The Sky at Night Phần 7 pptx

98 24 Scorpion in the Sky
and contains several hundred thousand stars. The full diameter is 95 light-years,
and near the centre the individual stars are so close together that collisions must
sometimes occur. In May 1861, a nova flared up in the cluster and briefly outshone
all the other cluster members combined; it was given a designation – T-Scorpii – but
has never reappeared. It was probably an ordinary nova, a one-off, but M80 is worth
monitoring.
Adjoining Scorpius are the beautiful clouds of the star Rho Ophiuchi, and the
whole region is exceptionally rich. So, when the sky is clear, go and seek out the
Scorpion, with its red leader, its chain of bright stars, and its wonderful star fields.
There is no other constellation quite like it.
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© Springer Science+Business Media, LLC 2010
The Perseids provide the most reliable of all the annual meteor showers. Conditions
in 2007 were expected to be good, so we planned a meteor watch at my observatory.
It turned out to be a distinct success.
Meteors can be seen on almost any clear night, and there can be few people who
have not been impressed with shooting stars flashing across the sky and vanishing
in a second or two. Yet not everybody knows what they are, and only during the past
couple of centuries have we been able to learn much about them. They are phenom-
ena of the upper atmosphere, and each streak indicates the last moments of a tiny
object, generally no larger than a grain of sand, dashing into the upper air from outer
Chapter 25
The August Perseids
Perseids 2006 (Credit: Pete Lawrence)
100 25 The August Perseids
space and burning away by friction against the atmospheric particles. It enters the air
at a speed of anything up to 45 miles per second and is destroyed by the time it has
penetrated to about 40 miles above sea level, ending its journey in the form of ultra-
fine “dust.”

Meteors are cometary debris. A comet has a nucleus made up of ice and solid
particles; its mass is very slightly compared with that of a planet or even an asteroid,
and I once described a comet as being “the nearest approach to nothing that can still
be anything”. Some move round the Sun in elliptical orbits, in periods of a few
years; these so-called short-period comets are old friends, and we know when and
where to expect them. Others have much longer periods, so that we cannot predict
them. I well remember the lovely green comet of 1996, discovered by the Japanese
astronomer Yuji Hyakutake, and named after him. It will return to the inner Solar
System in about 15,000 years time; remember to look out for it!
As a comet draws in towards the Sun, its outer ices begin to evaporate, and the
comet may develop a tail of immense length, so that it may become spectacular
despite its negligible mass. However, a comet loses material at ever perihelion
passage, and it wastes away; some periodical comets have been seen to disintegrate.
The most famous case of this was that of Biela’s Comet, which is seen every
6 ¾ years – until 1846, when it broke in half. The pair came back on schedule in
1852, but this was their final appearance. The comet is dead, but when the Earth
passes close to the place where it ought to be, we pick up particles which it has left
behind – and the result is a shower of shooting stars. Actually, it is more accurate
to say “when the Earth passes through the orbit of the defunct comet,” because
dust particles have been left all along the orbit.
Few periodical comets ever become bright enough to be seen with the naked eye
even when they are relatively close to us, and only one – Halley’s Comet – can ever
become brilliant. It returns to perihelion every 76 years, but at its last visit, in 1986,
it was badly placed and some people were disappointed (it will be back again in
2063). But comets are clearly linked with meteor showers, and this brings me on to
the August Perseids and our Sky at Night programme for that month in 2007.
The particles left behind by a comet are moving along in parallel paths, and so the
resulting meteors appear to emanate from one definite point in the sky, known as the
radiant. The best demonstration I can give you is to picture the scene from the top of
an arc overlooking a motorway. The lanes seem to diverge from a point near the

