Stars that go Bang in the Night

Talk given By Dr. Robert Smith at the Society's May meeting on Wednesday 17th.  

      Robert Smith lectures in Astronomy at Sussex University and he based this evening's talk on one of his great interests; Novae and Dwarf Novae, both of which he told us are Cataclysmic Variables (CVs).

      He began by telling us that they are all binary stars with mass transfer from a red star, normally a red dwarf to a compact star; either a white dwarf or a neutron star and this transfer takes place in an arc due to angular momentum.  This matter forms a circular orbit in the form of an accretion disk, spiralling in towards the centre.

      CVs are recognised by their light curves; Novae produce an outburst usually about 10,000 times their normal brightness and recurring about every 10,000 years, whereas Dwarf Novae produce outbursts about 100 times their quiescent brightness but recurring every few months to a few years.

      As an example of a Dwarf Novae, Robert showed a light chart of SS Cygni.  Although the average outburst take place about every 2 months, within this, peaks varied in size and shape.

      Another example is the Z Camelopardolis type outburst where the Z Cam can produce periods of "standstills" during which there are no outbursts.  Another example are SU UMa stars,  where there can be a mixture of normal and superbursts.

      Robert then posed the question, - why do outbursts occur and why are they not all the same?  The main answers are to do with mass transfer, friction and instabilities in the accretion disk.

      In most cases, as the red star orbits the compact star, mass is transferred via an accretion stream into a flat accretion ring resulting in a high intensity white spot at the point of impact with the disk's edge.  The disk is much hotter than the red star, typically between 20 and 30 thousand degrees, particularly at the centre from which there is considerable ultra-violet radiation, and we were shown an artist's impression of the shadow this would cause on the surface of the red star.

      In some cases the compact star has a very strong magnetic field which causes the mass transfer to take place along lines of magnetic force and are directed into the poles, often causing X-ray emission from the impact points.

      Mass transfer occurs because the two stars orbit each other in just a few hours, with the distance between them being less than the diameter of our Sun, resulting in strongly interacting gravitational fields.  This was diagrammatically represented as each star sitting at the bottom of its gravitational potential well, mass transfer occurring when one well "fills up" and overflows.  At a certain point, material from the red star reaches what is called the Roche Lobe and spills into the other star.

      An animation of the two stars showed the possibility of there being eclipses of the bright compact star by the Roche Lobe.  This provides a lot of information about the two stars and confirms that there really are two stars.

      Using light graphs of these eclipses helps to provide "eclipse mapping" and from this it has been possible to determine that the central area of the accretion disk is hotter than the disk itself, which is again hotter than the eclipsing star.

      Robert went on to explain that hydrogen from the red star spirals down through the accretion disk forming an ever-increasing layer around the compact star until the temperature builds to a point where nuclear fusion occurs resulting in a huge hydrogen bomb.  This explosion removes the layer and the cycle can begin all over again.

      The energy source in a Dwarf Nova is just gravity.  Material from the source falls into the white dwarf's "well" and into the accretion disk, producing a lot of energy, but due to friction, the material spirals in and eventually falls on to the white dwarf's surface.  This friction, or viscosity, is also the cause of the brightness of the disk.

      In old novae, this is all that happens and there is no outburst producing "standstills" in Z Cam stars.

      In Dwarf Novae, the flow is much slower and mass appears to get trapped and then is released in one great lump, releasing a burst of energy.  Robert said that this was not fully understood and appeared to be quite complicated.

      It would depend on the mass flow through the disk related to the mass flow from the red star.  Not only that but the temperature in the disk, which depends on the mass flow would also be a significant factor.

      We saw a graph of density plotted against temperature, which showed the stable state, with low viscosity and low mass flow rate and the hydrogen un-ionised and the peak of an outburst with high viscosity and high mass flow rate where the hydrogen is fully ionised.  Between these two states the system is unstable as in Dwarf novae where the hydrogen is partially ionised.

      Robert explained that this was a crude explanation and needed a more detailed treatment to account for the real light curves.

      He then spoke of detecting star-spots on the red dwarf, superoutbursts and superhumps, but said enough is as good as a feast, and after such an excellent talk with plenty of food for thought, I for one, was beginning to suffer from intellectual indigestion...

      During his talk, Robert mentioned art work by Mark Garlic which can be found on website www.space-art.co.uk


    Wednesday 21st June 2006 is "The Solstice Telescope Evening" and is an opportunity for members to bring their telescopes and talk to newcomers, novices and knowledgeable observers. The general public are being invited to join us. Many would very much like to buy a 'scope if only they had the opportunity to talk to amateur astronomers like yourselves who can explain the pros and cons of various instruments without any ulterior motive.

      If any member would be like to give a short talk of just a few minutes, they would be very welcome.  The subject does not need to be pure astronomy as experiences and visits to locations can be just as interesting to members.

    The meeting will be held in the Upper Room of the Methodist Church, Wadhurst High Street and will commence at 7.30 pm.


    Wednesday 19th July 2006.  Gilbert Satterthwaite FRAS will be talking to us about "Sir George Airy's Contribution to Positional Astronomy".

    Wednesday 20th September 2006.  We welcome back Konrad Malin-Smith FRAS.  This time he will be giving a talk about White Dwarfs.

