Three extra stars have been discovered orbiting members of the Southern Cross, which graces the flag of five nations. Beta Crucis, the boxed star on the left of the constellation, has two companions, and alpha Crucis, on the bottom, has one. The image at bottom left shows beta Crucis and its most recently-discovered companion.

Credit: Swarthmore College

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SYDNEY: The discovery of three extra stars in the Southern Cross threatens the accuracy of the flags of five nations, including Australia.

Beta Crucis, the left hand star in the famous constellation, and the nineteenth brightest star in the sky, appears to have a companion that orbits it once every 2000 years, according to observations made by scientists using the U.S. Chandra X-ray Observatory satellite.

A team led by astronomer David Cohen of Swarthmore College in Pennsylvania uncovered the companion star largely by accident, while the group was using the satellite to study the X-rays emitted by Beta Crucis itself. "We were interested in how the highly supersonic stellar winds of hot, luminous stars produce X-rays," said Cohen. "We were surprised to see two strong X-ray sources where we had expected to see only one."

Nick Lomb, an astronomer at the Sydney Observatory, said the massive star's smaller companion had never been noticed because it was lost in the glare from Beta Crucis. "It would be like looking for a glow worm next to a floodlight," he told The Sydney Morning Herald.

Beta Crucis and its companion orbit each other at a distance of about 60 billion kilometres - 400 times the distance from the Earth to the Sun. But from our vantage point 352 light years away, the two stars are indistinguishable - the same apparent distance apart as a car's headlights seen from 100 kilometres away.

Beside Australia, the Southern Cross appears on the flags of New Zealand, Brazil, Samoa, and Papua New Guinea. Adding a sixth star to the Aussie flag to improve its accuracy would be difficult, though, Lomb told the Herald, because astronomers already know of two other ignored companions in the constellation.

Alpha Crucis, the star at the base of the cross, is a binary star system whose other member is invisible to the naked eye, and even before the Swarthmore team's discovery, scientists had identified another stellar companion to Beta Crucis, meaning that the system actually has three members. Beta Crucis' other companion completes an orbit once every five years - so close that the individual stars can only be resolved by noting the changes in their spectra.

According to Lomb, showing all eight stars to scale on the flag would require enlarging it slightly. "The flag would have to be huge, probably the size of Sydney," he told the Herald. He added that, in a few million years, the Cross would not exist, three of its stars having exhausted their fuel and burned out. Even sooner than that the perfect symmetry of the Cross will disappear "as all five stars are moving in very different, random directions. In 20,000 years the difference will be quite noticeable," he explained.

Michael Kuhn, an undergraduate member of Cohen's team, presented the findings at a recent meeting of the American Astronomical Society in Seattle, Washington state.