http://www.nytimes.com/2007/05/07/science/space/08novacnd.html?ref=science
Astronomers Report Biggest Stellar Explosion
By DENNIS OVERBYE
Published: May 7, 2007
Kaboom, indeed.
In a cascade of superlatives that belies the
traditional cerebral reserve of their profession,
astronomers reported Monday that they had seen
the brightest and most powerful stellar explosion ever recorded.
The cataclysm a monster more than a hundred
times as energetic as the typical supernova in
which the more massive stars end their lives
might be an example of a completely new type of
explosion, astronomers said. Such a blast
proposed but never seen would explain how the
earliest and most massive stars in the universe
ended their lives and strewed new elements across
space to fertilize future stars and planets.
It is quite possibly the most massive star that
has ever been seen to explode, said Nathan Smith
of the University of California, Berkeley, who
estimated the star as freakishly massive, about
150 times the mass of the Sun.
Were really excited about this, Dr. Smith
said. If it really is what we think it is, it
forces us to rethink how massive stars die. He
led a team of astronomers from the University of
California, Berkeley, and the University of
Texas, who have submitted a paper about the
supernova to the Astrophysical Journal and
discussed the results in a news conference from NASA headquarters today.
Astronomers have been following the star since
last September, when it was discovered in a
galaxy 240 million light years away in the
constellation Perseus by Robert Quimby, a
University of Texas graduate student, who was
using a small robotic telescope at McDonald
Observatory near Fort Davis, Tex., to troll for supernovas.
The star bears an eerie resemblance to one in our
own galaxy, Eta Carinae, which has been burbling
and bubbling in the last few centuries as if
getting ready for its own outburst. The
observations suggest that the troubled and
enigmatic star, thought to weigh in about 120
solar masses, could blow up sooner than theorists
had thought. Mario Livio a theorist at the Space
Telescope Science Institute who was not involved
in the research, said the death of that star
could be the most spectacular star show in history.
Cautioning that theorists still do not know for
sure what caused the explosion announced today,
Dr. Livio said, Here we have the brightest
supernova we have ever observed and we dont know
the explosion mechanism. It doesnt get any more exciting for a theorist.
Such supermassive stars are extremely rare in the
modern universe but are believed to have been
common among the first stars that formed when the
universe was less than a billion years old.
We may be witnessing an example in the local
universe of a process quite common in the early
universe, said Alex Filippenko, a team member
also from the University of California, Berkeley.
The explosion raises astronomers hopes that the
next generation of bigger telescopes, like NASAs
coming James Webb Space Telescope, will be able
to detect these stars by their explosions.
Ironically, we might first detect the first
generation of stars by their deaths, Dr. Filippenko said.
Supernovas come about in two basic ways:
explosions of small stars about one and half
times the mass of the Sun, known as White Dwarfs,
and which are uniform enough to serve as
cosmological distance markers; and the collapse
of the cores of more massive stars into black
holes or neutron stars when their thermonuclear fuel has run out.
The astronomers first suspected that the
supernovas dramatic output was caused by the
shock wave of a white dwarf exploding into a
dense cloud of hydrogen. When observations with
the Chandra X-ray Observatory failed to find
enough X-rays to support that scenario, the group
was forced to consider the alternative that the
luminosity was produced by the decay of
radioactive nickel. But to match the
observations, the star would have to produce 22
solar masses of radioactive nickel way off scale for the core collapse model.
To get more than 20 solar masses of nickel, you
need one heck of a huge star, Dr. Smith said. In
this case, he said, The core did not collapse, it was blown to smithereens.
In desperation, the astronomers turned to a
theory proposed nearly 40 years ago by Zalman
Barkat of the Hebrew University of Jerusalem and
his colleagues. The intensity of radiation in the
cores of such supermassive stars could be so
great, they said, that pairs of electrons and
their antimatter opposites, positrons, would be created.
That is bad news for the star, Dr. Livio said,
explaining that the disappearance of the
radiation would sap the cores energy and cause
the star to collapse. But in this case the star
still has plenty of fuel and blows up.
The core is still composed of explosive oxygen,
explained Craig Wheeler of the University of
Texas and another of the papers authors. The
oxygen ignites and blows the star to smithereens
with no remnant, no black hole left.
The pair instability, as it is known, is
particularly relevant to the very first stars,
made of pristine hydrogen and helium fresh from
the furnace of the big bang. According to theory,
they could grow to large size because they lacked
the heavier elements, dubbed metals, which are
very efficient in catching light and thus make
modern stars more susceptible to fragmenting
during their formation, limiting their sizes.
Those metals have been produced by thermonuclear reactions in stars.
It is very convenient, as Dr. Smith and his
colleagues pointed out, that the pair mechanism
produces an explosion that scatters all the
stars ashes enriched by thermonuclear
processing, outward into space instead to down into a black hole.
Dr. Filippenko said, It effectively fertilizes
the material from which second and third generation was made.
The astronomers stressed that this diagnosis,
while thrilling, was far from definite. Dr.
Wheeler said, We dont have a good alternative
explanation for the source of luminosity, but we need some smoking gun.
The star is now going behind the Sun but when it
comes out, more observations are planned. The
results of those observations could also have
implications for those later generation stars, like Eta Carinae.
Astronomers had presumed that heavy stars shed
heavy envelopes of hydrogen by winds and burps
before reaching the final stage where they
implode into black holes. It could be, however,
that the most massive stars just cant shed mass fast enough, Dr. Smith said.
Eta Carinae could blow up sooner than we thought,
Dr. Smith said, noting that it could be tomorrow,
it could be thousands of years from now. Astronomers have no way of telling.
Even if it did blow as the new supernova did last
fall, at a distance of around 7,500 light years,
Eta Carinae would be unlikely to cause any
serious harm to Earth, astronomers said. The
explosion would be visible in the daylight and at
night you would be able to read a book by its light.
As for extinguishing life on Earth, We can sleep
quietly tonight, Dr. Livio said, adding that the
puzzle of the supernova would keep astronomers "awake for quite a while."
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