Astronomers Discover Third Tail on Hale-Bopp

BOSTON (AP) _ Astronomers say they have found a third tail trailing behind the Hale-Bopp comet _ a thin, straight jet of sodium gas unlike any other seen before, The Boston Globe reported today.

The discovery was made Friday by a team of astronomers at the Isaac Newton Group of telescopes in the Canary Islands. The scientists were at a loss to explain how the sodium tail was created.

The astronomers used a filter over a telescope that allowed them to detect the light given off by sodium gas, the same yellow glow seen in ordinary sodium-vapor street lamps.

Astronomers have long known that comets have two types of tails _ one made of dust and the other of electrically charged gas called plasma. They have also known that comets contain sodium, but have not yet seen it in the form of a tail.

"It's a bit of a surprise," said Brian Marsden, an astronomer who runs the Cambridge-based International Astronomical Union's clearinghouse for discoveries. "It may be the most surprising thing we've had with Hale-Bopp."

The sodium tail looks very different compared with the Hale-Bopp's plasma and dust tails. The dust tail is the broad, bright band that is most visible from Earth. The plasma tail is made of narrow wisps and is seen to the left of the dust tail.

The sodium tail, on the other hand, is described as a straight, thin line farther left of the two tails. Unlike the others, it doesn't grow wider as it gets farther from the nucleus. The tails are made of very fine particles, about the consistency of cigarette smoke.