horizon – the “radiant” of the lanes, and approaching cars will come from that point.
The August meteor issue from a radiant in the constellation Perseus, and I always
regard this as the richest of all annual showers – of which there are many.
The Perseids were not identified until 1835, when attention was drawn to them by
a Flemish scientist, Adolphe Quetelet, who was an interesting man; he was an enthu-
siastic astronomer, but was also a leading criminologist and statistician. After he drew
attention to the August shower, others took notice of it, and in 1864 the Italian astrono-
mer Giovanni Schiaparelli (best remembered, perhaps for his observations of the
“canals” of Mars) found that the meteors moved in the same orbit as a comet, Swift-
Tuttle, which had been discovered in 1862, and was thought to have a period of well
10125 The August Perseids
over 100 years (Lewis Swift and Horace Tuttle were both renowned comet hunters).
The inference was obvious; Swift-Tuttle was the parent comet of the Perseids.
Every year, the shower meteors begin to appear around July 23 and become
more and more plentiful until maximum activity is reached on August 12–13.
After that subsides, though a few Perseids may still be seen as late as August 20.
The ZHR, or Zenithal Hourly Rate, may be as high as 80 (the ZHR is defined as
the number of naked-eye shower meteors which would be seen by an observer
under ideal conditions, with the radiant at the zenith or overhead point. In practice,
these conditions are never attained, so that the observed rate is always less than the
theoretical ZHR).
There are three more points to be noted here. First, although the meteors radiate
from Perseus – but are not confined to Perseus – they flash along to any point in the
sky. Second, not all the meteors that you see will be Perseids; some belong to other,
less prolific showers, while there are also sporadic meteors, which may appear from
any direction at any moment, and are not linked with known comets. You can identify
Perseids by plotting their paths against the stars and tracing them “backwards” to
their starting point. Finally, what we see in the sky is not the particle itself, which
is too small to be visible, but the luminous effects which it produces during its
headlong dash through the atmosphere.

For 2007, the prospects were as good as they could possibly be. The weather
forecast was favourable, and – most important of all – there was no interference
from moonlight; the Moon was new. Maximum was due at 2 a.m. on the morning
of August 13. So for a Sky at Night “special,” we assembled a group of experienced
meteor observers, headed by John Mason, plus others (such as me) who were less
dedicated, but were determined to enjoy the display of cosmic fireworks. The two
groups were doing different programmes. I was merely carrying out a television
commentary plus counting the meteors, noting their magnitudes, their colours
(if any) and other exceptional features; some meteors leave trains which persist for
some seconds, or even less frequently, a minute or two. The more serious members
of the party were carrying out photography. The method here is to point the camera
in a suitable direction and take time-exposure. You will record star trails, and perhaps
an artificial satellite or two. We were lucky; we had barely settled down when the
ISS – the International Space Station – passed overhead. You could not possibly
miss it; it shone more brightly than Venus.
What did we hope to see? Well, a decent shower; the Perseids are usually
co-operative. There was always the chance of a “fireball” which would light up the
landscape; I have seen a few of these, produced by particles the size of grapes or
even golf-balls, but they are rare. Otherwise, we had to wait and see. The sky was
cloudless and transparent, and the lawn surrounding my observatory is shielded
from any obtrusive artificial lights, so that it was pleasingly dark.
Meteors began to appear; apart from one brief period, the clouds stayed
away. Gradually activity increased. There were some bright meteors, though
no fireballs. None of us felt inclined to give up; as usual on these occasions,
there was rather a party atmosphere. Coffee was most welcome, and we
102 25 The August Perseids
remained until the approach of dawn, when we adjourned indoors to refresh
ourselves with drinks which were rather more potent than coffee. The end of
an enjoyable evening.
What had we achieved? Scientifically not a great deal, though routine observations

are always useful; our main point was that we had spoken to a large audience of
around a million people, some of whom went outside and watched a spectacle
which they would otherwise have missed. That in itself, I feel, justified our special
programme. Of course, the Perseids will be with us again next year, but in 2008 the
conditions will not be nearly so favourable, as meteor watchers will have to contend
with strong moonlight.
Finally, spare a thought for Comet Swift-Tuttle, responsible for it all. It was not
seen for many years after 1862, and calculations indicated that although it had
been missed at several intervening returns, it should be back in 1981. Careful
searches gave negative results, and I thought that it had simply been overlooked.
John Mason disagreed; he believed that the orbit had been wrongly worked out, so
that the real date of the next perihelion passage would be early in the 1990s. We
had a modest bet about this (a bottle of Irish Whisky, I recall) and I was confident –
until 1992, when the comet turned up. The period is now known to be 135 years,
Swift-Tuttle has not been conspicuous lately, but a good deal will be heard about
it in the twenty-second century, when it will pass alarmingly close to us. Wait for
an end-of-the world scare then – but don’t blame the Perseids!
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Everybody is fascinated by black holes. Who better to give us the latest news than
professor John Brown, the Astronomer Royal for Scotland, who as well as being a
world-renowned solar physicist is also an expert amateur magician. Joined by
Drs Fiona Spititz and Chris Lintott, we attired ourselves suitably and did our best
to forge a link between black holes and Dark Forces.
Chapter 26
Black Holes: And Black Magic
Einstein’s cross (Credit: ESA)
104 26 Black Holes: And Black Magic
Black Holes and Black Magic? No true connection, needless to say, but black

