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Astronomers team up with public to solve star system puzzle

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An extremely precise measurement of the distance from the Earth to a star system has enabled astronomers to better understand the way exotic objects, like black holes, swallow material they rip off the surface of nearby stars.

Published in the prestigious journal Science, a team of astronomers headed by Dr James Miller-Jones from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), have measured the distance to star system SS Cygni to be 372 light years, much closer than a previous measurement made by the Hubble Space Telescope in the 1990s.

The measurement was made possible by amateur astronomers from the American Association of Variable Star Observers (AAVSO) who alerted the local team to changes in the star system, triggering the team to start observations with two of the world’s most accurate radio telescopes.

Dr Miller-Jones’ team then measured the annual ‘wobble’ of the system compared to distant background galaxies, allowing them to measure the distance to SS Cygni with unprecedented precision.

“If you hold your finger out at arm’s length and move your head from side to side, you should see your finger appear to ‘wobble’ against the background. If you move your finger closer to your head, you’ll see it starts to ‘wobble’ more,” Dr James Miller-Jones said.

“We did the exact same thing with SS Cygni – we measured how much it appeared to move against some very distant galaxies.

“The wobble we were detecting is the equivalent of trying to see someone moving in New York from as far away as Sydney.”

The distance to SS Cygni had previously been measured using the Hubble Space Telescope, producing a puzzling result that was much further than theoretical studies predicted.

“If SS Cygni was actually as far away as Hubble measured, then it was far too bright to be what we thought it was, and we would have had to rethink the physics of how systems like this worked,” Dr Miller-Jones said.

“We would then have expected to see material swallowed more continuously, rather than in the individual, short-lived gulps that we actually observe.

“Our new distance measurement has solved the puzzle of SS Cygni’s brightness, reconciling the source distance and brightness with what is expected from our theories.”

SS Cygni is a double star system containing a normal low mass star and a white dwarf star.

Dr Miller-Jones explained that a white dwarf was the remnant of a star like our Sun that had run out of fuel and collapsed into an object about the size of Earth.

“Because it’s so dense, its strong gravity strips gas off its companion star, which then swirls around the white dwarf,” he said.

“Every couple of months, we see a dramatic outburst in visible light as the gas gets dumped onto the white dwarf surface.  That’s when the system emits radio waves.”

The team used groups of telescopes called the Very Long Baseline Array (VLBA) in the United States and the European Very Long Baseline Interferometry Network (EVN) in Europe and South Africa to pinpoint the exact location of the system relative to the background galaxies.

This research was carried out in conjunction with the University of Alberta, University of Southampton, Radboud University Nijmegen and AAVSO.

ICRAR is a joint venture between Curtin University and The University of Western Australia providing research excellence in the field of radio astronomy.

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