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When
the skies are clear and the Moon is not too bright, the Rev Robert
Evans, a quiet and cheerful man, lugs a bulky telescope onto the
back deck of his home in the Blue Mountains, about 50 miles west of
Sydney, and does an extraordinary thing. He looks deep into the past
and finds dying stars. Looking
into the past is the easy part. Glance at the night sky and what you
see is history and lots of it — not the stars as they are now but
as they were when their light left them. For all we know, the North
Star, our faithful companion, might actually have burnt out last
January or in 1854 or at any time since the early 14th century —
its light takes 680 years to reach us — and news of it just
hasn’t reached us yet. Stars
die all the time. What Bob Evans does better than anyone else who
has ever tried is spot these moments of celestial farewell. By day
he is a kindly and now semi- retired minister in the Uniting Church
in Australia, who does a bit of locum work and researches the
history of 19th-century religious movements. But by night he is, in
his unassuming way, a titan of the skies. He hunts supernovas. A
supernova occurs when a giant star, one much bigger than our own sun,
collapses and then spectacularly explodes, releasing in an instant
the energy of 100 billion suns, burning for a time more brightly
than all the stars in its galaxy. “It’s like a trillion hydrogen
bombs going off at once,” says Evans. If
a supernova explosion happened too close to us, we would be goners,
according to Evans. “It
would wreck the show,” as he cheerfully puts it. But the universe
is vast and supernovas are normally much too far away to harm us. And
supernovas are significant to us in one decidedly central way.
Without them we wouldn’t be here. They are the link between Big
Bang and the creation of our own solar system. The
story of their discovery, and of their role in creating life on
Earth, involves two of the most singular figures in 20th-century
science, one of them a Yorkshireman whose recent obituary accused
him of putting his name to “rubbish”. But for the moment let’s
stick with the quiet and cheerful Bob Evans. Most
supernovas are so unimaginably distant that their light reaches us
as no more than the faintest twinkle. For the month or so that they
are visible, all that distinguishes them from the other stars in the
sky is that they occupy a point of space that wasn’t filled before.
It is these anomalous, very occasional pricks in the crowded dome of
the night sky that Evans finds. To
understand what a feat this is, imagine a dining-room table covered
in a black tablecloth and someone throwing a handful of salt across
it. The scattered grains can be thought of as a galaxy. Now imagine
1,500 more tables like the first one — enough to fill an Ikea car
park — each with a random array of salt across it. Now add one
grain of salt to any table and let Bob Evans walk among them. At a
glance he will spot it. That grain of salt is the supernova. Evans’s
is a talent so exceptional that Oliver Sacks, in An Anthropologist
on Mars, devotes a passage to him in a chapter on autistic savants
— quickly adding that “there is no suggestion that he is
autistic”. Evans laughs, but he is powerless to explain quite
where his talent comes from. “I
just seem to have a knack for memorising star fields,” he told me,
with a frankly apologetic look, when I visited him and his wife
Elaine in their picture-book bungalow on a tranquil edge of the
village of Hazelbrook, out where Sydney finally ends and the
boundless Australian bush begins. “I’m
not particularly good at other things,” he added. “I don’t
remember names well.” “Or
where he’s put things,” called Elaine from the kitchen. THE
term supernova was coined in the 1930s by the first of our eccentric
scientists, a memorably odd astrophysicist named Fritz Zwicky. Born
in Bulgaria and raised in Switzerland, Zwicky went to the California
Institute of Technology (Caltech) in the 1920s and there at once
distinguished himself by his abrasive personality and erratic
talents. He
didn’t seem to be outstandingly bright, and many of his colleagues
considered him little more than an irritating buffoon. A fitness
fanatic, he would often drop to the floor of the Caltech cafeteria
and do one-armed press-ups. He was also notoriously aggressive,
threatening to kill his closest collaborator, a gentle man named
Walter Baade, on at least one occasion. But
Zwicky was capable of insights of the most startling brilliance. In
the early 1930s he turned his attention to a question that had long
troubled astronomers: the appearance in the sky of occasional
unexplained points of light, new stars. Improbably
he wondered if the neutron — the subatomic particle which had just
been discovered in England and was thus both novel and rather
fashionable — might be at the heart of things. It
occurred to him that if a star collapsed to the sort of densities
found in the core of atoms, the result would be an unimaginably
compacted core. Atoms would literally be crushed together, their
electrons forced into the nucleus, forming neutrons. You would have
a neutron star. Imagine
a million really weighty cannonballs squeezed down to the size of a
marble and — well, you’re still not even close. The core of a
neutron star is so dense that a single spoonful of matter from it
would weigh 90 billion kilograms. A spoonful! But there was more.