holes are so bizarre that they really do seem to be magical. Even now we cannot
pretend that we have anything like a full understanding of them.
Then first concept of them seems to have been due to an English amateur scientist,
the Rev. John Michell, who lived from 1724 to 1793 (of course, most people mis-
spell his name as “Mitchell”). His activities were very varied – someone ought to
write a really good biography of him – and one of his suggestions was that a body
of sufficient mass would pull hard enough to prevent even light escaping from it.
A similar comment was made later by the great French mathematician Laplace,
but the term “black hole” dates from only 1968, when it was introduced by the
American scientist John Wheeler. It caught on and is now part of our language, but
in a way it is misleading because a black hole is not black at all. It emits no light
and so cannot really be said to have any colour.
If we cannot see a black hole, we have to locate it by means of its gravitational
pull upon objects that we can see. Black holes are incredibly massive – thousands
of millions of times more massive than the Sun – so that they can certainly make
their presence felt. We have every reason to believe that there is a black hole in the
centre of our Galaxy, because we can measure the speeds at which objects fairly
close to it whirl around it, and this allows us to calculate its mass.
It seems that a black hole is the end product of a very massive star. Normal stars
create their energy by nuclear reactions taking place inside them, as our Sun is
doing now. Eventually the supply of available nuclear “Fuel” will run out, and the
whole situation must change. A modest star such as the Sun will lose its outer layers
and subside into a white dwarf, where the atoms are crushed and packed together
so tightly that the star is extremely dense; the final state will be as cold, dead black
dwarf. We know plenty of white dwarfs, one of which, the faint companion of
Sirius, is no larger than the Earth but is as massive as the Sun, but the whole course
of stellar evolution is so slow that the universe may not yet be old enough for
any dwarfs to have formed. After all, the Big Bang happened a mere 13.7 thousand
million years ago!
A star much more massive than the Sun will die in a much more spectacular

fashion; it will explode as a supernova, blowing much of its material away into
space while the remnant, now composed of neutrons, will spin round and send out
beams of radio emission. If these beams sweep over the Earth, just as the beams of
a rotating lighthouse will sweep across the watcher on the beach, we pick up pulsed
radio waves – hence the term “pulsar” (there is a pulsar in the famous Crab Nebula,
6,000 light-years away; the supernova responsible was seen to blaze out in the year
1054). Pulsars, too, must end up as black dwarfs. But if the mass of the dying star
is greater still, it cannot even produce a pulsar. Once the final collapse starts, nothing
can stop it. The star becomes smaller and smaller, denser and denser – and the
escape velocity goes on increasingly until it reaches 186,000 miles per second, the
speed of light. Light is the fastest thing in the universe, at least so far as we know,
and so nothing can break free from the doomed star. It has become a black hole.
The underlying principle is not the same as was believed by Michell, because the
modern idea of gravity is different. To Michell, and also to Newton, gravity was a
10526 Black Holes: And Black Magic
force which enabled one body to affect another even when the two were widely
separated; this “action at a distance” was often regarded as a form of scientific
black magic. Albert Einstein changed all this and interpreted gravity as a distortion
of “space-time.” Consider a bowl with paper stretched across its top; roll a marble
across the paper, and it will follow a straight line. Now imagine an object inside the
basin which could in some way pull the sheet of paper downwards (I admit that I do
not see quite how this could be managed, but never mind). The marble will no
longer roll straight; its path will be distorted. I know this is a poor analogy, but it is
better than nothing. In space, the object at the bottom of the bowl represents our
black hole.
Black holes can therefore affect the paths of light-beams, and this is shown by
the phenomenon of gravitational lensing (though other massive bodies can act simi-
larly; large galaxies and clusters of galaxies, in particular). A good example of this
is what is termed the Einstein Cross (because it was Einstein who realised that this
sort of thing could occur). At a distance of 8,000 million light-years, we find a

