Monday 4th December 2006 Meeting Report
Making Models Of Stars


Continuing on from last months meeting aimed at beginners, EAAS committee member Paul Evans gave a brief introduction to astrophotography which was titled "Don't be afraid of the M setting". This talk covered the camera basics - how an exposure is a combination of an image focused to a point, through an aperture, onto a sensor, chemical or electronic, for a particular length of time. He also covered why automated camera settings go spectacularly wrong when applied to astronomy and how to take control with the "M" manual setting and get better results. Paul gave a very informative and entertaining talk. Click here for the shockwave file from his topic.

Our chairman, Mark Stronge then introduced the main speaker, Dr John Eldridge. John studied in Cambridge for his Physics undergraduate and post-graduate degrees, which he finished in 2004. He earned his Astro-physics PhD on stellar evolution, improving the stellar evolution code and then modelling supernova progenitors. After this he moved to Institute d'Astrophysique de Paris to work on Gamma-ray bursts and their circumstellar environments, and making models of star wind bubbles. After a year Dr Eldridge moved to Belfast and is now working with Steve Smartt on massive stars and supernova progenitors.

The title of his talk was “Making models of stars”. The aim of this talk was to outline the way we model stars and then relate them to what we observe. Special focus was given to the most massive stars with the most dramatic evolution during their lives and the most violent end when they die in a spectacular supernova.

Dr Eldridge

Dr Eldridge started his talk by taking a look at our nearest star – the Sun. He showed pictures of total and annular eclipses and a composite picture taken by the SOHO spacecraft to show the different features of the sun. He then moved on to talk about the Orion Nebula which is a very active star forming region. He highlighted the star Betelgeuse in Orion. Betelgeuse is a red super giant star. Super giant stars have masses between 10 and 70 solar masses. The Sun has a solar mass of 1. These stars are more than 30,000 times more luminous than the Sun. If Betelgeuse was placed in the centre of our solar system, its outer surface would extend almost out to the orbit of Jupiter.

John outlined the process of star formation which is caused by nuclear fusion when Hydrogen is converted into Helium in the star’s core. He pointed out that all the hydrogen in the Earth’s oceans would burn up in only four years in the Sun. In larger stars like super giant stars, the fuel in the core of the stars gets burned up much more quickly and these stars have a much shorter life than main sequence stars like our Sun.


Dr Eldridge then talked about Wolf Rayet Stars. These are massive stars, over 20 solar masses with pure helium cores and are 10,000 times more luminous than the Sun. They are losing their mass rapidly by means of a very strong stellar wind. Massive red super giant stars are thought to evolve into these Wolf Rayet Stars. John outlined the processes which are going on in the cores of stars. To convert Helium into the heavier elements Carbon and Oxygen, temperatures of 200 million degrees centigrade are required. For the conversion from Carbon and Oxygen into Silicon, temperatures of 1 billion degrees centigrade are required. For the ultimate conversion of Silicon into Iron, temperatures of 10 billion degrees centigrade are required. Only stars of 10 solar masses or more can form iron cores. Our Sun will only form a Carbon/Oxygen core and it will take 4.5 billion years.

Dr Eldridge explained what are stellar bubbles. This is where the star expels material. The bigger the star, the more mass is lost. 8 solar mass stars lose only half a solar mass during its lifetime, 15 solar mass stars lose two solar masses and any star of 20 solar masses or more will always end up as an 8 solar mass Wolf Rayet star.

He then moved on to the death of the massive stars – Supernovae. A supernova is caused when the pressure on the iron core of a super giant star from the material surrounding it increases until it blows. One of the most famous supernovae of recent times was Supernova 1987A. As the name suggests it blew in 1987. It was located in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy visible only from the Southern Hemisphere. It was visible to the naked eye and had a peak brightness of magnitude +3. It was the closest supernova since SN 1604, which occurred in the Milky Way galaxy itself. The star that went supernova was approximately 168,000 light-years from Earth and as its light took that time to get to the Earth, the event actually happened when modern humans were only beginning to evolve on Earth. As mentioned above SN1604 was the last supernova to occur in the Milky Way galaxy. It was first observed by Johannes Kepler. On average there is 1 supernova every 100 years in the Milky Way galaxy, so we are overdue. Dr Eldridge noted that recent discoveries have indicated that it is not just massive stars with iron cores that go supernova, but also massive stars with oxygen/neon cores and that current theories will have to be re-examined. He concluded his talk by pointing out the difficulties of star modelling. This is because 50 % of all stars are binary stars and due to the interaction between the two stars, standard models are harder to establish. He noted in addition that 25 % of all stars are singles, 20 % triples, and 5 % with 4 or more stars interacting together.


Dr. Eldridge finished his talk by taking questions from the audience who all enjoyed an excellent talk.

Next month Mr Martin Campbell, IAA member from Dungannon and regular contributor to the EAAS photo gallery will be giving us an “Introduction to Widefield Astrophotography” on Monday 9th January 2007.

Until Next Month and Happy Christmas
Neill McKeown



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