MIRA 51

Christmas 1999

Editors Bit

Things are never quite what they seem to be, you only have to look at the night sky to see that.  How can you prove that the Earth turns when you can't feel it?  You can see the Sun move round the sky by day and the stars wheel across the heavens at night, but you can't feel it.  No wonder it took thousands of years too prove it was us moving and not the sky.

Jupiter looks brighter than Mars, but it's three times further away.  The Sun and the Moon are the same size in the sky.  By a fluke of nature the Sun happens to be 400 times the size of the Moon but it is 400 times further away, therefore a close fit in size in our sky.  A feature which some of us appreciated last August.

William Herschel spent years looking at the night sky through the largest telescopes of the day and he eventually decided that the shape of the universe was a brick like shape with the Sun near the centre.  Where there were gaps in the general spread of stars across the Milky Way he assumed that there were fewer stars in that part of the heavens.  He never thought that less stars in an area meant a obstruction of the star light by dust or gas.  This fact took another couple of centuries until telescopes and photography in the early years of this century proved the existence of clouds of dust and gas blocking the light of distant stars.

Stars that look close together, such as in many double stars, can in fact be many light years apart.  The sky contains many pairs of such line-of-sight doubles, stars which are completely unattached to each other.  Other stars which do form a binary can vary wildly in size and brightness with many double stars presenting a challenge to observers to split them in scopes.

Constellations with neat arrangements of evenly bright stars are not necessary close to each other or at the same distance from us.  A good example is the seven main bright stars in the constellation of Orion the Hunter which are actually spread over 2,000 light years in depth!  Lets start with the red star Betelgeuse at the top left, it is 310 ly from us, Bellatrix (top right) 360 ly, Rigel, a large blue B8 giant at bottom right lies 925 ly away and Saiph (bottom left) only 70 ly.  The three belt stars which look nearly the same brightness to us are (L to R) Alnitak, 1100 ly, Alnilam 1220 ly and Mintaka a bright O9 star, is a massive 2300 ly away!  This star has a luminosity of 53,000 times our sun.  You can see from just this one constellation that the shape and brightness of its stars depends very much on where we are looking at it.  For instance if we could look down on Orion from above at 90° to how we see it now, we would not recognise it at all!  None of the stars would be in a shape we would recognise.

Another example is the Plough, Ursa Major, the seven  bright stars of this asterism look similar in brightness but the two stars at each end are not part of the group and lie much further out.  Dubhe the pointer star nearest Polaris is at 124 ly, Merak, the bottom pointer is 79 ly, next Phad at 84 ly, Megrez 81 ly, Alioth 81 ly, Mizar 78 ly, Alcor 81 ly and the tail star Alkaid at 101 ly.  These distances are the latest from the Hipparcos mission, and you can see that the middle five stars of the Plough are part of the same group and all are moving through space together in the same direction. 

Ivor Clarke

Editor

So may I present the original, uncut, version of

The Accidental Death of an Anarchist
by Mike Frost

Introduction
On Thursday, February 15th 1894, Martial Bourdin, a young Frenchman of diminutive stature, ate his lunch at the International restaurant in Fitzroy Square, London, then made his way to Westminster, where he caught the 3.10pm horse-drawn tram to the East Greenwich terminus.  His was the only through ticket, and the conductor later remembered that the Frenchman seemed nervous and concerned about a package on the seat beside him.  In Bourdin's pockets, it later transpired, were thirteen pounds sterling in gold (a substantial sum in 1894), a membership card for the Autonomie club, a ticket for a ball ”in aid of funds for revolutionary purposes", and pieces of paper on which were written, in Latin, chemical formulae for explosives.

The tram arrived at its destination at 4.19pm.  Bourdin asked the depot checker the way to Greenwich park.  It had been a rainy day and the light was failing as the Frenchman made his way into the park from King William Street, shortly before the closing time of 5pm.  A gardener called Burchill noticed him and remarked to a companion about a little man in a brown overcoat and felt hat, hugging a parcel the size of a brick.  Bourdin made his way across the park on the main avenue, and began to climb the hill towards the observatory at the crest.

In 1998, of course, the Greenwich Observatory forms part of the National Maritime Museum, and is no longer a professional observatory, although a telescope is still available for use by the people of London.  But in 1894, the buildings were still home to the Royal Greenwich Observatory, an institution of international renown, established on the Greenwich site for 219 years.  Since 1884, Greenwich had been the undisputed site of the international meridian, the arbitrary line of longitude marking the transition from east to west hemispheres.  The chief purpose of the observatory was the production of almanacs and astronomical predictions; to this end, careful measurements of stellar transit times were made in the observatory and laboriously reduced, by human "computers", into tabular form.  On February 15th 1894 at nine minutes to five, most of the staff had finished for the day, as Martial Bourdin approached the observatory, carrying a bomb.