quasar, catalogued as Q2237 = 030; quasars, as we know, are the immensely luminous
cores of active galaxies. En route to Earth the quasar’s light passes by the very massive
galaxy called Huchra’s Lens. The light from the quasar is “bent”; we see four
images of the background quasar, and you will agree that the effect is remarkably
striking. Many other cases of gravitational lensing are known, though not many are
as symmetrical as the Einstein Cross.
I must not forget to say something about Hawking radiation, first proposed by the
famous Cambridge cosmologist Stephen Hawking. What we call a vacuum is not
actually empty; it is seething with “virtual particles” which appear in pairs, but vanish
again so quickly that they are truly ghostlike. A particle and its antiparticle will anni-
hilate each other – but if a pair appears at the extreme edge of a black hole, the so-called
event horizon, one of the pair may enter the black hole, leaving its partner marooned
outside. With no partner, the stranded member of the pair becomes a “real” particle,
and the black hole is forced to emit a certain amount of radiation, which results in loss
of mass. It has been suggested that the emission of enough Hawking radiation might
finally make the black hole explode and destroy itself, but whether a major black hole
has yet perished in this way seems somewhat uncertain….
Exotic theories about black holes are plentiful. They have been regarded as passages
between our universe and a completely different universe, in a different dimension,
with which we can normally have no contact whatsoever; on the “multiverse” picture,
there may be many of these – perhaps an infinite number. Approaching a black hole
in the hope of a free ticket to the universe next door would be rather hazardous, and
there would be all manner of curious effects involving both space and time.
Incidentally, what is the fate of a star which collapses to produce a black hole? Does
it crush itself out of existence altogether, or does it turn up elsewhere, either in our
universe or in another? Will we Earthmen ever be threatened by a maverick hole able
to creep up and take us by surprise?
At present these are the problems which we cannot solve. Perhaps we will find
the answers eventually, but meantime it is not hard to see why some people still feel
that there must be at least a tenuous link between black holes and black magic!

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P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_27,
© Springer Science+Business Media, LLC 2010
In October 2007, the great radio telescope at Jodrell Bank had been in action for
half a century. Together with Chris Lintott, I went there and talked not only to
Bernard Lovell but also to Bernard Baruch, Ian Morrison and Phil Diamond of the
Jodrell Bank team. Bernard and I are very old friends. I remember thinking – would
this be the last time we would meet face to face? Bernard, now 94, is totally fit in
every way; thanks to the activities of our gallant German allies, long ago, I am not
really mobile. We must see. Incidentally, both Bernard and I are cricket fanatics. He
played to a really high standard; I did my best to spin my unorthodox leg-breaks.
Sadly, we never actually took the field together, and now fear it is rather too late!
Go to Jodrell Bank, near Macclesfield in Cheshire, and you will see the 250-ft
Lovell radio telescope. In fact you cannot possibly miss it, because it dominates the
entire landscape. It is a miracle of engineering even today and was even more so in
Chapter 27
Jodrell Bank: Fiftieth Anniversary
The Lovell Telescope (photo by Patrick Moore)
108 27 Jodrell Bank: Fiftieth Anniversary
1957, when it first came into action. It was the brainchild of Professor Sir Bernard
Lovell; but for him it would never have been built, and radio astronomy would not
be so advanced as it actually is.
Radio astronomy began in the 1930s, but was slow to develop because most
astronomers were suspicious of it. Lovell was not; he could see the possibilities,
and at the University of Manchester he began research as soon as he was free of his
wartime work. Meteor trails, for example, could be studied by radar. Manchester
was a hopeless site, because of pollution from electric trams and other sources.
Lovell was given the use of a site well away from the city and was told that he
would be allowed to stay there for 15 years.

