This finding provides two important breakthroughs. First, it gives far more accurate measurements of the brightness of each individual star than had ever previously been possible.

Second, by comparing two photographs of each star taken up to 60 years apart, it becomes easy to identify stars whose brightness has slowly changed.

Bryan Gaensler and Greg Madsen took up this challenge by combining photographic and digital data from two major astronomical surveys, separated by 60 years, according to a University of Syndey release.

Gaensler, professor at the ARC Centre of Excellence for All-sky Astrophysics, University of Syndey, and Madsen from the University of Cambridge, began by re-examining a collection of 7,400 old photographic plates.

They had previously been combined by the US Naval Observatory into a catalogue of more than one billion stars and galaxies.

The scientists then painstakingly matched all the objects in this catalogue with more modern measurements from the Sloan Digital Sky Survey.

Using stringent criteria, they produced a final catalogue of 44 million stars and galaxies that had definitely been seen twice: both in old photographs and with modern cameras.

"Thanks to clever computer algorithms, we thankfully didn't need to inspect all billion stars and galaxies individually," said Gaensler.

"What is special about this catalogue is that it carefully combines historical data with modern measurements. This is a unique way to study objects that gradually change over years or even decades," says Madsen.

The researchers are making their entire catalogue public on the internet, in the lead-up to the next generation of telescopes designed to search for changes in the night sky.


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