Watching a dying star transform. (Astronomy).

Just before dying, sunlike stars blossom into beauties. They set aglow cocoons of gas that they've previously hurled into space. These shimmering gas bubbles, which come in a rainbow of colors, take on intriguing shapes including teardrops, shells, a cat's eye, doughnuts, and hourglasses.

Astronomers have for the first time caught one of these dying stars at the very beginning of this brief, shining period, when it's known as a planetary nebula.

So named because astronomers a century ago thought their shape resembled that of planets, planetary nebulas have been studied since the 1890s. But never before has a planetary nebula been imaged so soon--only about 15 years--after it formed, reports Yolanda Gomez of the National Autonomous University in Mexico City and her colleagues in the Nov. 15, 2001 NATURE.

A planetary nebula arises when an old, sunlike star contracts under its own gravity to become a compact object called a white dwarf. The contraction heats the star, prompting it to pour out enormous amounts of ultraviolet light. As it penetrates gas that the star previously ejected, the radiation tears molecules into their constituent atoms and rips electrons from those atoms.

Although the radiation rapidly destroys molecules, astronomers studying a doughnut-shaped planetary nebula called K3-35 detected significant amounts of water vapor. Theorists calculate that water molecules can't survive longer than 100 years after a nebula forms, "so we are seeing this star during an extremely brief transition period of its life," says Gomez.

She and her colleagues used the Very Large Array radio telescope in Socorro, N.M., to study the nebula, which lies 16,000 light-years from Earth in the constellation Vulpecula. The researchers detected the water vapor because it amplifies radio waves of a particular frequency. The team's analysis of previous data indicates that the nebula formed no earlier than 1984.

"We infer that K3-35 is being observed at the very moment of its transformation from a giant star to a planetary nebula," the researchers say.

Some of the water vapor resides in the nebula's doughnut-shape ring of gas, which lies at a distance from the star that's roughly equal to the distance between the sun and Pluto. The vapor also infuses the tips of much larger lobes of gas that jut out from the star to distances 100 times the radius of the gas ring.

"We now have a laboratory for watching [the planetary nebula] process take place over the next few years," says study collaborator Luis F. Miranda of the Institute of Andulucia 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." --R.C.