The Measurement of Stars

Talk by Norman Walker at the Society meeting on Wednesday 18th May 2005

      The original speaker for this evening had to cancel his talk and Norman Walker stepped in at the last minute and gave a very interesting and detailed talk about "the measurement of stars".  Norman was a professional astronomer based at Herstmonceux with the Royal Greenwich Observatory and had been heavily involved with the Cygnus X - 1 project, which produced evidence for one of the very earliest Black Holes to be discovered.

      To begin his talk Norman stated that there was still an important place for the amateur astronomer particularly in the field of observing and measuring variable stars. He then introduced us to the Hertzsprung-Russell diagram, the stellar evolution chart in which stars are classified through their absolute magnitude set against their spectral class, which measures colour temperature.

      It was pointed out that the greater the mass of a star, the greater the energy it held, and gave as an example the white dwarf, Sirius B with a temperature of millions of degrees but is in fact thought to be only about the size of our moon.

      At the other extreme are the red giants where stars from the main sequence find their surface temperature reduced but their radius increases, as does their luminosity.

      The main sequence of stars are in a fairly well defined line between hot white stars and cool red stars.

      There is a band through the centre of the chart from the middle of the white dwarf region through to the centre of the red giants and this is called the Instability Strip in which pulsating variables are found.  Norman said that certain red giants can take only a few million years to cross through this strip to join the white dwarfs by throwing off shells of material.

      Richard Miles and Andy Hollis had produced light curves from a considerable number of measurements over a period of time, and it was concluded that variations in magnitude could be caused by the expanding and contraction of stars or by rotating odd shapes.  Sometimes these changes in magnitude could be as little as a thousandth of a magnitude.  These measurements are difficult to obtain using photometers and now CCD chips are able to detect these changes with some accuracy, although it is necessary to use a filter to even out angular discrepancies.  It was also noted that the periods were not always constant.

      Our speaker then talked about the Observed / Calculated values over time, showing that results became more accurate over long periods of time, although there was also the problem of missing some short-term significant feature by not taking constant measurements.

      Norman then went on to talk about neutron stars the size of cities and then black holes.  He then talked of his own involvement in the Cygnus X-1 project where he spent a total of 12 years in Spain and using a 12-inch telescope 25 miles from any town was able to produce evidence for the possible candidate for a black hole at the core of the Super Giant.  There appeared to be material from a larger object being pulled into a much smaller object where there were also large changes in x-ray emissions.

      Some variable stars can produce many peaks over very short periods but in some there is only one peak in 20 years.  Many of these changes needed special techniques to be observed such as using Furrier Analysis on the results and again the importance of many observations was stressed.

      Finally Norman talked about Cataclysmic Variables, which were more explosive in character and where material is being pulled into an accretion disk, but at a point close to the disk, there is a hot spot, which is the point from which most emissions are detected.

      The talk moved on to practical observing for the amateur beginning with the most basic method of monitoring the brightness by comparison with adjacent stars.  With practice Norman said that it is possible to get down to tenth magnitude changes and even to one-hundredth magnitude variations.

      With a telescope fitted with a CCD it is possible to get down to one-thousandth magnitude changes,  although Norman did say that using an ordinary video camera chip, one had to bear in mind that the use of anti-streak techniques caused non-linear results, and for best results it is important to us a linear form of detector.

      Our own Sun is a variable but fortunately is never hot enough to sterilize nor cold enough to freeze life on Earth.

      There is a relationship between the period and the real magnitude of Cepheid variable stars which makes them ideal for calculating their distance from the Solar System and therefore they become measuring sticks both within our own galaxy and even in other galaxies.

      Norman spent some time answering questions from the floor.  One question was about the source of variable-star data and catalogues. He suggested the AAVSO in the USA and BAAVSO of the British Astronomical Association.

www.aavso.org provides a wealth of information from charts and recent reports to observing tips and methods of submitting observations.

www.britastro.org/vss is the Variable Star Section of the BBA.  It is a useful source, but is really designed around members of the association.

      Norman Walker then informally spent the next hour or so with members both in the drama studio and also in the pub afterwards and was happy to give the benefit of his knowledge as a professional astronomer.


      The next meeting of the Society is on Wednesday 15th June 2005 when Dr. Andrew Coates from the Space Science Laboratory will be giving a talk called "The Cassini - Huygens Mission"

      The meeting takes place as usual in the Drama Studio of Uplands College, Wadhurst and begins at 7.30 pm.


      Wednesday 20th July 2005.  Konrad Malin-Smith will be guiding us around "Pulsars".

      There will be no meeting in August, but Michael Harte and his wife have again kindly offered to host a "bring your own food" barbecue on Saturday 27th August at Greenman Farm, Wadhurst.  More details of this event in the July Newsletter.




      This month is probably the poorest month of the year for observing the night sky.  The 21st of June marks the summer solstice and the shortest night is only a few days later than that.  The Milky Way is now well positioned and on a clear night is quite spectacular whether just looking at it with the naked eye or through binoculars.  Through a telescope the number of stars is quite astonishing and in the constellation of Scutum is the open cluster M11, the Wild Duck, where there are about 3,000 stars.

      The first object to be seen after sunset is Jupiter and the first three stars make up the summer triangle of Deneb, Altair and Vega.

      An interesting fact about Vega is that it indicates the direction in which the Solar System is moving in its orbit around the centre of the Milky Way at a speed of 220 kilometres per second.  Of course the Earth is orbiting the Sun at about 30 km per second as well.

      This makes one wonder what speed the Milky Way is moving - and relative to what.  The only slightly satisfactory method NASA have come up with is to use the cosmic background radiation frm the Big Bang as measured by the Cosmic Background Explorer (COBE) and relative to that they say we are moving at 360 km per second +/- 20 km per second in the general direction of the constellations of Leo and the Crater at present.

      On another interesting note I read in the June edition of "Astronomy Now" is that the solstice is very slowly getting earlier and earlier and in about 2,000 years time, it will take place on the 19th of June.  Just a thought...


      On July the 4th,  NASA probe Deep Impact is expected to crash into the core of comet Temple 1 in an attempt to find out what the comet is made of, whether it is a loose pile of rubble or a solid structure.  The US Planetary Society are holding a competition 'Guess the comet's width in metres'.

      The most accurate guess will win a plaque from Ball Aerospace who built Deep Impact.



Chairman          Tim Bance                   01732 832745                 timbance@hotmail.com
Secretary         Ian Reeves                        01892 784255
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 July Newsletter should be with the Editor by June 27th  2005