Ordinary Meeting, 2007 January 31

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Ordinary Meeting, 2007 January 31

held at New Hunts House, Guys Hospital, London Bridge, London SE1

Richard Miles, President

Ron Johnson, Hazel Collett and Nick James, Secretaries

The President opened the fourth meeting of the 117th Session and invited Mrs Hazel Collett to read the minutes of the previous meeting, which were approved by the audience and duly signed. He announced that 30 new members were proposed for election; those 26 who had been proposed at the previous meeting were approved and declared elected. Mr Nick James, Papers Secretary, announced that one paper had been approved for Journal publication:

[Barbados report – Title???], by Damian Peach

The President explained that it was customary for him, at this time in his Presidency, to propose a successor. He was pleased to recommend Mr Roger Pickard, Director of the Variable Star Section, for the post. Members applauded. He went on to give an update on the Association's recent star count exercise; members had been invited to count the number of stars visible in a patch of sky in Orion on certain dates in December and January and to submit their results together with the latitude and longitude of their observing site. The aim of the exercise had been to map the extent of light pollution across the country. Nearly 1,800 reports had been submitted, of which a preliminary analysis would be published shortly; initial indications were that a very good geographical coverage had been achieved. The Campaign for Dark Skies (CfDS) intended to conduct follow-up surveys in coming years, which would be compared against the current 'baseline' survey to monitor changes.

The next Ordinary Meeting would be held on March 28, together with a Special General Meeting. Before then, an Observers' Workshop would be held on February 24 at the Open University, and the Deep Sky Section's Annual Meeting would be held on March 3 at the Humfrey Rooms, Northampton.

The President then introduced the evening's first speaker, Prof Michael Rowan-Robinson of Imperial College, London. Prof Rowan-Robinson's research interests included infrared and sub-millimetre astronomy, and he was currently President of the Royal Astronomical Society.

The Spitzer Space Telescope

The Spitzer Space Telescope, Prof Rowan-Robinson explained, was the fourth and final spacecraft of NASA's Great Observatories programme. Its most famous sibling had been the Hubble Space Telescope (HST), launched in 1990, which remained operational, observing at near-infrared, optical and ultraviolet wavelengths. The speaker noted with regret, however, that its main camera, the Advanced Camera for Surveys (ACS) had failed only the previous day. Spitzer's other siblings were the Compton Gamma Ray Observatory and the Chandra X-Ray Observatory, both launched in 1991. Spitzer had had a somewhat chequered history, which explained the long delay between the launch of these counterparts and its own in August 2003.

Spitzer observed at mid-to-far-infrared wavelengths, between 3.6 and 160 μm. Astronomy in the near-infrared had a long history, which could be traced back to William Herschel's initial discovery that the solar spectrum contained radiation at wavelengths longer than that of red light – the first indication that infrared light existed. Progress had been slow, however, especially in mid-to-far-infrared astronomy, because the Earth's atmosphere had been found to be highly opaque to infrared light in all but a few narrow wavelength windows; essentially, there was permanent cloud cover. Though some high-altitude balloon/aircraft-based experiments had achieved useful imaging in the 1960s and 1970s by getting above the Earth's atmosphere, it had not been until the launch of the space-based InfraRed Astronomical Satellite (IRAS) in 1983 that it had become possible on a large scale.

IRAS, a collaborative mission between the US, UK and the Netherlands, had been a survey instrument, which had mapped the entire sky at far-infrared wavelengths (25-100 μm). It had orbited along the Earth's day/night terminator, perpendicular to the ecliptic, pointing vertically away from the Earth to minimise any scatter of the Earth's thermal radiation into its tube. This configuration had mapped the whole sky over six months, as the Sun moved along the ecliptic. Over its eleven months of operation, IRAS had mapped most of the sky twice.

Most of the radiation detected by IRAS had been the thermal emission of small solid particles in inter-stellar space, called dust. Before IRAS, such particles had only been known to exist in dense clouds such as the Coalsack Nebula, and around star-forming regions such as M42. Perhaps the most significant discovery of IRAS had been the omnipresence of dust: thin wispy clouds of dust had been found to be spread throughout the whole of the Milky Way. Though almost invisible at visible wavelengths, their faint thermal radiation could be clearly detected in the far-infrared. By meteorological analogy, these clouds were now termed 'interstellar cirrus'.

The principal scientific interest in infrared astronomy since IRAS had lain in the fact that infrared emission seemed to trace very closely the formation of new stars. Stars seemed to form preferentially in dusty regions, M42 being a prime example. The optical opacity of such sites limited the amount which could be learnt about them at visible wavelengths; much more could be learnt from their dust emission – its temperature and morphology.

After IRAS, the next major advance had come with the building of the James Clerk Maxwell Telescope (JCMT) on the summit of Mauna Kea in Hawaii. This had been a UK project, seeing first light in 1987. Deep surveys with the JCMT had, quite unexpectedly, revealed a large number of distant galaxies which were incredibly bright in the infrared. These were typically so distant that their light had taken more than 10 billion years to reach us, and so we saw them as they had been only 2-3 billion years after the Big Bang. They were at least a hundred times – perhaps a thousand times – more luminous than the Milky Way. It was thought that they were forming vast numbers of new stars, and their brightness could be attributed to the many hot young stars within them. The Universe appeared to have been filled with such galaxies when it was 2-3 billion years old, and it seemed likely that most of the heavy elements seen today, such as carbon and oxygen, had been forged within the hot young stars in them.

