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Tuesday, 27 November, 2001, 16:11 GMT
Star caught in the act
K3-35, VLA
Ultraviolet light makes the expelled gas fluoresce
By BBC News Online science editor Dr David Whitehouse

A star has been caught during the very brief period in its life when it is changing into a so-called planetary nebula - a shining bubble of glowing gas surrounding the hot remnant of the star.

"This is the first time that anyone has seen a star that is so clearly going through this transformation stage," said Yolanda Gomez, an astronomer at the Institute for Astronomy at the National Autonomous University in Mexico City, Mexico.

"We believe this star began to enter its planetary-nebula phase only after 1984," she added.

Astronomers believe that our own Sun will someday "puff out" in the way, probably destroying the Earth in the process.

Brief lives

Towards the end of their lives, stars like our Sun eject gas into space before contracting under their own gravity into white dwarf stars.

The gravitational contraction raises the star's temperature, causing it to pour out energetic ultraviolet light. This light disrupts molecules in the gas ejected earlier making them glow. The glowing gas can form beautiful glowing shells, and other shapes, of gas.

However, once the remnant star has heated sufficiently to produce yet larger amounts of ultraviolet light, molecules in the gas are destroyed. This means that the glowing shell period is a short one.

Using the Very Large Array (VLA) of radio telescopes in New Mexico astronomers observed radio waves emitted by water molecules in a planetary nebula.

Ring of gas

According to Gomez: "The water molecules, we believe, are all destroyed within only 100 years of the beginning of this stage, so we are seeing this star during an extremely brief transition period of its life. This is the first time that water has been detected in a planetary nebula."

The planetary nebula is called K3-35 and is some 16,000 light-years from Earth in the constellation Vulpecula. It is a doughnut-shaped ring of gas with lobes of outflowing material, similar to structures seen in other planetary nebulae.

The researchers were surprised to find regions near the star in which water molecules were amplifying, or strengthening, radio-wave emissions in the same way that a laser amplifies light waves.

They found these regions, called masers, in the doughnut-shaped structure surrounding the central star, as well as at the end of much larger lobes of gas extending from the star.

Space 'laboratory'

The doughnut-shaped ring has a radius of more than twice the distance from the Sun to Pluto. The masers at the ends of the lobes are more than 100 times more distant from the star.

By analysing their VLA observations as well as earlier observations of the object by other astronomers, the research team concludes that K3-35 has only just begun its transformation into a planetary nebula.

"This is extremely exciting, because we now have a 'laboratory' for watching this process take place over the next few years," said Luis Miranda, of the Astrophysical Institute of Andalucia (IAA) in Granada, Spain.

"We don't fully understand everything we see in this object, but know that we are going to learn much valuable information about this process by watching it develop."

He added: "We are very lucky to have caught this star during such a very brief but important period of its life."

The term planetary nebula is a misnomer; early astronomers mistakenly thought these objects to be planets.

See also:

24 Jan 01 | Science/Nature
06 Apr 00 | Science/Nature
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