I well remember those early days. Jodrell Bank really was a grassy bank, and we
lay there on our backs after dark plotting meteor trails. Then, thanks to Lovell, came
the first radio “dish”; it was an improvement upon anything previously produced, and
it was soon producing valuable results, but it could point only straight upward….
Lovell needed a steerable “dish,” and with engineer Charles Husband he planned
one…. It was to be 250 ft in diameter and would stretch the powers of 1950s tech-
nology to its very limit – perhaps even beyond. The budget, rather grudgingly
given, amounted to a million pounds.
There were problems galore. Midway during the construction it was realised that
the resistance to wind had been grossly underestimated, and this necessitated exten-
sive redesigning, which cost a great deal. Government accountants, those sworn
enemies of research, were always hovering like vultures, and on the fourth of
October 1957 the situation was desperate. The telescope was not complete and the
debt had spiralled to a quarter of a million pounds. To a colleague, Lovell said
sombrely: “Only a miracle can save us.”
Sputnik 1 was the miracle. The “bleep! bleep!” signals could be picked up easily,
but the rocket launcher could not, and of course this launcher was the first
Intercontinental Ballistic Missile (IBM). America panicked; outside the USSR only
the Jodrell Bank telescope could track the rocket, and overnight Lovell was trans-
formed from a reckless spendthrift into a national hero. Lord Nuffield paid the
outstanding debts; the crisis was over and Jodrell Bank was safe.
Since then research has been constant; the 250-ft dish is still the third largest in
the world and arguably the best. Quasars, pulsars, gravitational lenses and other
bizarre objects, unknown in 1987, have received particular attention. The telescope
is never idle. It has been updated, and a new dish, made of galvanized steel, has
been built on the site, notably the Mark 2 dish, the Jodrell Bank heads the MERLIN
network of seven radio telescopes working together.
Jodrell Bank has its place in history, and it will be in forefront of research for
many years to come. I am proud to have been around to watch its development, and
particularly proud to have been at its 50th anniversary with Bernard Lovell who,

like his telescope, will never be forgotten.
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Chapter 28
The Grand Collision
Andromeda (Credit: Robert Gendler)
110 28 The Grand Collision
Look into the future, and you can visualise a collision which will be utterly
devastating – far greater than a head-on clash between two planets, or two stars.
Our Galaxy, with its hundred thousand million stars, will collide with an even
larger system, the Andromeda Spiral. To talk about this coming cataclysm, I was
joined by two regular and welcome guests, Professors Carlos Frenk and Derek
Ward-Thompson, while in my observatory Chris Lintott and Nik Szymanek turned
my 15-in. telescope towards Andromeda.
Pegasus, the Flying Horse, is the main autumn constellation, but is still on view
during evenings later in the year. Its four main stars make up a square, which is easy
to locate even though maps tend to make it look smaller and brighter than it really is.
The upper left-hand star of the square, Alpheratz, has for some illogical reason been
transferred from Pegasus to the adjacent constellation of Andromedae, and has
become Alpha Andromedae instead of Delta Pegasi. Andromeda is marked by a line
of stars extending from Alpheratz in the direction of Capella. Not far from the
second brightish star in the line, the orange Mirach, you will find the dim blur that
marks the position of the Andromeda Galaxy, M31.
It is just visible to the naked eye when the sky is really dark, and binoculars
show it clearly, but photographic or electronic images obtained with adequate tele-
scopes are needed to bring out details of its structure. It is in fact a spiral, much
larger than the Milky Way Galaxy, and containing more than our quota of one
hundred thousand million suns. It has been known since very early times and was
the 31st object in Charles Messier’s catalogue of star-clusters and nebulae. It used

