Astronomers using the National Science Foundation’s Very Long
Baseline Array (VLBA) radio telescope have found that an aging
star is spewing narrow, rotating streams of water molecules into
space, like a jerking garden hose that has escaped its owner’s
grasp. The discovery may help resolve a longstanding mystery
about how the stunningly beautiful objects called planetary
nebulae are formed.

The astronomers used the VLBA, operated by the National Radio
Astronomy Observatory, to study a star called W43A. W43A
is about 8,500 light-years from Earth in the direction of the
constellation Aquila, the eagle. This star has come to the end of
its normal lifetime and, astronomers believe, is about to start
forming a planetary nebula, a shell of brightly glowing gas lit by
the hot ember into which the star will collapse.

“A prime mystery about planetary nebulae is that many are not
spherical even though the star from which they are ejected is a
sphere,” said Phillip Diamond, director of the MERLIN radio observatory
at Jodrell Bank in England, and one of the researchers using the VLBA.
“The spinning jets of water molecules we found coming from this
star may be one mechanism for producing the structures seen in many
planetary nebulae,” he added.

The research team, led by Hiroshi Imai of Japan’s National
Astronomical Observatory (now at the Joint Institute for VLBI in
Europe, based in the Netherlands), also includes Kumiko Obara of the
Mizusawa Astrogeodynamics Observatory and Kagoshima University;
Toshihiro Omodaka, also of Kagoshima University; and Tetsuo
Sasao of the Japanese National Astronomical Observatory. The
scientists reported their findings in the June 20 issue
of the scientific journal Nature.

As stars similar to our Sun reach the end of their “normal”
lives, in which they are powered by nuclear fusion of hydrogen
atoms in their cores, they begin to blow off their outer
atmospheres, then eventually collapse to a white dwarf, about
the size of the Earth. Intense ultraviolet radiation from the
white dwarf causes the gas thrown off earlier to glow, producing
a planetary nebula.

Planetary nebulae, many visible to amateurs with backyard
telescopes, have been studied by astronomers for years. About
1600 planetary nebulae have been found, and astronomers believe
many more exist in our Milky Way Galaxy. Some are spherical,
but most are not, displaying a variety of often intricate, beautiful
shapes. The fact that many of these objects are not spherical was
long known, but a series of spectacular images made with the Hubble
Space Telescope in 1997 reinforced that fact dramatically.

“The problem for scientists is, how do you get from a star that
we know is a sphere to a planetary nebula that is far from being a
sphere and yet came from that star,” said Imai. Some theorists
suggested that old stars must be somehow producing jets of material
that help form the odd-shaped planetary nebulae, but such jets had,
until now, never been seen.

W43A was known to have regions near it in which water molecules
are amplifying, or strengthening, radio emission at a frequency
of 22 GigaHertz. Such regions are called masers, because they
amplify microwave radiation the same way a laser amplifies light
radiation. Imai’s team used the VLBA, the sharpest radio “eye”
in the world, to find out where these masers are. To their
surprise, they found that the maser regions are strung out in
two curved lines, moving in opposite directions from the star
at about 325,000 miles per hour.

“The path of the jets is curved like a corkscrew, as if whatever
is squirting them out is slowly rotating, or precessing, like a
child’s top wobbles just before it falls down,” said Diamond.

What is producing the jets? “We’re not sure,” Diamond said.
“Traditional wisdom says that it takes a disk of material
closely orbiting the star to produce jets, but we don’t yet
know how such a disk could be produced around such an old
star,” he added.

The astronomers are probably very lucky to have caught W43A
in what they believe is a brief transitional stage of its
life. “Our analysis of the water jets indicates that they
are only a few decades old,” Imai said. “Once the star
collapses of its own gravity into a dense white dwarf, its
intense ultraviolet radiation will rip apart the water
molecules, making observations such as ours impossible,”
he added.

Planetary nebulae may be the worst-named class of objects in
astronomy, because, despite the name, they have nothing to do
with planets. The French astronomer Charles Messier discovered
the first one, now known as the “Dumbbell Nebula” to amateur
astronomers, in 1764. Sir William Herschel, who discovered the
planet Uranus in 1781, later began a systematic survey of the
entire sky and found more objects similar to the Dumbbell.
Because their appearance resembled, to him, the appearance of
Uranus in a telescope, he coined the term “planetary nebula,”
a name that has stuck ever since. Astronomers have long known
that these objects are not actually related to planets, but
the name has remained to confuse generations of students.

The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative
agreement by Associated Universities, Inc.


NOTE: A graphic to accompany this release is at:

Caption: Artist’s conception of W43A, with the aging star
surrounded by a disk of material and a precessing,
twisted jet of molecules streaming away from it
in two directions.

Credit: Kirk Woellert/National Science Foundation.

For more information:

Dr. Phillip Diamond

44 1477 571 321