Zwicky realised that after the collapse of such a star there would
be a huge amount of energy left over — enough to make the biggest
bang in the universe. He called these resultant explosions
supernovas. They would be — they are — the biggest events in
creation. Interestingly,
Zwicky had almost no understanding of why any of this would happen.
And he was held in such disdain by most of his colleagues that his
ideas attracted almost no notice. When, five years later, the great
Robert Oppenheimer turned his attention to neutron stars in a
landmark paper, he made not a single reference to any of Zwicky’s
work, even though Zwicky had been working for years on the same
problem in an office just down the corridor. Zwicky
was also the first to recognise that there wasn’t nearly enough
visible mass in the universe to hold galaxies together, and that
there must be some other gravitational influence — what we now
call dark matter. But his deductions concerning dark matter
wouldn’t attract serious attention for nearly four decades. We can
only assume that he did a lot of press-ups in this period. Supernovas
are extremely rare. A star can burn for billions of years, but it
dies just once and quickly, and only a few dying stars explode. Most
expire quietly, like a camp fire at dawn. In a typical galaxy,
consisting of 100 billion stars, a supernova will occur on average
once every 200 or 300 years. Looking for a supernova, therefore, is
a little bit like standing on the observation platform of the Empire
State Building with a telescope and searching windows around
Manhattan in the hope of finding, let us say, someone lighting a
21st-birthday cake. So
when the hopeful and softly spoken Evans got in touch with the
astronomical community more than 20 years ago to ask if they had any
usable field charts for hunting supernovas, they thought he was out
of his mind. In
the whole of astronomical history before Evans started looking in
1980, fewer than 60 supernovas had been found. From 1980 to 1996 he
averaged two discoveries a year — not a huge payoff for hundreds
of nights of peering and peering. Once he found three in 15 days,
but another time he went three years without finding any at all.
This year he recorded his 36th. Only
about 6,000 stars are visible to the naked eye from Earth, and only
about 2,000 can be seen from any one spot. With a 16in telescope
such as Evans uses, however, you begin to count not in stars but in
galaxies. From his deck, he supposes he can see between 50,000 and
100,000 galaxies. Before
I visited him, I had imagined that he would have a proper
observatory in his back yard, with a sliding domed roof and a
mechanised chair that would be a pleasure to manoeuvre. In fact, he
led me not outside but to a crowded storeroom off the kitchen where
he keeps his books and papers and where his telescope — a white
cylinder that is about the size and shape of a household hot-water
tank — rests in a home-made, swivelling plywood mount. When
he wishes to observe, he carries them, in two trips, to a small sun
deck off the kitchen. Between the overhang of the roof and the
feathery tops of eucalyptus trees growing up from the slope below,
he has only a letterbox view of the sky, but he says it is more than
good enough for his purposes. On
a table beside the telescope were stacks of blurry photos with
little points of haloed brightness. One he showed me depicted a
swarm of stars in which lurked a trifling flare that I had to put
close to my face to see. This, Evans told me, was a star in a
constellation called Fornax from a galaxy known to astronomy as NGC
1365. (NGC stands for New General Catalogue, where these things are
recorded.) For 60m silent years, the light from this star’s
spectacular demise travelled through space until one night in August
2001 it arrived at Earth in the form of a puff of radiance, the
tiniest brightening, in the night sky. It was, of course, Evans on
his eucalypt-scented hillside who spotted it. “There’s
something satisfying, I think,” Evans said, “about the idea of
light travelling for millions of years through space and just at the
right moment as it reaches Earth someone looks at the right bit of
sky and sees it. It just seems right that an event of that magnitude
should be witnessed.” I
couldn’t get away from the nagging question: what would it be like
if a star exploded nearby? Our nearest stellar neighbour is Alpha
Centauri, 4.3 light years away. I had imagined that if there were an
explosion there we would have 4.3 years to watch the light of this
magnificent event spreading across the sky, as if tipped from a
giant can. What would it be like if we had four years and four
months to watch an inescapable doom advancing towards us, knowing
that when it finally arrived it would blow the skin right off our
bones? Would people still go to work? Would farmers plant crops?
Would anyone deliver them to the shops? Back
in the town in New Hampshire where I live, I put these questions to
John Thorstensen, an astronomer at Dartmouth College. “Oh no,”
he said, laughing. “The news of such an event travels out at the
speed of light, but so does the destructiveness, so you’d learn
about it and die from it in the same instant. But don’t worry
because it’s not going to happen.” The
reason we can be reasonably confident of this, Thorstensen explained,
is that it takes a particular kind of star to make a supernova in
the first place. A candidate star must be 10 to 20 times as massive
as our own sun, and “we don’t have anything of the requisite
size that’s that close. The universe is a mercifully big place”.