to be popularly called the Andromeda Nebula, but Edwin Hubble’s work in the
1920s proved that it is an independent galaxy, far beyond ours. Its distance is now
given as 2.6 million light-years, making it the most remote object distinctly visible
without optical aid.
Its distance was not easy to measure. Hubble made use of the stars known as
Cepheid variables, some of which are to be found in M31. They do not shine
steadily; they brighten and fade in short periods, generally a few days, and they are
absolutely regular, so that we always know how they will behave. A Cepheid’s
period is linked to its real luminosity; the longer the period, the more powerful the
star – which means that we can find the luminosity of a Cepheid, and hence its
distance, simply by watching it. Hubble’s original value for the distance of M31,
900,000 light-years, turned out to be too low, because Cepheids are more powerful
than he had expected, but at least he had taken the essential leap. Our Galaxy is only
one of many.
Earlier, Vesto Slipher, at the Lowell Observatory in Arizona, had examined the
spectra of galaxies and had found that almost all of them showed red shifts; the dark
absorption lines were shifted over to the red or long-wave end of the background
rainbow. This is the familiar Doppler effect; a red shift indicates a velocity of reces-
sion, and Hubble concluded, correctly, that the entire universe is expanding. Yet there
are some exceptions. Galaxies tend to be gregarious, forming groups; the so-called
Local Group, to which we belong, contains three large systems (the Milky Way, M31,
and the Triangulum Spiral M33 – plus several of moderate size and more than two
11128 The Grand Collision
dozen dwarfs. These are not racing away from us. In fact, it is wrong to say that every
galaxy is receding from every other galaxy – but it is true to say that the groups
of galaxies are receding from the other groups. (Even this may be too sweeping,
but it will suffice for now.) At the moment, M31 is 2.6 million light-years away,
which works out to something like 16 million miles, but it hurtling towards us at
almost 200 miles per second.
Will this continue into the foreseeable future – and if so, will M31 eventually

hit us? So far as we can tell the answers to these questions are “Yes” and “Yes.”
But please do not be alarmed. The collision will not happen for at least a 1,000
million years, and probably longer than that; estimates range between 2,000 and
10,000 million years. By then the Earth will no longer exist, at least in its present
form. The Sun will have evolved through hits red giant stage, and it is hardly likely
that the Earth will survive. Even if it does, all life on it will long since have been
wiped out.
M31 will not rush straight towards our Galaxy and hit it, in the manner of two
cars meeting head-on. They will first orbit each other and take part in what we may
call a cosmic waltz; only then will they merge, and individual stars will seldom
collide – after all, a star is a comparatively small target in the vastness of space. The
situation may be likened to that of two orderly crowds passing through each other.
But the material between the stars will be colliding all the time, and this will cause
chaos, with energetic star formation triggered off. When the main encounter is over,
after perhaps a 1,000 million years, the graceful spirals will have been destroyed,
replaced by a single elliptical system.
Galactic collisions are not so rare as we once thought, and neither are they so
widely separated relative to their actual dimensions. Represent the Earth by a pinhead,
and the nearest star, Proxima Centauri, will be another (even smaller) pinhead over
four miles away. But is we represent the Milky Way Galaxy by a dinner-plate,
Andromeda will be another plate on the far side of the dining-room table. We can
see other encounters, too. Look at the Antennae Galaxies in Corvus (NGC4038-9)
which are well within range of amateur owned telescopes; the images shown here
tell the whole story, with the ejected gas and dust giving the appearance of insect
antennae. The two systems passed through each other about 600 million years ago.
Collisions between galaxies are by no means unusual today and were more frequent
in the early history of the universe, when aggregations of galaxies were much
closer together than they are now. It is only direct collisions between individual
stars which are so rare.
Quite apart from encounters between giant galaxies, we must also note that

smaller systems will be absorbed; for example, our Galaxy is at present “swallowing”
the Sagittarius dwarf, which will lose its identity long before the menacing swoop
of Andromeda. In fact, large galaxies act as cannibals. I am particularly intrigued
by the Mice (NGC 4676) where two huge galaxies are pulling each other apart; they
have not yet merged, but it is only a matter of time, and the Hubble Space Telescope
can show what is happening, even though the Mice are 3,000 million light-years
away. The long streamers are due to the relative difference between the gravita-
tional pulls on the near and far parts of each galaxy.
112 28 The Grand Collision
Making forecasts is always dangerous, but so far as the future career of our
Galaxy is concerned, we are confident that there is no major mistake. Go outdoors
tonight, if the sky is clear, and seek out the innocent-looking smudge of light which
marks the Andromeda spiral; it takes an effort of the imagination to accept that you
are seeing a colossal star-system hurtling towards us at breakneck speed. It is certain
to hit our Galaxy. The collision may happen in a 1,000 million years, 2,000 million
year, 5,000 million years, perhaps not for 10,000 million years – but happen it will.
There can be no final reprieve.
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We had actually recorded the December programme when Holmes’ Comet suddenly
hit the headlines. We could not possibly ignore it, so we fixed a last-minute recording
session. Fortunately, the sky was clear, and the comet shone down, blissfully igno-
rant of the fact that its abrupt flare-up had played havoc with our carefully planned
broadcasting schedule.
Comets are the most erratic members of the Solar System. We never quite know
how they are going to behave, and one of them, known officially as Comet 17P/
Holmes, has given us a real surprise. Last October, it suddenly flared up from
Chapter 29
Holmes’ Comet