As we shall see, we can only guess at the reasons behind his journey to the Greenwich Observatory.  What is better known is the curious way in which his story came to be told, first in print by one of the great novelists of the twentieth century, and latterly on screen by an impressive array of Hollywood's finest actors.  And, recently, his story has had bizarre echoes in the actions of latter-day revolutionaries.

Martial Bourdin
Martial Bourdin was born in 1868, and moved to London in 1888 to help his brother Henri, who was a tailor in Soho.  Bourdin had already developed anarchist tendencies in France; in London, he joined the Autonomie Club, based in Windmill St. off Tottenham Court Road, where English and foreign anarchists dined and swapped ideas and plots.  He was sent to America in 1891 or 92 on behalf of the anarchist movement, and lived in New York and Chicago. On his return to London he became a zealous and trusted member of the movement.  There are even reports of him giving a lecture on explosives to the Autonomie club.  Other authors, however, paint a picture of Bourdin as a trusting simpleton.  It is difficult to gain a clear picture of the man, particularly as his fellow anarchists had their own reasons to obscure the truth.

On February 12th, 1894, just 3 days before Bourdin's trip to Greenwich, a bomb was thrown in the cafe Terminus in Paris, killing one man and injuring others.  The French police believed that the materials for the bomb came from London, but were slow to notify their British counterparts of their suspicions.  The Autonomie club almost certainly knew they were due for a raid.  Perhaps the London anarchists decided to launch a hastily prepared outrage of their own, or maybe Bourdin was fleeing London for France, and simply chose Greenwich park as somewhere to dump incriminating explosives (although this seems unlikely given the number of parks Bourdin could have chosen closer to central London, and his reported determination to make an unfamiliar journey).  Finally, there is the possibility that Bourdin was simply duped into carrying the bomb into the park by some unknown agitator whom he trusted.

Nowadays the path to the observatory from the river slopes from west to east in front of the observatory buildings.  In 1894, no such path existed, and instead much steeper steps, not in good repair, zig-zagged up the hill to the west of the observatory.  Perhaps Martial Bourdin tripped on a tree root, perhaps he fumbled as he tried to prime the bomb with a phial of acid.  Whatever the reason, Bourdin had only reached the second bend from the top when his bomb went off prematurely.

The aftermath of the bomb
W. G. Thackery and H. P. Hollis, two assistants, were talking in the lower computing room, when, to quote Thackery, "we were suddenly startled by a loud explosion, the detonation of which was sharp and clear, apparently followed by a noise something like a shell going through the air.  I immediately remarked to Mr Hollis That is dynamite! Spot the time".

The explosion was heard throughout the park, and several people came running.  First on the scene were two schoolboys, George Frost and Thomas Winter.  They found Bourdin, covered in blood, glass and black powder, kneeling on the pathway, unable to walk away.  He called to the boys "Come here! Come here! Call me a cab", but the boys were too scared to come close.

Shortly afterwards park-keeper Sullivan arrived and a park constable came running up the path from the avenue.  Again, Bourdin cried out "Take me home, take me home", in a foreign accent.  The constable noticed that he had wrapped a handkerchief around his left hand.  Fearing that the bomber carried a revolver, he removed the handkerchief, and was horrified to find that the Frenchman's left hand had been blown off clean above the wrist.  Dr Willes, who lived nearby in Crooms Hill, was summoned.  He opened Bourdin's waistcoat and found that the poor man's stomach had been blown open.  Bourdin was taken by stretcher to the Seaman's hospital at 5.15pm, where he lingered, to the surprise of the doctor, for 25 minutes, and then died.

Sensational Reporting
The attempted bombing caused a sensation, occupying the press for several days.  It was reported that "excitement at Greenwich is very great and many people have made their way along the zig-zag path where the accident occurred".  Park keeper Sullivan revelled in the publicity, remarking that "in all his experience he had never known so many people in the park on a single day, except on Bank Holidays, even when the band was playing in the summer months."

Although some eye-witness accounts spoke of damage to the observatory, in fact there was none  a trail of blood and bone fragments stopped well short of the observatory railings. Extra guards went on duty at the Woolwich Arsenal, Dockyards and Barracks, and the Autonomie club was raided and documents seized.

On Monday February 19th, an inquest took place in the Lecture Hall, Royal Hill.  Several Autonomie club members were present, also Martial's brother Henri, who created a stir by refusing to swear on the bible.  When the inquest jury visited the Greenwich park, they were met by a 'decently-dressed young fellow' who cried "Vive L'Anarchie!", and denounced the existing order.  When asked to desist, he walked off, making uncomplimentary remarks about the then Home Secretary Herbert Asquith.  Colonel Majendie, government inspector of explosives, carried out an enquiry into the security of the observatory and concluded, after some deliberation, that nothing needed to be done.

