By Dr David Whitehouse BBC News Online science editor

Astronomers have combined the visible light from six telescopes to produce a single, high-resolution image of a distant multiple-star system.

The feat is being described as a major advance in humanity's ability to probe the Universe.

The synthesized image shows the three component stars of Eta Virginis, located about 130 light-years from Earth.

To obtain such detail in a single, normal telescope would have required a massive mirror that measured 50 metres (164 feet) across. Nothing on such a scale currently exits.

Scientists are delighted because the picture shows how the technique known as interferometry can be used to study stars in the optical (visible) region of the spectrum.

Separation distance

"This development makes it possible to 'synthesize' telescopes with apertures in excess of hundreds of metres," said Dr Kenneth Johnston, Scientific Director of the US Naval Observatory (USNO), which has been working on the project with the Naval Research Laboratory (NRL), and Lowell Observatory.

"It will lead to the direct imaging of the surfaces of stars and of star spots, analogous to the sunspots on the Sun.

A single, massive telescope would have to be built to pick out the three stars that make up Eta Virginis

"This technology can also be applied to space systems for remote sensing of the Earth and other objects in the Solar System, as well as stars and galaxies."

Optical interferometers combine the light from several independent telescopes to form a "synthetic" telescope whose ability to make a high-resolution image depends not on the size of any individual telescope in the network but on the maximum separation between the telescopes.

Radio interferometry has been carried out for almost 50 years but the field of optical interferometry is a new and rapidly developing branch or astronomy.

Distant planets

Many scientists believe optical interferometers could be the answer to the prohibitive costs and immense technical difficulties that currently hold back the development of extremely large, single-mirror telescopes.

That said, it is no easy task linking optical telescopes together either. To merge the six beams from the six telescopes focused on Eta Virginis, the researchers had to design a new type of hybrid beam combiner

The twin Keck 10-metre (32 feet) telescopes on Hawaii were combined for preliminary observations last year and the European Southern Observatory's Paranal Observatory will eventually combine the light from four eight-metre (26 feet) telescopes.

Stellar astrophysics should be revolutionised by the capability to directly image stars.

Ultimately, when employed in space with the experience collected from ground-based instruments, optical interferometry may develop the capability to image Earth-sized planets orbiting distant stars.

'On the brink'

We could then study the atmospheres of these worlds to see if the telltale signs of life are present.

"Remember the early days of radio interferometry and look at the world- wide arrays we routinely use today," said Dr Johnston.

Close-up on Eta Virginis: The technology may eventually lead to pictures of distant planets

"We've gone from simple two-element arrays to continent-sized ones with 10 or more antennae that produce extremely fine-scale images of distant quasars.

"We are standing on the brink of achieving similar results for visible-light sources."

In the near future, the researchers will be commissioning all of the remaining stations on to which any of the six telescopes can be mounted for a maximum array size of 430 metres (1,410 feet), the largest of all current imaging interferometer projects.