In 1995, the European Space Agency (ESA) had followed up the success of IRAS with a new space-based infrared observatory, the Infrared Space Observatory (ISO). It had carried both cameras and spectrographs, though it was best remembered for the latter. The spectra of dusty regions provided clues about the chemical composition of the dust. Broad consensus had now been achieved: dust grains measured around 0.5 μm across and were composed of graphite and amorphous silicates – the best terrestrial analogues were, respectively, soot and sand. In addition, there was also evidence for complex organic molecules termed polycyclic aromatic hydrocarbons (PAHs) – these were toxic species, commonly found in car exhaust.

Prof Rowan-Robinson then outlined the scientific questions which Spitzer would address. Using dust emission as a tracer of star formation, and looking at galaxies as a function of distance, a principal aim was to map out when, in the history of the Universe, most of the stars formed. It was currently thought that stars had been formed in the greatest numbers when the Universe had been about 6 billion years old, around the time that our own Sun had formed. Ever since, star formation appeared to have been in decline. It was yet to be seen, however, how this model would change in the light of Spitzer observations.

The speaker then outlined the long and troubled history of Spitzer. It had been proposed in 1979, under the name of the Space InfraRed Telescope Facility (SIRTF). In 1984, a team of project scientists had been chosen, and it had been envisaged that the observatory would be launched aboard the Shuttle, in common with the other Great Observatories. In 1990, however, in the aftermath of the Challenger disaster of 1986, an unmanned launch had become the preferred option. It was thus decided to use a Titan rocket – one of NASA's standard heavy-lift launch vehicles. To fit the launch capacity of a Titan, a 1-m telescope, weighing 5.7 tonnes, had been designed; the budget had been $2 billion. The majority of the telescope's weight had been liquid helium, required to keep its optics cool so that their own thermal infrared emission would not swamp that of the sky.

In 1993, however, budget cuts had seen this design 'descoped' to a 2.7-tonne instrument which could be launched on an Atlas rocket – a smaller and cheaper launch vehicle. Further budget cuts in 1995 had forced the design to be cut back even further, to a 0.75-tonne telescope of 85-cm aperture which could be launched aboard a small Delta rocket for $450 million. It had been this design which had been launched in August 2003.

However, this descoping had not been as unfortunate as it might sound. Ingenuity on the part of the engineers had allowed the final telescope to achieve a very similar specification to that of the original 5.7-tonne design. This had been achieved by launching Spitzer not into Earth orbit, but into solar orbit; it followed the Earth around the Sun, trailing slightly behind it. Without the warming effect of Earthshine, and with a very efficient sunshield, Spitzer was in a very cold environment, minimising its need for active refrigeration. Some liquid helium was still required, but the amount much reduced. The supply had been designed to last for 2.5–5 years; in practice, Spitzer's natural cooling seemed even more efficient than originally thought, and its helium supply was likely to last for 5.5 years. Spitzer was slowly moving further away from the Earth, and eventually radio communication would be lost, in around 2013. It would pass by the Earth again in around 2700.

Turning to present an overview of Spitzer's early results, the speaker opened with an image of the star Formalhaut, which was seen to be surrounded by an infrared-bright disc. This was divided into two parts, the inner being apparently hotter than the outer. The speaker suggested that these two structures might be comparable to the Oort Cloud and zodiacal dust in our own Solar System.

The speaker then showed an image of the Elephant Trunk Nebula, a dark nebula super-imposed on the emission of the Garnet Star emission nebula (IC 1396). Spitzer images, as well as revealing the thermal emission of the obscuring dust which made the nebula dark at visible wavelengths, also showed a number of stars embedded within it. These were apparently newly-formed stars, obscured from view at visible wavelengths.

Prof Rowan-Robinson explained that his own research interest was in a survey called the Spitzer Wide-area InfraRed Extragalactic (SWIRE) survey. This was one of six Legacy projects which had been chosen by NASA to be allocated unusually long periods of observing time. Over a period of 851 hours, SWIRE had mapped 50 square degrees of sky – an area 250 times that of a Full Moon – and had detected nearly two million galaxies.

He hoped to use this to investigate which types of galaxies hosted most of the star formation in the Universe. He hoped to establish whether there was any systematic difference between the star formation seen in large galaxy clusters and that seen in more solitary galaxies. He also hoped that the survey would shed some light on the nature of the exceptionally luminous star-forming galaxies which had been detected by the JCMT.

To close, he remarked that there were several forthcoming space infrared telescopes due for launch within the next few years. A Japanese observatory, ASTRO-F, would fly later in 2007; ESA would fly the Herschel observatory in 2008; and the James Webb Space Telescope (JWST) was scheduled for launch in 2013 – this would be particularly exciting: a 6.5-m space infrared telescope.

Following the applause, Dr Nick Hewitt asked if Spitzer would be able to make any useful observations in the time between its running out of helium and the loss of radio communication. Prof Rowan-Robinson replied that Spitzer would lose its ability to observe in the far-infrared, but would still be able to take mid-infrared images. In due course, NASA would invite observing proposals.

The President then invited the evening's second speaker, Mr Geoffrey Johnstone, to present the month's sky notes.




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