The Home Office took steps to ensure that Bourdin's funeral was not a rallying point for the anarchist cause.  The police had to protect the few mourners who did attend from a crowd who attacked the cortege in Fitzroy Square.  Martial Bourdin's spectacularly unsuccessful career should have ended with his burial, on Feb 23rd 1894, in St Pancras cemetery, Finchley.  Yet his story was to undergo an unexpected renaissance.

Conrad's Novel
Over the next thirteen years, the Greenwich Park bomb faded from memory.  Then, in 1907, the writer Joseph Conrad (Polish born but living in England) happened to discuss the story with a friend.  Conrad recalled he had been appalled by the futility of Bourdin's gesture, "a blood stained inanity of so fatuous a kind that it was impossible to fathom its origin...".

His friend happened to remark that Bourdin had had a sister who committed suicide after Bourdin's demise. Whether or not this was true is not clear; there is no mention of a sister in contemporary accounts.  But Conrad was struck by the effects of an essentially political act on family relations.  The grain of an idea formed from his discussion  to retell the story of Bourdin's attack on the Greenwich Observatory from the point of view of the bomber and his family.

What emerged was a novel called "The Secret Agent", which is considered one of his finest.  The nihilistic attack on the observatory remains at the heart of the novel, though it is not described explicitly.  However, for dramatic purposes, the characters have been developed somewhat from the original story.

The Secret Agent
The Secret Agent of Conrad's book is Adolf Verloc, a shopkeeper who lives with his wife Winnie and her simple-minded brother Stevie, above a London book-shop selling dodgy material.  Verloc is the agent of a foreign power (never named) and was once the provider of military secrets, but has now no wish other than to live in domestic comfort.  His one contribution to international politics is to run a society, the grandly named Future of the Proletariat, who are in fact nothing more than a talking shop.  Verloc's nationality is never explicitly defined, but it is interesting that Conrad makes one member of the secret society, Alexander Ossipan, a French anarchist.

Verloc's comfortable existence is disrupted when he is called to his employer's embassy to be torn down a strip by the first secretary, Mr Vladimir.  The diplomat is displeased by Verloc's lack of action and demands that he carry out an act of terrorism.  Perhaps, in Vladimir's choice of target, we gain an idea of Conrad's view of the Greenwich incident.  The intended site of the outrage must be a building, not an individual. Royalty and religion are ruled out, as is Verloc's suggestion of a bombing campaign against the embassies.  Instead, Verloc' controller has a more precise target:-
"..The sacrosanct fetish of today is science... what do you think of having a go at astronomy?"
Why astronomy?  "I defy the ingenuity of journalists to persuade their public that any given member of the public can have a personal grievance against astronomy ... the whole civilised world has heard of Greenwich.  The very boot-blacks in the basement of Charing Cross Station know something of it."

Verloc, shaken out his complacency, is forced to plan the bombing.  He consults the mysterious Professor, a one-time chemistry technician, who, slighted by his superiors, has withdrawn in contempt from society, and now harbours a hatred of all institutions, and scientific institutions in particular.  The Professor (drawing on another facet of Bourdin' character) provides Verloc with the materials for a bomb.  But instead of planting the bomb himself, Verloc imposes on his dim-witted brother-in-law Stevie to do the deed.  It is Stevie' clumsiness which leads to the bomb exploding prematurely, and Stevie' death which drives the story to its tragic conclusion.

Unexpected Inspiration
Conrad's novel was a great critical success; Penguin Popular Classics bill it as "arguably one of the greatest novels of the twentieth century".  Chillingly, it has been reported that Theodore Kaczynski, the UnaBomber, drew on the novel as inspiration for his bombing campaign, identifying with the antisocial and anti-scientific character of the Professor.  Kaczynski allegedly used the pseudonyms Conrad and Konrad when on his bombing missions.

The Secret Agent has been filmed twice. Alfred Hitchcock was first in 1936, under the title "Sabotage" (NOT "The Secret Agent", which was a later Hitchcock film).  "Sabotage" was a free adaptation of the book, with no mention of the Greenwich Observatory  the bomb goes off accidentally in a bus on the Strand (which co-incidentally was what happened to an IRA bomb over sixty years later).

