44 million stars and counting: play Snap and remap the sky 29 November 2013

The astronomers then set out to painstakingly match all the objects in this catalogue with more modern measurements from the Sloan Digital Sky Survey. Using very stringent criteria to be absolutely sure of a match, Professor Gaensler and Dr Madsen produced a final catalogue of 44 million stars and 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 Professor Gaensler. "But even so, processing the data and then testing everything to make sure we got it right took us more than a year." (Phys.org) —Tens of millions of stars and galaxies, among them hundreds of thousands that are "This cosmic game of Snap provides two important unexpectedly fading or brightening, have been breakthroughs. First, it gives far more accurate catalogued properly for the first time. measurements of the brightness of each individual star than had ever previously been possible. Professor Bryan Gaensler, Director of the ARC Second, by comparing two photographs of each Centre of Excellence for All-sky star taken up to sixty years apart, it becomes easy (CAASTRO) based in the School of at the to identify stars whose brightness has slowly University of , , and Dr Greg changed." Madsen at the University of Cambridge, undertook this formidable challenge by combining The researchers found that approximately 250,000 photographic and digital data from two major objects in their new catalogue, or about 0.6 percent astronomical surveys of the sky, separated by sixty of all the stars in the sky, change in their brightness years. by quite large amounts over a human lifetime.

The new precision catalogue has just been Some of these discoveries appear to be new cases published in The Astrophysical Journal of stars known as 'Mira variables': red giants in a Supplement Series. It represents one of the most late stage of stellar evolution that pulsate in comprehensive and accurate compilations of stars brightness before collapsing into a dense white and galaxies ever produced, covering 35 percent dwarf. Other stars are likely to be exhibiting rare of the sky and using data going back as far as and unusual behaviour that has never previously 1949. been identified.

Professor Gaensler and Dr Madsen began by re- "What is special about this catalogue is that it examining a collection of 7400 old photographic carefully combines historical data with modern plates, which had previously been combined by the measurements. This is a unique way to study US Naval Observatory into a catalogue of more objects that gradually change over years or even than one billion stars and galaxies. decades," says Dr Madsen.

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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, , such as the Panoramic Survey Telescope and Rapid Response System in Hawaii and the SkyMapper telescope in Australia.

"This catalogue comes at just the right moment for the next generation of telescopes. Using our measurements, astronomers who find interesting new stars in the sky can essentially go back in time, and see what the object they're studying was doing 60 years earlier," said Professor Gaensler.

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