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A DEFUNCT solar power station in the Pyrenees has been reborn as a gamma-ray
telescope, and now physicists are picking up signals from exotic galaxies and
exploded stars.

Celeste, as the new detector is called, rose from the ashes of the
Th茅mis experimental solar power station near Font-Romeu in France. The
station consisted of around 200 steerable mirrors which focused sunlight onto a
collector at the top of a tower to generate power.

The power station closed in the mid-1980s, never having produced power
commercially. But now physicists have revamped the equipment to pick up gamma
rays from space. When a gamma ray hits an atom in the upper atmosphere, it
dislodges a cascade of electrons which rain down to the Earth鈥檚 surface,
generating tiny flashes of light called Cerenkov radiation.

Using 40 of the mirrors at the Th茅mis site, physicists focus these
flashes on a light detector on the tower. The telescope went into action at the
end of last year, and the first analysis of its observations proves that Celeste
can see gamma rays in an energy range that astronomers have not been able to
study before. The telescope has already picked up gamma rays from the Crab
Nebula, the remains of a star seen exploding in the Middle Ages, and Markarian
421, a bright 鈥渁ctive galactic nucleus鈥�, thought to be a supermassive black hole
consuming surrounding gas, dust and stars.

鈥淲e鈥檙e really happy,鈥� says David Smith of the French research agency CNRS,
who heads the Celeste project. 鈥淲e鈥檙e expecting in the fall to start looking at
new sources.鈥