More interesting, perhaps, is a more recent and star-studded version directed in 1996 by Christopher Hampton, famous as the director of "Dangerous Liaisons" and its stage predecessor "Les Liasions Dangereuse" (he also wrote the screenplay for "Total Eclipse" which, despite its name, has emphatically no astronomical connections).  In the U.K. this was released (by Fox Entertainments) as "The Secret Agent", and in the U.S. under the more pedantic "Joseph Conrad's The Secret Agent".

One has to wonder what Bourdin would have thought of the string of household names employed to tell his story  would he have been flattered by the attention or contemptuous of the establishment film industry?  Verloc is played by Bob Hoskins; Ossipan, the french anarchist, by Gerard Depardieu; the Professor by Robin Williams (under the pseudonym George Spelvin). The support cast list is no less impressive  Mrs Verloc is played by Patricia Arquette, Stevie by Christian Bale and the first secretary Mr Vladimir by Eddie Izzard.  The film features a score by minimalist composer Philip Glass and was filmed on location in London and Loughborough, including scenes in Greenwich Parkand at the Greenwich Observatory.

With such an impressive pedigree it is a great shame that the film was a critical and box-office failure.  Perhaps the relentlessly downbeat nature of the story counted against it; some reviewers criticised the non-linear flow of the story, which followed the book' structure.  However, it would be a pity to miss the dramatisation of one of astronomy' more unlikely brushes with politics.

Watch out for it on TV!
 

The author would like to thank Maria Blyzinsky of the National Maritime Museum, Adam Perkins of the Cambridge University Library and Philip Taylor of the Royal Greenwich Observatory for their assistance in researching this article.
 
 

Sources:
Non fiction:
Royal Greenwich Observatory  archive RGO 7/58, Cambridge University Library.

Fireworks at the Royal Observatory  by Philip Laurie  (The Castle Review, Journal of the RGO Sports and Social Club, Vol 5, No 1, 1955)

Propaganda by Deed  The Greenwich Observatory Bomb of 1894  by Philip Taylor (Open Space No 9, P4, Nov 1996, PPARC)

http://www.ast.cam.uk/pubinfo/leaflets/secretagent/RO bomb.html

The International Anarchist Movement in Late Victorian London  by Hermia Oliver (St Martin' Press, 1983)

Unabomber 'Based his Life on Novel'  by Serge F. Kovaleski (Washington Post, reprinted in Guardian Weekly July 21st 1996)

Fiction:
The Secret Agent  by Joseph Conrad (first published 1907, Penguin Popular Classics 1994)

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Dust not only gathers on our book selves and on our windowsills, but it is also out there  in space!  It is present everywhere, so rejoice, do not feel guilty of not dusting; read about it.

There is a lot of dust about!  In the inner solar system, the Zodiacal light is a faint brightening or glow in the sky just after sun set and just before dawn; the French astronomer Cassini wrote about some aspects of this phenomenon.  Now we know that it is due to the scattering of the suns rays by the dust in the same orbital plane as the planets.  Scattering is the change in the direction of motion of a photon, almost without energy loss.

Sir William Herschel observed some gaps in the distribution of stars on the sky and thought that these where just holes in the halo of stars around us.  Then around the 1930's and 40's, when observation techniques and the theory of the universe became more refined, it was noticed that the distance between the stars in a cluster depends on the method of measurement.  It can be calculated, A, either from the luminosity of the stars or B, the linear dimension of the star.  It soon became apparent that the distance estimated from method A was greater than if estimated from method B and therefore, if something present in the interstellar medium absorb the stars light (obscuration), giving the impression the star being fainter and further away.

Further observations based on obscuration at different optical wavelengths, suggested the presence of solid particles of size similar to the optical wavelength i.e. about 0.1 - 1 µm (1 µm = 1x10^-6 m).  Advances in infrared spectroscopy revealed further properties; these small solid particles remained no more the bet noir of the astronomers for forming the smoke screen; instead these were hailed as:
I)        the building blocks' of star formation
II)       formation of molecules in the interstellar space
III)     material rejected by stars in its dying days
IV)      the formation of planetary system around the stars.
Now we know that the gaps in the star clouds noticed by Herschel is caused by obscuration by dust.  Some of the current information of this cosmic dust is discussed here.

Detection of Dust Particles, Spectroscopy
When electromagnetic radiation strikes a dust particle, the atoms of the particle may absorb the energy, go to an unstable exited state and then eventually fall back to their normal state by releasing the excess energy in the form of emission spectrum from the dust particle.  However sometimes the dust/gas cloud may absorb the incoming radiation, but the energy rise of the dust/gas is not enough for it to emit any spectrum.  The incoming spectrum now misses some wavelengths which appears as a few missing bands — the absorption spectrum.  These spectroscopic studies are useful in understanding the chemistry of the dust/gas cloud.  But an observer, whether in space or on terra firma, measures the combined loss of radiation — caused by scattering and absorption; this is called extinction.