Which
brings us to the real significance of supernovas. For a long time
the theory of Big Bang — the moment of creation — had a gaping
hole that troubled a lot of people. It couldn’t begin to explain
how we got here. Although
98% of all matter that exists was created with Big Bang, that matter
consisted exclusively of light gases: helium, hydrogen and lithium.
Not one particle of the heavy stuff vital to our own being —
carbon, nitrogen, oxygen and all the rest — emerged from the
gaseous brew of creation. But
— and here’s the troubling point — to forge these heavy
elements, you need the kind of heat and energy thrown off by Big
Bang. Yet there was only one Big Bang and it didn’t produce them.
So where did they come from?
Hoyle,
who died in 2001, was described in an obituary in Nature as a
“cosmologist and controversialist”, and both of those he most
certainly was. He was, according to Nature’s obituary,
“embroiled in controversy for most of his life” and “put his
name to much rubbish”. Hoyle
claimed, for instance, and without evidence, that the Natural
History Museum’s treasured fossil of an archaeopteryx was a
forgery along the lines of the Piltdown hoax, causing much
exasperation to the museum’s palaeontologists, who had to spend
days fielding phone calls from journalists all over the world. He
coined the term Big Bang, in a moment of facetiousness, for a radio
broadcast in 1952. He pointed out that nothing in our understanding
of physics could account for why everything, gathered to a point,
would suddenly and dramatically begin to expand in the way Big Bang
theory assumes. Hoyle
favoured a steady-state theory in which the universe was constantly
expanding and continually creating new matter as it went. He also
realised that if stars imploded they would liberate huge amounts of
heat — 100m Celsius or more, enough to begin to generate the
heavier elements in a process known as nucleosynthesis. In 1957,
working with others, Hoyle showed how the heavier elements were
formed in supernova explosions. For this work, WA Fowler, one of his
collaborators, received a Nobel prize. Hoyle, shamefully, did not. According
to Hoyle’s theory, an exploding star would generate enough heat to
create all the new elements and spray them into the cosmos, where
they would form gaseous clouds — the interstellar medium as it is
known — that could eventually coalesce into new solar systems.
With the new theories it became possible at last to construct
plausible scenarios for how we got here. What we now think we know
is as follows. About
4.6 billion years ago a great swirl of gas and dust some 15 billion
miles across accumulated in space where we are now and began to
aggregate. Virtually all of it — 99.9% of the mass of the solar
system — went to make the Sun. Out of the float- ing material that
was left over, two microscopic grains floated close enough together
to be joined by electrostatic forces. This was the moment of
conception for our planet. All
over the inchoate solar system, the same was happening. Colliding
dust grains formed larger and larger clumps. Eventually the clumps
grew large enough to be called planetesimals. As these endlessly
bumped and collided they fractured or split or recombined in random
permutations, but in every encounter there was a winner, and some of
the winners grew big enough to dominate the orbit around which they
travelled. It
all happened remarkably quickly. To grow from a tiny cluster of
grains to a baby planet some hundreds of miles across is thought to
have taken only a few tens of thousands of years. In just 200m years,
possibly less, the Earth was essentially formed. At
this point, about 4.4 billion years ago, an object the size of Mars
crashed into Earth, blowing out enough material to form a companion
sphere, the Moon. Within weeks, it is thought, the material had
reassembled itself into a single clump, and within a year it had
formed into the spherical rock that companions us yet. When
Earth was only about a third of its eventual size, it was probably
already beginning to form an atmosphere, mostly of carbon dioxide,
nitrogen, methane and sulphur. Hardly the sort of stuff we would
associate with life, and yet from this noxious stew, life formed.
Carbon dioxide is a powerful greenhouse gas. This was a good thing
because the Sun was significantly dimmer back then. Had we not had
the benefit of a greenhouse effect, Earth might well have frozen
over permanently, and life might never have got a toehold. Somehow
life did. For the next 500m years the young Earth continued to be pelted relentlessly by comets, meteorites and other galactic debris, which brought water to fill the oceans and the components necessary for the successful formation of life. It was a singularly hostile environment, and yet somehow life got going. Some tiny bag of chemicals twitched and became animate. We were on our way.
from: A SHORT HISTORY OF NEARLY EVERYTHING by Bill
Bryson |