association of Universities for Research in Astronomy.

The research was supported in part by the National Aeronautics and Space
Administration and the National Science Foundation.

[NOTE: Images and an animation supporting this release are available at
http://www.gemini.edu/node/11120 ]