Particles also scatter the incoming radiation (or wavelength).  For particles to scatter radiation, the wavelength and the particle size has to be comparable.  By studying the scattering process one can get information on the size of the particle.

In the early days, terrestrial based observation of the spectrum of astronomical objects identified the presence of cosmic dust.  A wide range of radiation, from X-ray to infrared to millimetre range radio waves, were tried.  The major obstacle to the terrestrial based observation is that the incoming radiation is either reflected back or absorbed by the everyday compounds in the atmosphere like water vapour, oxygen etc.  Now, orbiting satellites above the earth's atmosphere can give a more elaborate picture.  At one time x-ray films sent on rocket missions, lasting for a few minutes, was the state of the art.  Physical scientists have worked out the relationship between the wavelength, emissivity, absorption, particle size and the temperature; thus we can choose a suitable wavelength, to study a given set of properties of dust.

Below is a short list of radiation ranges used for the study of the dust and the information these can provide.

Millimetre wavelength range 0.35 to about a few mm
Emissivity and hence the temperature of the dust grain, and information about the molecules both circumstellar (around stars) and interstellar (general background) region.  If a star is far away, radiation takes long time to reach us, indicating an earlier epoch.  If the dust from such a region is heated by the radiation from the star it emits infrared, but by the time these have reached the earth, the wavelength has been red shifted to a longer wavelength than the infrared;  into the sub-millimetre wave length.  Earlier optical studies suggested that the peak rate of star formation took place when the universe was about three-quarter of its present age.  However, recent sub-millimetre wave length observations at the James Clerk Maxwell telescope in Hawaii indicate that the peak rate was much earlier (Modern Astronomer, Vol. 2, September, 1998, p 284).

Infrared range 1x10^-3 to about 1x10^-6m
Temperature (from emissivity data),  absorption or emission spectrum in the infrared helps to identify composition and the scattering data give an indication of the grain size of the circumstellar dust.

Optical range 7x10^-7 to about 4x10^-7m
It was the observations in the optical range, which first alerted the astronomers to the possible presence of dust obscuring the halo of stars.  For example, if one area of the sky is viewed with different filters, the images appear somewhat different.  This is caused by parts of the spectrum from the dust/gas cloud being absorbed by the filter.  If the light from a cluster of stars passes through the interstellar dust, it may illuminate it. Diffused glow of the sky is due to the scattering of starlight by the dust.  Individual stars are sometimes surrounded by a dust cloud; this dust cloud was either generated by the star or is leftover matter from the star formation.  In general, the optical range perhaps does not provide much quantitative information like the size of the dust particles, none the less, it, often gives information about the chemical nature of the dust.

Ultraviolet range 1x10^-7 to about 5x10^-10m
Hydrogen gas is very abundant in the universe and it absorbs radiation in the ultraviolet range, so the information does not reach the ground-based observer.  However detectors on board spacecrafts have produced useful information about both circumstellar and interstellar dust.

X-ray range 5x10^-10 to about 5x10^-13m
Spectrum in the range of 0.1 - 100A° (1A° = 10^-8mm) is examined.  Radiation in this range is not absorbed by hydrogen gas.  For a dust grain to emit x-ray radiation of a wave length of about 30A°, its temperature has to be around 10⁶k; at this temperature the dust should evaporate.  However if the dust cloud is near a x-ray source, it can scatter x-rays, and hence can point to the source.  X-ray observations studies have been useful in the study of interstellar dust.  The bombardment of ions from a supernova explosion can increase the temperature of the dust to emit x-rays.

Beyond the x-ray is the gamma ray, 5x10^-13  to about 5x10^-16m wavelength range.  Gamma ray sources are well known to astronomers, but these have not been used to study the cosmic dust.

To sum up:
i)       Dust grains in the range of about 1- 100µm are responsible for the zodiacal light and to detect dust particles 100µm and below, spectroscopy is the tool.
ii)      Particles above about 100µm do not take part in the commonly observed phenomenon.
iii)     Particles above 100 to 500µm leaves an ion trail when entering earth's atmosphere.
iv)     Heating and ionisation of gases on entering earth's atmosphere is caused by particles in the range of about 500µm and greater; shooting stars or meteor shower are particles upwards to several metres  ˜ entering the earths atmosphere.

Collision Detectors
After learning about the spectroscopy and the scattering process, astronomers attempted a close encounter with the dust by sending space probes to be stoned or bombarded by dust.  These studies were looking for those grains, which are too small to be detected by the spectroscopic methods.  Although these may be small, but they make their presence felt — Giotto mission to Hally's comet suffered from impact damage; some of the instruments stopped working, the space probe was tilted so that commucations were temparaly lost.  This was probably caused by larger particles.

