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.