Like a
cosmic Grim Reaper, a blast of ultraviolet light signals the violent death of
the universe's most massive stars. Now astronomers have viewed this heavenly
harbinger for the first time.
"Astronomers
have been dreaming about seeing the first light from the violent death of a
star for over 30 years," said lead researcher Kevin Schawinksi of the University of Oxford. "Our observations open up an entirely new avenue for studying
the final stages in the lives of massive stars and the physics of
supernovae."
Schawinksi
and his colleagues detected the ultraviolet signal of a hefty star on the verge
of explosion, which they detail in the June 13 issue of the journal Science.
Usually,
when astronomers see a supernova, the star has already been destroyed. "It's
very hard to tell much about precisely the kind of star that actually died
there," Schawinski told SPACE.com. "The really cool thing
about our observations is this light traveling ahead of the shock wave traveled
through the star before it was destroyed."
He added,
"It's telling us about the properties, the conditions, of the star at the
moment it died, but before the shock wave actually disrupted it."
Doomed
star
When a
massive star, weighing at least 10 suns, runs out of nuclear fuel, it can
collapse under its own weight, triggering an explosion called a supernova. The
explosion sends the stellar guts
spewing away at 20 million mph (10,000 km/sec) in a fireball that's a
billion times brighter than the sun, the researchers say.
It's this
fireball that scientists observe. What they haven't seen until now are the
final moments of the doomed star just before the visible explosion. For the
past 30 years or so, theorists have predicted a surge of ultraviolet light
should come before the actual visible
explosion.
There are
several problems for actually seeing this phenomenon. "By the time you see
the supernova, it's already days or weeks in the past," Schawinski said.
"If you see a supernova you'd have to go back in time. You'd have to be
already looking at the position."
The other
issue is the fact that Earth's atmosphere absorbs ultraviolet light, and so
you'd need a space telescope to actually be able to view the death beacon. The
space telescope GALEX, which orbits Earth about every 98.6 minutes and views the
universe in ultraviolet, was the answer.
With GALEX,
researchers recently got front-row seats to the pre-show of what they suspect
was a red supergiant star measuring somewhere between 500 and 1,000 solar radii
on the verge of explosion. A red
supergiant is a hefty star nearing the end of its life that can swell to
100 times its original size before exploding.
Schawinski
and his colleagues looked at GALEX images taken at the positions of supernovae
previously identified with optical telescopes in Hawaii.
"We found
a new source at the location of one supernova, suddenly outshining its galaxy host
in the UV," said Mark Sullivan of the University of Oxford. "It
appeared a couple of weeks before the optical discovery of the supernova and
marked the first stage in the death of the star."
The
final hours
The UV peak
represented a unique phase in the formation of the supernova SNLS-04D2dc, just
before the shock wave from its collapsed core reached the star's surface to
violently eject its shell of hot gas.
During a
red supergiant's final hours, a shock wave whizzes outward with the related
radiation moving even faster and heating up the star's surface. The temperature
at the surface ramps up from a few thousand degrees Celsius to several hundred
thousand degrees. Just before the shock wave catches up and reaches the surface
(triggering a supernova), the star is producing the same total luminosity as a
thousand billion suns, the researchers say.
Once the
shock wave catches up, it plows through the outer parts of the star,
accelerating several suns' worth of material outward. The surface of the
star explodes. A few days later, supernova hunters will spot the bright
visible light of the explosion.
Schawinski
describes the observations as looking inside of a semi-transparent star as it's
dying.
"We
saw the whole thing. We saw the radiative precursor, this UV light, moving
ahead [of the shock wave]," Schawinski said. "We saw that arrive and
then the point at which the shock wave comes to the surface and destroys the
star. In a sense we could see the shock move inside the star because the light
from the shock was moving ahead of it."
The new UV
peak findings, the astrophysicists say, will shed light on deathly details once
hidden beneath a star's outer cloak.
"This
is a whole new avenue into studying the late stages of massive stars,"
said Oxford researcher Christian Wolf. "Most of what we know today is
based on computer simulations. But as always when you test theory against
observations for the first few times, we may be in for surprises."
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