All this information has other importance as well.  An interplanetary spacecraft may suffer from an impact velocity of the order of 10 to 20km/s, whereas for a fly by or sling shot situation, the impact velocity may rise to about 70km/s.  It is expected that in future solar probe mission may encounter impact velocity of about 310km/s.  These probes are expected to provide information from about 4 times solar radius distance from the centre of the sun and if the sun is to act as gravitational sling shot, this information would be useful.

For real proof of the presence of dust grains in space:- the lunar rock brought back by the Apollo missions showed micron size crater impacts from small particles.  Such a dust detector essentially consists of  about 0.1mm thick foil, hemispherical in shape.  The maximum sensitive area is about 0.1m².  When a dust particle strikes the detector, plasma is produced.  There is an arrangement of electrodes, which separate the charge into constituents such as electron, positive and negatively charged irons.  These are analysed to obtain information not only on the chemistry of the dust particle, but on its velocity as well.

These instruments are calibrated in the laboratory before being put on board the space probe.  This is carried out by bombarding the instrument target with particles of a known mass and charge range one is expected to meet; the angle of impact is also an important consideration.

Part of the Galileo programme was aimed to study the mass, velocity and electric charge of particles in the range of 10^-16 to 10^-6g in the Jovian atmosphere.  The information obtained would provide information on the nature of the particle and interaction with magnetic and radiation fields and their role in forming the ring around Jupiter.

Now that we now how to look for dust, let us see what we know about the dust.

Origin of Dust
Roughly speaking the comic dust is of two types:
i)     Circumstellar:  associated with a single star and
ii)    Interstellar: uniformly distributed throughout the space.

The presence of the  circumstellar dust is either due to
A:     the star is relatively young and the observed dust is remnant material from the star formation,
B:     stellar wind (a radiation induced movement of fine particles) has attracted the dust around the star.
There is also a  third possibility, chemical reaction occurring in some of the mature stars can lead to the formation of dust around the star.

Since this type of dust is associated with that of a single star, its properties are dominated by the star itself.  The temperature of the dust is higher than the interstellar background.  The circumstellar dust shell is of the order of about 1pc (the distance, at which the object would form 1 second of arc ~ 3.09 x 10¹⁶m = 3.25 light years).  If we know the optical spectrum of a given star, we can calculate theoretically the expected temperature and the expected infrared radiation from the star; but when we measure the infrared radiation from the star, it sometimes turns out that the star is emitting excess infrared radiation (this is called infrared excess).  This infrared excess is a combined effect of infrared radiation from the star and the surrounding dust.

Chemistry of Interstellar dust
There are two types of interstellar dust distributions,
i)      general distribution of dust in background and,
ii)     dense cloud.
Dust in the interstellar cloud may settle on a core of ice, thus making the dust grains appear larger than commonly observed in the general background.  We have discussed that the interstellar dust may appear to radiate more in the infrared regions.

Absorption and extinction behaviours for various galaxies show a similar pattern; this tends to suggest that the interstellar dust medium is generally uniform in nature.  The mathematics of extinction in the optical region (7x10^-7 to about 4x10^-7m wavelength) gives us the following mass or density distribution (in kg m^-3) that can be expected for interstellar matter.
Free electron                                 4.1x10^-22
Molecules                                     4.4x10^-26
Small particles <<wavelength>>    1.5x10^-27
Particles  ~ wavelength>>             1.5x10^-27
Particles >> wavelength                1.2x10^-23

If we assume that the composition of the sun, in weight as, 73% hydrogen, 25% helium and 2% heavy elements, (mass greater than those of hydrogen and helium), to be the average composition of the gases in the interstellar media. Now if we calculate the density ratio of dust to gas, it turns out that the only way we can account for the heavy elements such as carbon, nitrogen, oxygen, magnesium, silicon, sulphur and iron, if these are present as chemical compounds.

In this simplistic model, heavier inert gases such as neon are not considered.  However there is one problematic compound, water ice; it is not present in the interstellar medium, but it can form, for example, by nucleation of water ice on a grain e.g. a silicate grain.  The ice can form by direct reaction between hydrogen and oxygen or by accretion of OH radicals to form ice.

Spectroscopic observations tend to show somewhat lower concentration of heavier elements (i.e. elements other than hydrogen and helium), than we can expect from theoretical calculations.  It has been suggested that these elements are tied up in gas, for example carbon monoxide, CO, or as compounds as in dust grains.

Often the spectral pattern of the interstellar dust is not completely understood; this makes positive identification of the dust somewhat difficult.  It is now agreed that the major constituents are carbon and silicates, however these may not be present as separate particles but as one compound nucleating and growing on another particle.

