WASHINGTON — Astronomers at 25 observatories around the world, including Hawai'i, began aiming their telescopes this week at a preview of our sun's eventual death.

Their target is a slowly cooling "white dwarf" star in the constellation Virgo that eventually will become a cold, black cinder.

A similar fate is forecast for the sun, but not to worry. That won't happen for at least 4 billion years.

"Someday the sun will be a white dwarf," said Judith Provencal, an astronomer at the University of Delaware in Newark. "It's forming the white dwarf in its core right now."

Most stars become white dwarfs after they exhaust their nuclear fuel. They aren't burning anymore, as the sun is, but glowing like embers in a dying fire. Dwarfs are extremely dense, holding as much material as the sun in a body the size of our planet. Astronomers say that a teaspoon of white dwarf material would weigh about a ton on Earth.

The series of white dwarf observations, scheduled to run until May 1, is a project of an international astronomical network known as the Whole Earth Telescope.

The viewings began Wednesday night at the Southern African Large Telescope, a 39-foot-wide mirror in Sutherland, South Africa. Observatories in Spain, Delaware, Texas, Arizona, Hawai'i, New Zealand, Australia, China and so on around the globe will provide around-the-clock coverage, ending May 1 in Brazil.

"We like to have two telescopes at each longitude," said Provencal, the coordinator of the project. "That way, if one is cloudy, hopefully the other won't be." A trial run at four European observatories last November failed because Europe was socked in by snow and clouds the entire month.

The target, a white dwarf known as IU Vir, some 300 trillion miles from Earth, alternately brightens and dims as huge blobs of material in its interior rise and fall, rather like a lava lamp.

The goal of the observations is to determine the rate of changes in the star's brightness over time, which will let astronomers figure out how fast it's cooling. The rate slows slightly as the star cools.

"Once a white dwarf forms, all it does is sit there and cool," Provencal said. "So we can measure the temperature of a white dwarf, and we can figure out how long it took to cool to that temperature and hence determine how old it is."

Currently, the temperature of IU Vir is thought to be about 21,500 degrees Fahrenheit. The coolest known white dwarf is about 2,500 degrees.

Astronomers think that white dwarfs are the final stage in the evolution of a low- or medium-mass star, such as our sun. When the sun burns up all its hydrogen, it will swell into an enormous "red giant" that will swallow everything in the solar system as far out as Mars.

In time, the red giant will shed its outer layers, forming a ringlike object called a planetary nebula. Its core will be a white dwarf.

By the time that happens, the Earth will have been destroyed and mankind with it, unless our descendants have found a way to reach a planet circling another, younger star.