WADHURST ASTRONOMICAL SOCIETY
JUNE NEWSLETTER 2006
INDEX: MEETINGS, OTHER NEWS, CONTACTS
that go Bang in the Night
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,
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
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
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
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
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".
20th September 2006. We welcome
back Konrad Malin-Smith FRAS. This
time he will be giving a talk about White Dwarfs.
18th October 2006. Jerry
Workman will be paying us another visit and this time he talks about the latest
news from Mars.
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".
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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 -29º 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 -24º23' 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!
a Moment Wasted
Dr. Tony Phillips
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/,
article was provided by the Jet Propulsion Laboratory, California Institute of
Technology, under a contract with the National Aeronautics and Space
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|Chairman||Tim Bance||01732 email@example.com
|Secretary||Ian Reeves||01892 firstname.lastname@example.org|
|Secretary||Phil Berry||01892 email@example.com|
|Treasurer||Mike Wyles||01892 firstname.lastname@example.org
|Publicity& Web site||Michael Harte||01892 email@example.com
|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
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