The MNR model, (theoretical calculations by Mathis, Nordsieck and Rumpel) suggests the interstellar  dust grain size to be of the order of 5 - 250nm (5x10^-9 - 25x10^-8m), (weighing about 10^-18 to 10^-13g).  In this calculation it is assumed that this fine size distribution is the result of inter-particle collision.  But such a collision process is difficult to imagine, since the dust density is very low (number of grains about 10^-6 m^-3).  It is now proposed that the dust grains are first formed by the stars, then break down to smaller sizes and eventually travel to the interstellar medium.  Ulysses and Galileo probes have recently identified bigger dust grains 3x10^-13 to 1x10^-7g.  Calculations based on the availability of hydrogen and other heavier elements, (the latter take part in the dust formation), suggest that to form the commonly available interstellar grains of about 0.1µm size, the time needed is of the order of 10¹⁰ years, which is far higher than the age of the universe; this also indicates that the origin of the interstellar dust is perhaps somewhere else.

The interstellar dust can lead to some interesting phenomenon such as
i)     a near by star can light up the dust grains: this is called reflecting nebula,
ii)    light echo from variable stars: the direct light from the star and the light scattered by the dust appear at different times.  To the observer the scattered light appears to come from a circle, the radius of which increases with time and the net effect is like an expanding ripple in a pond.  The magnetic and electric fields and the ionised state of the particles often cause the particles to be aligned in certain direction.  This can give rise to the light (radiation) beam to appear polarised.

Circumstellar Dust
We have seen earlier that this is associated with a given star and like the interstellar dust, the circumstellar dust can also make the star appear cooler, that is, more to the red end of the spectrum.  The presence of this dust reduces the luminosity of the star and by examining the spectral properties we can determine the nature of the dust particles.

We have discussed earlier that the circumstellar dust can either be matter condensing to form the star or remnant matter after the star formation.  From the temperature around stars, around 2000K (1700°C) we can expect the formation of carbon monoxide, CO. After the formation of CO, we can have excess of, either carbon, or oxygen atoms and if we have excess oxygen, compounds of oxygen such as oxides of say aluminium, magnesium, iron, silicon or silicates, (silicate is a oxygen-silicon compound) form.  The spectral signature from silicates prepared in the laboratory and from the circumstellar dust show only partial resemblance; it is not always possible to say with confidence, if these silicates are crystalline; however the silicates in comets appear to be crystalline.

Let us now consider the stars where we have carbon excess.  As the star ages, the nuclear fusion reactions within the star synthesise carbon and silicon which move to the outer region of the star and the spectral lines suggests the possible presence of silicon carbide, SiC, molecules. Some spectral data has been explained as the presence of about 50A° size carbon particles, but its confirmation is open to interpretation.  (Silicon carbide is almost as hard as diamond, and a lot of it is produced in the factories for making grinding tools).

When a collapsing protostar collects material from the accretion disc and forms a star, the material from the disc flows out at right angle to the disc.  The presence of strong magnetic field may cause the dust grains to align preferentially and this may lead to polarisation.  As the gas and the dust pass between the observer and the star, the pre-main sequence stars appear to be dusty, and the polarised nature of the beam may make the stars to appear as variable stars.

When the molecules form, these are initially  in the gaseous state; these then grow by the process of nucleation and growth.  The nucleation is by heterogeneous nucleation i.e. gaseous molecules forming on another dust grain and then it slowly grows.  The situation is similar to seed crystal used to grow larger crystal in chemistry laboratorys.  This model can explain some of the features of the dust formation around the star.  As early as the 1930s it was recognised that novae (a white dwarf star in a binary star system, on the surface of which, a runaway thermonuclear reaction is occurring and material is being thrown out) are the sites for the formation of dust and gas.  By late 1980s some interesting observations were made.  The optical luminosity of the novae goes through a dip and with time recovers again.  This was explained by the fact that
i)      molecules like CN formed, and
ii)     these molecules obscured the novae.
Additionally dust also formed and it caused further obscuration of the novae.  The problem with this explanation was, molecules in the gaseous state can not block out a wide range of wavelengths; so if we keep on looking at all wavelengths, then at certain wavelengths, we should we should not see any obscuration.

Finally the infrared observations suggested the sequence to be:
i)      as the dust forms, it covers the surroundings and the star appears dim;
ii)     then the dust absorbs some of the radiation and re-emits in the infrared range;
iii)    on dispersion of the dust cloud the optical luminosity is restored again.
The infrared luminosity therefore first decreases, rises and then finally drops.  To put things in perspective a few numbers, about 10²²kg of material may be thrown out in such an explosion, once it has reached about 10¹³m from the star, nucleation and growth process forms dust grains of size of about 0.5µm and the whole process may take 150 to 190 days.

