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Boston

RECREATING an exploding star in your lab sounds impossible, not to mention
foolhardy. But that鈥檚 what Bruce Remington of Lawrence Livermore National
Laboratory in California has done in a bid to understand the biggest shockwaves
in the Universe.

When a huge star runs out of fuel and collapses, its core rebounds outwards
in a huge explosion called a supernova. The edge of the explosion is marked by a
shockwave, where the debris is moving faster than the speed of sound. On Earth,
shockwaves travel spherically out from an explosion. But astronomers see sinewy
fingers of debris spreading out from supernovae, indicating that their
shockwaves are convoluted.

One possible explanation is that supernovae shockwaves shake the surrounding
remnants of the star violently enough to radiate photons in front and behind the
wave. 鈥淩adiative shockwaves鈥� like these would be much less stable than regular
shockwaves, says Remington, and irregularities in the star鈥檚 debris could easily
seed finger formation.

Remington and his colleague Paul Drake at the University of Michigan decided
to make a miniature supernova to see if this theory holds water. Their method,
which they proposed several years ago
(快猫短视频, 18 April 1998, p 30),
uses a laser to vaporise a tiny plastic target. The explosion blasts into a
compressible foam, which ripples with the shockwave passing through it.

Supernovae impart 10⁴¹ kilojoules of energy to the surrounding debris and
take years to spread out. But by scaling down to a target a tenth of a
millimetre across, which is blasted to bits in a nanosecond, Remington and Drake
found they could duplicate the physics in supernova shockwaves.

In a paper submitted to Physical Review Letters, Drake and Remington report
that their shockwave did indeed radiate photons. 鈥淭his is the first time there鈥檚
been clean evidence of a radiative shock,鈥� he says.

Remington鈥檚 shockwave didn鈥檛 last long enough to produce the characteristic
finger shapes seen in supernova blasts. But he says the conditions were similar
to those in experiments by fusion researchers where finger shapes have been seen
before. These researchers may have unwittingly created radiative shockwaves too,
says Remington.

He is now working with Peter Norreys at the Rutherford Appleton Laboratory in
Oxfordshire to replicate another mysterious astronomical event鈥攇amma ray
bursts. Remington says a plasma fireball created by vaporising a
micrometre-sized gold target with their laser in a picosecond should generate,
for an instant, the same temperature as the photons released by a gamma ray
burst. The experiment could be used to test theories about what causes these
bursts.

Barry Ripin of the Naval Research Laboratory in Washington DC says
Remington鈥檚 experiments are impressive. But he cautions that 鈥渋t鈥檚 important
that this is coupled with real astrophysics to ensure you don鈥檛 stray too far
from reality.鈥