The University of Southampton

Astronomers team up with the public to solve decade-old puzzle

Published: 24 May 2013
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A Southampton astronomer is among a team of international researchers that has finally solved a decade-old puzzle about the way exotic objects such as black holes swallow the material they rip off the surface of nearby binary companions.

Christian Knigge, Professor in Physics and Astronomy at the University of Southampton, worked with colleagues from around the world to observe an extremely precise measurement of the distance to a star system containing such an exotic object, in this case a white dwarf.

Their findings have been published today in the prestigious journal Science, and show how the team, led by Dr James Miller-Jones, from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), has measured the distance to star system SS Cygni to be 372 light years. This is much closer than a previous measurement made by the Hubble Space Telescope. The new distance finally explains why this system swallows the material it steals from its companion in big gulps rather than in a steady slurp.

The measurement was made possible by amateur astronomers from the American Association of Variable Star Observers (AAVSO) who alerted the researchers to changes in the compact star system.

The academics used two of the world’s most accurate radio telescopes – 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 measure the annual wobble of the system compared to distant background galaxies, allowing them to measure the distance to SS Cygni with unprecedented precision.

James said: “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. We did the exact same thing with SS Cygni - we measured how far it moved against some very distant galaxies as the Earth moved around the Sun. The wobble we were detecting is the equivalent of trying to see someone stand up in New York from as far as 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 predicted. At this distance, the white dwarf in SS Cygni should have swallowed material continuously rather than in gulps.

“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,” James said.

SS Cygni is a double star system containing a normal low mass star and a white dwarf star. A white dwarf is the remnant of a star like our Sun that has 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.

Occasionally the flow of gas onto the white dwarf will increase dramatically, causing the system to appear up to 40 times brighter in visible light. It’s only during these rare periods that the star system emits radio waves, which allow for a much more precise measure of the distance.

“Since exactly the same kinds of processes occur when more massive stars like neutron stars and black holes pull gas from orbiting companion stars, white dwarf systems like this provide ideal laboratories to help us understand what is happening,” explained Christian.

The revised measured distance of just over 370 light years to SS Cygni has solved the puzzle of the system’s brightness and explains why it exhibits such a dramatic outburst in visible light.

“We can now have much more confidence in our understanding of what happens as gas falls onto these exotic objects. Not just white dwarves but neutron stars and black holes as well,” said James.

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