Holmes’ Comet (Credit: Pete Lawrence)
114 29 Holmes’ Comet
magnitude 17 to 2.5, and in less than 48 hours had brightened by a factor of over
a million. Nothing quite like this had ever been seen before.
The comet was originally discovered in 1892 by an English amateur, Edwin
Holmes, who was casually scanning the region of the Andromeda Galaxy. It bright-
ened up to the fringe of naked-eye visibility, and developed a short tail; it faded,
brightened again and then became very dim. It proved to be a member of Jupiter’s
comet family, with a period of 6.9 years; the distance from the Sun ranged from
2.05 astronomical units (191,000,000 miles) at perihelion out to 5.18 astronomical
units (482,000,000 miles) at aphelion. The orbital eccentricity (0,43) and inclina-
tion (19.1°) were modest, and the comet could not make any close approaches to
the Earth. It returned on schedule in 1899 and 1906, but was always faint, and
caused no particular interest. It was not seen at the next few predicted returns, and was
regarded as lost. But in 1964 E. Roemer, at the Lowell Observatory in Arizona,
recovered it after its position had been determined by Brian Marsden, who makes
a habit of locating long-lost comets. Since then, it has been observed at every
return, but has been dim and unremarkable – until now.
On 23 October 2007, J.A. Enriquez, in the Canary Islands, found that the comet
was much brighter than expected. The outburst was violent, with material sent out
from the tiny, icy, 2.2-mile nucleus so rapidly that by the 1st week of November the
coma had expanded to become larger than the Sun. It lay in the constellation of
Perseus, and completely altered the look of the whole of that part of the sky; with
the naked eye it looked like a slightly fuzzy star, and on 25 October I estimated the
magnitude to be 2.4, much brighter than Epsilon Persei. With binoculars it looked
rather like a globular cluster, but telescopically it was clear that the brightest part of
the huge coma was displaced from the centre. It was strange to realise that this
flimsy object, of negligible mass, had become the largest member of the Solar
System!
What makes the event even more remarkable is that perihelion had been passed

as long ago as 4 May, that the comet was receding from both the Sun and the Earth;
at the time of the outburst it was more or less at opposition, so that when a definite
tail developed it pointed away from us, and was so foreshortened that it was difficult
to make out at all. Visually it was never conspicuous, but images from the Hubble
Space Telescope in late November showed a bluish tail; at one stage the tail became
disconnected, and there was also a section of material, which broke away from the
coma itself. Evidently, there was considerable activity, and the magnitude fell
surprisingly slowly (it was a pity that moonlight interfered with observations during
a particularly interesting period). On 1 December, my estimated magnitude of the
comet was 2.9, but I must add that seeing conditions were not very good.
There does not seem to be anything very unusual in the comet’s composition,
and the cause of the outburst is not entirely easy to explain. A collision with
another body was suggested, but personally I do not believe than an impact was
responsible, because of the previous outbursts in 1892. True, these were minor
compared with that of 2007, but for the same comet to be struck three times would
really be too much of a coincidence. Surely, the explosion must have been internal.
Other comets have behaved in rather the same way, and it is worth recalling Comet
11529 Holmes’ Comet
P/Schwassmann-Wachmann (better known as Schwassmann-Wachmann 3) which
has a period of 5.4 years, and has been seen regularly since discovered in 1930; at
the return of 1995 it began to show obvious signs of breaking up. This has contin-
ued; fragments passed within eight million miles of us during the return of 2006,
and in the foreseeable future the disintegration is likely to be complete. But
Holmes shows no sign of breaking up completely, and all we can say is that very
unusual activity is taking place.
What next? Obviously it will be more difficult to follow events as the comet
moves out beyond the asteroid belt toward the orbit of Jupiter, and what will happen
to it before the next perihelion passage, in 2011, remains to be seen, but all observers,
both amateur and professional, will watch it as closely as they can. It is certainly
one of the weirdest things I have ever seen in the sky.