    Wednesday 18th October 2006.  Jerry Workman will be paying us another visit and this time he talks about the latest news from Mars.

    Wednesday 15th November 2006.  David Rooney, who is the Deputy Horologist at Greenwich Museum, is to give us a talk called "A Brief History of GMT".




    As mentioned in previous Newsletters, there will be no meeting in August BUT Michael Harte and his wife are again kindly offering to hold an astro-barbecue evening on Saturday 26th August 2006.  Last year was a great success with members bringing their telescopes and binoculars or just coming to join in.

    The barbecue evening will be held at Greenman Farm, Wadhurst - on the south side of the B2099 immediately to the west on the railway bridge.  All Society members are invited and Michael suggests that members aim to arrive at 7.00 pm.

    You need to bring your own food and drink, but everything else will be provided.  More details in the July Newsletter.


    With the summer solstice occurring on the 21st of June, astronomical twilight lasts just about all night here in the south of the UK, but this is the one time when one can look towards the centre of our own galaxy, and wonder.  It is in Sagittarius at RA 17h45m40.04s, Dec -29 00' 28.1" and at a distance of about 26,000 light years.  It is thought that a massive black hole, 2.6 million times the mass of the Sun, exists here, although because of cool intersteller dust this part of the galaxy cannot be observed visually.  Available information about the centre of the Galaxy is obtained from gamma rays, hard x-rays and radio waves.  In the near future it may be possible to produce direct images of the event horizon of the black hole (if it exists) using astronomical interferometers.

    Not far from this part of the sky at RA 18h3m48s, Dec -2423' is M8, the bright Lagoon Nebula with an integrated magnitude of 5.8 and should be able to be seen with quite a small telescope.

    On the 20th of June, Mercury reaches greatest elongation and sets about 90 minutes after the Sun and will be able to be seen through binoculars as a half illuminated disk with magnitude 0.2.  Always ensure that the Sun has set before trying to find the planet to avoid catching direct sunlight which can instantly damage eyesight!

    Mars and Saturn are not far behind Mercury and are visually just a degree apart, although they are now less spectacular than they have been earlier in the year.  But Jupiter has a magnitude of about -2.4 and is dominant in the southern sky.  Venus rises just before the Sun in the morning sky with a magnitude of nearly -4 and with a phase of 80%.


Not a Moment Wasted

by Dr. Tony Phillips

      The Ring Nebula.  Check.  M13.  Check.  Next up: The Whirlpool galaxy. 

    You punch in the coordinates and your telescope takes off, slewing across the sky.  You tap your feet and stare at the stars.  These Messier marathons would go much faster if the telescope didn't take so long to slew.  What a waste of time!

    Don't tell that to the x-ray astronomers.

    "We're putting our slew time to good use," explains Norbert Schartel, project scientist for the European Space Agency's XMM-Newton x-ray telescope.  The telescope, named for Sir Isaac Newton, was launched into Earth orbit in 1999.  It's now midway through an 11-year mission to study black holes, neutron stars, active galaxies and other violent denizens of the Universe that show up particularly well at x-ray wavelengths.

    For the past four years, whenever XMM-Newton slewed from one object to another, astronomers kept the telescope's cameras running, recording whatever might drift through the field of view.  The result is a stunning survey of the heavens covering 15% of the entire sky.

    Sifting through the data, ESA astronomers have found entire clusters of galaxies unknown before anyone started paying attention to "slew time."  Some already-known galaxies have been caught in the act of flaring-a sign, researchers believe, of a central black hole gobbling matter from nearby stars and interstellar clouds.  Here in our own galaxy, the 20,000 year old Vela supernova remnant has been expanding.  XMM-Newton has slewed across it many times, tracing its changing contours in exquisite detail.

    The slew technique works because of XMM-Newton's great sensitivity.  It has more collecting area than any other x-ray telescope in the history of astronomy.  Sources flit through the field of view in only 10 seconds, but that's plenty of time in most cases to gather valuable data.

    The work is just beginning.  Astronomers plan to continue the slew survey, eventually mapping as much as 80% of the entire sky.  No one knows how many new clusters will be found or how many black holes might be caught gobbling their neighbours.  One thing's for sure: "There will be new discoveries," says Schartel.

    Tap, tap, tap. The next time you're in the backyard with your telescope, and it takes off for the Whirlpool galaxy, don't just stand there. Try to keep up with the moving eyepiece.  Look, you never know what might drift by.

    See some of the other XMM-Newton images at http://sci.esa.int .  For more about XMM-Newton's Education and Public Outreach program, including downloadable classroom materials, go to http://xmm.sonoma.edu.  Kids can learn about black holes and play "Black Hole Rescue" at The Space Place, http://spaceplace.nasa.gov/, under "Games."

This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.



Chairman  Tim Bance  01732 832745 timbance@hotmail.com
Secretary Ian Reeves  01892 784255  ian-reeves@tiscali.co.uk
Secretary Phil Berry 01892 783544 phil.berry@tiscali.co.uk
Treasurer  Mike Wyles  01892 542863 mikewyles@globalnet.co.uk
Publicity& Web site Michael Harte  01892 783292 michael@greenman.demon.co.uk
Editor Geoff Rathbone 01959 524727 Geoff@rathbone007.fsnet.co.uk

Any material for inclusion in the June Newsletter should be with the Editor by June 28th  2006