LJMU hosts international workshop on exploding stars


18 September 2012

A group of over 20 astronomers from the US, South Africa and Europe met at LJMU's Astrophysics Research Institute (ARI) for an intensive few days of discussions and presentations on the latest findings and future plans on how best to study the class of exploding stars known as novae.

This was the latest in an annual series of workshops which previously have been held at the University of Minnesota. The principal meeting organisers this time were Professor Mike Bode and Dr Matt Darnley of the ARI.

A nova system comprises two stars held close together by the mutual pull of gravity on each other. They are usually in such a very tight orbit that they can go once around each other in only a few hours. The more massive of these two stars is an exotic object known as a White Dwarf star. A White Dwarf is the burnt out core of a star like our Sun that has used up all its hydrogen fuel. Such an object is about the size of Earth but it still has about the mass of the Sun. Indeed, the two stars are so close together that hydrogen-rich material is stripped from the more normal companion by the same kind of tides that we witness at ocean shores. The stripped material falls onto the surface of the White Dwarf and when enough material has fallen onto the White Dwarf a runaway thermonuclear fusion explosion occurs and blows these gases into space. When we see such an explosion we call it a nova. Studies of these explosions at the ARI show that the White Dwarf is not destroyed in the explosion and will undergo these explosions many times in its life.

One of the big issues discussed at the workshop was the link between nova explosions and an even more titanic and catastrophic explosion called a 'carbon deflagration supernova.' If during the long term evolution of the White Dwarf, its mass grows with time, there comes a point where the entire star starts to collapse and becomes so hot that its carbon rich gas can fuse and make an entire progression of heavier nuclei. Enough energy is released from this gigantic explosion so that the supernova can become as bright as all the other stars in the galaxy put together - for a short time. Moreover, a lot of the iron in your bodies - think red blood cells which carry oxygen through your blood stream - is produced in these explosions. We are therefore literally star stuff. These White Dwarf supernovae are also important to our studies of the evolution of the universe. They are so bright and so easily identified that we can find them in distant and young galaxies at times when the universe was only a few billion years old - much younger than our current age of nearly 14 billion years. Studies of these distant supernova now show that there is an unknown 'force' pushing the universe apart so that it will expand forever at an accelerating rate. We have not yet identified this force but wanting for a better term we call it Dark Energy.Artist's impression of the outburst of nova RS Ophiuchi

In order to address this and other important issues in the field, the workshop discussed how to better simulate nova outbursts and how best to utilise future observational opportunities. Among the more exciting possibilities are those provided by the SOFIA observatory. As described by one of the attendees, Professor Robert Gehrz, Director of the University of Minnesota Observatories and Leader of the NASA SOFIA Community Task Force (SCTF), SOFIA is a large telescope that is flown at high altitude aboard a highly modified Boeing 747.  Professors Brian Warner and Patrick Woudt of the University of Cape Town reported on possible observations with the South African Large Telescope (SALT). A large contingent from the University of Leicester were present to alert the attendees about using the currently flying Swift observatory that has expended more than three million seconds on nova observations. Other space-based and ground-based facilities were discussed and proposals about how to best use them were described, including those for LJMU's own Liverpool Telescope, of which one of its original science drivers is nova research.

Another of the attendees, Sumner Starrfield, Regents' Professor at Arizona State University, a long-time collaborator with ARI astronomers and who also provided an ARI science seminar on the eve of the workshop, commented: "This was one of the best workshops I have attended."

Professor Mike Bode added: "The workshop was a great success scientifically and also socially. Many of the delegates had not been to the area before and several remarked how impressed they were with Liverpool. One memory that will stay with me however was when Sumner Starrfield told me his mother and her family had emigrated from Russia to Canada in the early 20th century and that they had found the record of her boarding the ship in Liverpool.  The hotel on the dock front where Sumner stayed was within walking distance of the very spot where his family set sail on 6 March 1913.  The hairs stood up on the back of my neck as he told me."

At the end of the workshop, the invitation was extended by Prof Steve Shore of the University of Pisa to hold next year's event at his institution.

Photo caption:  Artist's impression by David Hardy of the outburst of nova RS Ophiuchi in 2006 showing with the explosion occurring on the surface of the White Dwarf star in a binary star system.



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