44 million stars and galaxies mapped

Astronomers have created a new catalogue of the universe that covers 35 per cent of the sky and includes 44 million stars and galaxies that have been seen at least twice.
This is the first time that tens of millions of stars and galaxies, among them hundreds of thousands that are unexpectedly fading or brightening, have been catalogued properly for the first time, researchers said.
Professor Bryan Gaensler, Director of the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO) based in the School of Physics at the University of Sydney, Australia, and Dr Greg Madsen, formerly at the University of Sydney and now based at the University of Cambridge, UK, undertook this formidable challenge by combining photographic and digital data from two major astronomical surveys of the sky separated by sixty years.

The new precision catalogue represents one of the most comprehensive and accurate compilations of stars and galaxies ever produced, covering 35 per cent of the sky and using data going back as far as 1949.
Gaensler and Madsen began by re-examining a collection of 7,400 old photographic plates, which had been previously been combined by the US Naval Observatory into a catalogue of more than one billion stars and galaxies.
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, Gaensler and Madsen produced a final catalogue of 44 million stars and galaxies that had definitely been seen twice: both in old photographs and with modern cameras.

The catalogue provides two important new 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 sixty years apart, it becomes easy to identify stars whose brightness has slowly changed.
Gaensler and Madsen found that around 250,000 objects in their new catalogue, or about 0.6 per cent of all the stars in the sky, change in their brightness by quite large amounts over a human lifetime.
Some of these new discoveries appear to be new cases of stars known as "Mira variables": red giants in a late stage of stellar evolution that pulsate in brightness before collapsing into a dense white dwarf.

Other stars are likely to be exhibiting rare and unusual behaviour that has never previously been identified.
"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," said Madsen.
The catalogue has been published in The Astrophysical Journal Supplement Series.