A similar phenomenon also occurs with supernovae, but the explosion is on a much higher scale; the molecules flow out with greater speed and it takes longer, about one year, to form dust grains and form particles.  It may be mentioned here that the light echo as discussed with the interstellar dust may start its life as circumstellar dust around a supernovae.

There is a special group of stars called R Coronae Borealis, which are rich in carbon molecules and not rich in hydrogen, the most commonly occurring element.  These show an optical wavelength dependent absorption, but no such absorption spectrum is observed in the infrared range.

Only the novae explosions give rise to specific isotope ratios in the dust grains and some of the meteorites also show specific isotope ratios, therefore it may be possible that the meteorite had its origin in the given stellar activity.

In addition to the inorganic compounds like silicates, oxides or silicon carbides, spectroscopic evidence suggests the presence of organic molecules.  Positive identification is not always possible, all we can say is the presence of carbon-hydrogen, C-H, bond.  In general, both the interstellar and circumstellar dust is about 1µm in size; this corresponds to about 3 - 4x10⁶ atoms.  The problem of positive identification is due to the fact that in the laboratory we deal with a much higher concentration (hence size) of the molecules and we do not know how the spectrum would look like if we have only a very small amount, say about — 100 atoms in 10A°.
While trying to correlate the possible grain size and the observed and predicted temperature from spectroscopy, it emerged that about 10A° size stellar dust particles can be intermittently heated to a higher temperature due to being  bombarded by gas jets.  Since the matter is not plentiful, particle temperature would rise and then fall again before the next strike.  This process makes these particles hotter than if these are heated by radiation alone.  Now, if this calculated temperature is the true temperature of the dust particles, compounds like water ice, silicates would evaporate.

However one group of carbon-hydrogen compounds like (C6H6)n, C₂₄H)₁₂, can explain quite a few of the features of the infrared spectrum.  Some of these compounds can also occur as cluster and platelets.  These compounds are called Polycyclic Aromatic Hydrocarbon or PAH.  Again like many other observations, some of the features remain unexplained and part of the reason is that the compounds synthesised in the laboratory may be similar but may not be the same.  These molecules might have been responsible for starting the carbon based life as we know it.

Another well known candidate is large carbon molecules, C₆₀ and C₇₂  — Bukminsterfullerin.  Its origin has been explained by the radioactive decay process in the stars.  The structure looks like a soccer ball.  (Theoretical calculations predict that materials based on this molecule can bring about a new generation of strong materials.)

Dust nearer to home (astronomical scale)
We now know that the dust from the nucleus of the comet scatters sunlight and makes it visible.  The radiation pressure causes the dust to fly away from the nucleus of the comet.  The heat of the sun causes release of the volatile matters.  Some of the dust particles are charged, i.e. ionised — (remember the ion trial of Hale-Bopp).  This way comets may account for the dust particles in the inner solar system.  Fragmentation of asteroids and meteors and mutual collision is the source of dust nearer to Earth.  If this fragmentation of the larger bodies into smaller particles is the source of dust, then it is likely that the dust in the solar system is of recent origin and not of the same age as the solar system.

Some of the solar dust, under the influence of solar radiation pressure, will leave the solar system and move into interstellar space.  As the solar system moves through interstellar space, some of the interstellar dust enters the solar dust system, for example Kuiper belt (icy planetisimals just beyond the orbit of Pluto, confirmed 1991 - 92), and may also take part in further fragmentation of solar dust.  Dust particles under the influence of electric and magnetic forces can align themselves and eventually form planetary rings.  Some recent observations suggest that the rings around the Jupiter have been formed by micrometeorite collision with its innermost moons.

Future
We can see that by getting to know the cosmic dust we can learn about the evolution of the stars, planetary rings, planets and eventually us.  They knew it all along when they said dust to dust.  We can not say anymore  obscured,  buried under a cloud of dust.  We can now see through the dust.  We have different wavelength ranges to look into the dust cloud, each complimenting the other.  Ground based observations are just important as those aboard spacecraft.  We can now send probes to interact with the dust and may be able to bring back samples as well.  At last we have learnt to treasure the dust.
 

Bibliography

Dusty Universe  Evans, Aneurin, Praxis Publishing Ltd, 1994
Graphs, Amara personal communication
EOS, number 4  Mann, Ingrid, 27 January 1989
The Cassini/Huygens Mission Clarke, Ivor, MIRA 43, 1997

Thanks are due to Ivor Clarke for suggestions on the manuscript and daughter Twisha for reading the proof..

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