To study and encourage popular interest in all branches of Science.

Newsletter April 2007

Dear Member,

The last 2 lectures this session are as follows:

April 26 Professor Robert Weale (President) on Broccoli and Eyesight.

May 24 Nick Ward (Institute of Neurology, UCL) on The Plastic Brain: Adaptation with Age, Experience & Injury.

ANNUAL GENERAL MEETING on Thursday June 21st at 8 pm. This will start with wine and cheese (£2 each) and the AGM itself will start at 8.45 and if time allows will be followed by a scientific entertainment on Domesday Scenarios.

The agenda will include the usual reports from Officers and Sections and the Election of Officers and five Ordinary Members of Council for the coming year. Council proposes the following:

Secretary Julie Atkinson
Treasurer Peter Wallis
Membership Secretary Elisabeth Fischer
Programme Secretary Jim Brightwell
Ordinary Members Aileen Cook
Nayna Kumari
John Tennant
Hemant Desai
Elizabeth Davies

Council invites further nominations to the above officer posts and ordinary members; such nominees shall be duly proposed and seconded and have agreed to serve if elected.

A special item will be the ratification of the MoU with the BA. Please see the website for details.

Visit to the Regional Planetary Imaging Facility

Sunday afternoon, 27th May at University College London, organized by Julia Daniels.

Dr Peter Grindrod of UCL has agreed to come in specially to host the joint visit by members of the HSS and the Amateur Geological Society. He will present some of the amazingly detailed images of planetary surfaces that we are now getting from NASA and ESA for study by geology students. There will also be a display of maps and globes of Mars, Venus, The Moon and other satellites with solid rocky surfaces.

It may be necessary to limit the number of visitors. Dr Grindrod said that about 30 would be possible but the room is small and with only 10 chairs. He will therefore stay open from 12 to 4pm and Julia will allocate staggered arrival times. There will be 4 groups, arriving at 12noon, 1pm, 2pm, and 3pm. You can stay longer than an hour; it's just the seat that you are booking.

To book a place and time slot, and to receive a map and directions to the RPIF, please give your name, phone number and address to Julia Daniels (tel: 020-8346 1056, 25 Village Rd, London N3 1TL) or see her at the next meeting.

The Centre of the Galaxy.

Peter R Wallis

An article in Nature [1] reviewed recently our knowledge about the mysteries at the centre of our galaxy. For many years astronomers have suspected that a massive black hole is there, seen by us in the direction of the constellation of Sagittarius. The first hint came from Shapley's study of globular star clusters [2] which he found to be concentrated there. In the 1930s Karl Jansky of Bell Labs detected a strong source of radio noise from that direction but it was not until 1968 that the source was precisely located and seen in the infra-red to be the 'star' Sagittarius A (Sgr A*). This was a time when astronomers had seen many very bright objects in the universe called 'Quasars' which were so powerful that it seemed possible that they were powered by black holes. Donald Lynden-Bell [3] suggested in 1969 that our galaxy and its neighbours could all have 'dead quasars' at their hearts. The implication was that Sgr A* could be a massive black hole.

Confirmation of this was not easy as telescopes using visible light could not see clearly through the clouds of dust. It is only in recent years that large radio arrays, infra-red and X-ray telescopes, detectors in orbit and adaptive-optics telescopes on earth have been able to reveal the region close to the central engine. It seems to be very small, perhaps no larger than the solar distance but containing a mass of 4 million suns. Yet it isn't very bright! Perhaps that is as it should be for a 'black' hole, which cannot itself emit any radiation. But radiation can be expected from the matter falling onto the accretion disc around it. Maybe this emission is variable according to the amount of gas and stars available at any time.

There is some information that Sgr A* was perhaps 100,000 times brighter in the X-ray spectrum in the 1950s than today, probably because it swallowed a planet's or star's mass of gas in one gulp. But we had no X-ray telescopes then as they only work in space. But all is not lost: at the January 2007 meeting of the American Astronomical Society in Seattle, Michael Muno of Caltech announced that he and his team had managed to see part of the 1950s outburst reflected off clouds on the far side of the galactic centre [4]. The clouds were a few tens of light years away from the center, so the reflected X-rays took half a century longer to reach NASA's Chandra X-ray telescope.

New instruments in radio, infra-red and millimeter wave bands have revealed much detail of the region near the black hole but have not so far been able to image its horizon the boundary beyond which no light can escape. The best we have done is to link radio telescopes across the world, very long base line interferometry. In principle this should be able to resolve details as small as the earth's orbit. But the radio waves come through intervening clouds of ionized gas which distort the image. It is hoped that future interferometers in the millimeter and sub-millimetre bands, which are less subject to distortion, will reveal the event horizon. This would be the final part of the proof that Sgr A* is a massive black hole.


[1] Jeff Kanipe, "A long time ago, in a galaxy not so far away", Nature, 5th April 2007. Back

[2] Shapley H, Contributions from the Mount Wilson Solar Observatory No. 152, 1918. Back

[3] Lynden-Bell D, Nature 223, 690-694, 1969. Back

[4] Muno M P, Baganoff F K, Brandt W N, Park S & Morris M R, Astrophys. J,656, L69-72, 2007. Back


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