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Cosmic dust disc to force rethink

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The discovery of a large disc of dust around a binary star system could force astronomers to rethink their computer models of the Universe.


Old and new: It is now thought a gaseous disc (top) is surrounded by a dusty disc (bottom)

Previous observations turned up no sign of the disc at WZ Sagittae.

But data from Nasa's Spitzer infrared telescope confirmed there was much more to this object than previously thought.

The discovery may have implications for the study of everything from supermassive black holes to the formation of planets.

The remarkable find around WZ Sagittae was made by teachers and students from across the US, working with scientists from the Spitzer Science Center in California and the National Optical Astronomy Observatory in Tucson, Arizona.

The results of the work were presented at the 211th meeting of the American Astronomical Society in Austin, Texas.

The Spitzer Space Telescope is one of Nasa's four Great Observatories, a companion to the Hubble Space Telescope.

WZ Sagittae consists of not one but two stars, which are in orbit around each other, known to astronomers as a binary.

The first is a type of dense, dead star known as a white dwarf; and the other is its low mass companion, a brown dwarf.

Even though white dwarfs are only about the size of the Earth, they may have as much mass as the Sun, and that means they have a strong gravitational pull.

The white dwarf star in the WZ Sagittae system uses this pull to grab material from its companion at a rate of a trillion kg per second, which is enough to cause some spectacular events.

As the material builds up, it will eventually cause a thermonuclear explosion, powerful enough to make the normally faint star visible in binoculars. Such explosions seem to occur every few decades; the last was in 2001.

Important effects

The teachers and students who made the latest discovery wanted to study the brown dwarf in the system. From our perspective, the two stars that make up WZ Sagittae happen to pass in front of each other in a series of eclipses.

In visible light, these eclipses last just eight minutes, but in the infrared light which Spitzer can detect, they took over 20 minutes.

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Nasa's Spitzer infrared telescope


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Astronomers look in the infrared region of the spectrum to detect cool gas and dust, so this surprising finding - analysed by high school students - means that a large disc of dust must be obscuring their view.

Steve Howell, the astronomer who presented the team's results in Austin, explained that this had important implications.

"Firstly, discs are everywhere, from systems which are forming planets through to supermassive black holes in the centres of galaxies," he explained.

Astronomers use the same computer models to analyse each of these disparate systems, and so the presence of an extra dusty disc could make a huge difference to their results.

Infrared observations also provide information about distant galaxies, many of which are much fainter in visible light. If dust like that detected by the students and teachers is important in these galaxies too, then many of the results of those studies may need recalibrating.

The National Optical Astronomy Observatory and Spitzer Space Telescope project incorporated schools in Montana, California and Pennsylvania.

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