Did antimatter 'factory' spark brightest supernova?
07 May 2007
The brightest supernova ever recorded may have been triggered
by an exotic process involving antimatter in an extremely
massive star, a new study says. The explosion may offer a
rare glimpse of how the universe's first generation of stars
The explosion was first spotted on 18 September 2006 and
named SN 2006gy. It quickly became apparent that it was something
out of the ordinary.
To begin with, it broke the record for the intrinsically
brightest supernova ever recorded. Other events, like SN
1987A, have appeared brighter to us, but only because they
took place much closer to Earth.
An early analysis of the explosion suggested it might be
the result of a stellar corpse called a white dwarf star
smashing into the core of a bloated red giant star.
But new evidence suggests it was something even more exotic.
It now appears to have been an extremely massive star meeting
its end in a highly unusual way that involves the production
of antimatter, according to a team of astronomers led by
Nathan Smith of the University of California in Berkeley,
The researchers used visible and infrared observations from
facilities such as the Lick Observatory on Mount Hamilton,
California, as well as X-ray measurements from NASA's Chandra
space telescope to investigate the nature of the explosion.
Watch an animation comparing
views of SN 2006gy at different wavelengths.
With 100 times the energy of a typical supernova explosion,
SN 2006gy was simply too energetic to be explained by the
explosion of a lightweight object like a white dwarf star,
even if it were to collide with the core of a red giant,
says team member Craig Wheeler of the University of Texas
in Austin, US.
"That kind of explanation could not produce the energy we're
seeing," he told New Scientist.
The researchers argue instead that it was the explosion
of a very heavy star that was born with as much as 150 times
the mass of the Sun. Heavy stars normally collapse to form
black holes at the end of their lives, but it has long been
theorised that especially heavy ones could instead be completely
ripped apart by an exotic process called pair instability.
In the bowels of such a stellar titan, the high temperature
and pressure conditions are ripe for the conversion of light
into particle pairs in which one particle is an electron
and the other is its antimatter counterpart, a positron.
This causes a drop in pressure that makes the star unstable.
It begins to contract, which eventually ignites runaway nuclear
reactions that rip the star to shreds. Watch an animation
of a pair instability supernova.
The huge amount of radioactive material spewed into space
from the shredded core of such a star could explain the extreme
brightness of SN 2006gy, the researchers say.
"It isn't quite proof yet, but it smells kind of like a
pair formation supernova," Wheeler says. "I think it's the
opening of a new chapter in supernova research."
Avishay Gal-Yam of Caltech in Pasadena, US, a member of
the team that initially suggested the white dwarf collision
scenario, says in light of the new data, a scenario involving
a massive star looks more likely to be correct.
He and colleagues had initially been sceptical of that possibility
because only old, relatively lightweight stars appeared to
lie within the galaxy that SN 2006gy exploded in, NGC 1260,
which is 240 million light years from Earth.
"Additional, very sensitive observations of the core show
that it has just a [few] young, massive stars, right where
the supernova exploded," Gal-Yam told New Scientist. "So
an explanation of the supernova which requires a massive
star, which initially seemed unlikely, now becomes more plausible."
He says the idea that the massive star exploded as a result
of the exotic pair instability scenario is an exciting but
speculative possibility that would need confirmation with
There had previously been some speculation that another
unusual supernova, SN 2006jc, was also
the result of pair instability.
But Alex Filippenko of Caltech, who is a member of Smith's
team, says that is unlikely. SN 2006jc was intrinsically
much dimmer, suggesting that it spewed far less radioactive
material into space, he says. "I think it very unlikely that
the same physical mechanism operated in SN 2006jc as in SN
2006gy," he told New Scientist.
SN 2006gy may have offered an unprecedented view of the
process that killed off the universe's first stars, he says.
The event shows that some extremely massive stars can avoid
collapsing to form a black hole, and instead seed the universe
with heavy elements when they die. Heavy elements are needed
for the formation of planets and life as we know it. "The
first-generation stars may have produced and dispersed heavy
elements in this manner," Filippenko says.
The event may also presage an even more spectacular explosion
in our cosmic backyard. The star that produced SN 2006gy
appears to have blown off a lot of material prior to the
One of the most massive stars in our own galaxy, Eta Carinae,
has also been shedding large amounts of material, suggesting
that it, too, might be about to die in a pair instability
supernova, Filippenko says. If it does go, it will appear
amazingly bright because at 7500 light years, it is much
closer than SN 2006gy was.
article on Eta Carinae ]