Thirty million to 50 million years. That’s all the time it took
to form the star we call the Sun. This may sound like a long time, but let’s
put it in perspective. Since the last dinosaurs walked the planet, enough time
has passed for at least one and possibly two stars like the Sun to have formed,
one after the other – utterly from scratch. The details of this miraculous
creation are not exceptionally well understood, but astronomers at least have a
good grounding in the basics. Perhaps ironically, one star’s birth starts at
the other end of the line – when other stars die.
Generally speaking, stars make their exit in one of two ways. A
low-mass star like the Sun eventually expands its outermost layers until the
star becomes a gross, bloated caricature of itself: a red giant. Gradually, the
star’s envelope expands outwards, all the time becoming thinner, until the
dense core of the star is revealed. Such an object is known as a white dwarf.
It is a tiny and, at first, white-hot object with a stellar mass – yet confined
to live out the rest of its existence within the limits of a planet’s radius.
The rest of the star meanwhile, the cast-o¤ atmosphere, grows larger and
larger. Eventually it becomes nothing but a thin fog of gas spread over more
than a light-year. This is the fate that awaits our Sun, as we shall see in
detail in Part 4. By contrast, a heavier star dies much more spectacularly. It
blows itself to smithereens in a star-shattering explosion called a supernova.
The star’s gases are jettisoned into space where, again, they disperse.
Whichever way a star finally meets its doom, much of its material has the same
ultimate destiny: it is flung back into the galaxy. Over billions of years,
these stellar remains accumulate and assemble themselves into the enormous
clouds that astronomers refer to collectively as interstellar matter.
But that is not the end of the story. In fact, it is our
starting point. For the Universe is the ultimate recycling machine. Starting
around 4660 million years ago, from the ashes of dead stars, a new one
eventually grew: a star known as the Sun.
Time zero
Giant Molecular Cloud
Before 4660 million years ago, our Solar System existed as
little more than a cloud of raw materials. The Sun, the planets, trees, people,
the AIDS virus – all came from this single, rarefied cloud of gas and dust
particles. These patches of interstellar fog were as common billions of years
ago as they are now. They are known as giant molecular clouds.
Orbiting the nucleus of a galaxy called the Milky Way, about
twothirds of the way out from the centre, this ancient cloud from which the Solar
System sprang was about 50–100 light-years across, similar in size to its
modern cousins. And again, like today’s giant molecular clouds, it presumably contained
enough material to outweigh millions of stars like the Sun. Most of its mass,
about 73 per cent of it, was made up of molecular hydrogen, a gas in which the
hydrogen atoms are glued together in twos to make simple molecules. The rest of
the cloud’s material was in the form of helium, with traces of heavier elements
such as carbon, nitrogen and oxygen, and particles of silicate materials –
fragments that astronomers like to lump under the category of ‘dust’. With
between a few thousand and a million gas molecules per cubic centimetre, the
cloud would have been recognised as better than a first-class vacuum by today’s
standards. And it was very cold, around _250 Celsius, barely hotter than interstellar
space itself. Molecular hydrogen cannot survive at very much higher
temperatures, because the energy shakes the molecules apart. So the cold kept
the molecules intact. But the cloud was nevertheless in danger of destruction.
A molecular cloud is like an interstellar house of cards,
forever on the verge of disintegration. A push, a pull, anything could have
triggered this ancient cloud’s demise – and there are lots of potential
triggers spread over 100 light-years of interstellar space. The cloud might have
passed close to a massive star whose gravitational tug stirred up the molecules
within the nebula. Or the cloud could perhaps have drifted within close range
of a supernova explosion, the shockwaves from the dying star burrowing into the
cosmic smog and compressing its gases. It would have taken only one such event
to collapse the house of cards, to make the cloud fall in on itself under
gravity.
Something like this must have happened to our ancient molecular cloud
about 4660 million years ago. It was the first step in the process that would
eventually lead to the formation of a certain star.
2 000 000 years
Solar Globule
Once the collapse of the giant molecular cloud had started, it
continued under its own momentum. By the time two million years had passed, a multitude
of nuclei had developed in the cloud, regions where the density was higher than
average. These concentrations began to pull in more gas from their surroundings
by virtue of their stronger gravity, and the original cloud fragmented into
hundreds or even thousands of small, dense cores. Most of them would later form
stars. One of them was destined to become the Sun.
By now, the cloud core from which the Sun would form was perhaps
a tenth of a light-year across, more than a hundred times the present size of the
Solar System out to Pluto. Gradually, this tight clump of gas continued to fall
in on itself like a slow-motion demolished chimney stack, a process known as
gravitational freefall. The innermost regions fell the fastest; they were
closest to the central condensation where the gravitational pull was greatest.
The outermost edges of the cloud core took longer to succumb to their
inevitable fall. Thus, because of these differences in infall rates, the cloud’s
contraction essentially amounted to an implosion, an explosion in reverse. In
time, as the gas closest to the centre plunged inward and accelerated, the
material there grew steadily hotter, the atoms and molecules within it rubbing
against each other frantically. After perhaps millions of years in a deep
freeze, the molecular cloud was finally warming up. The
eventual result was a
gas and dust cocoon: a shell of dark material surrounding a denser, warmer
core. Such an object is known as a globule. It was the Sun’s incubator.
As with all globules, the solar globule was dark. It emitted no
light at all. But a bit later in its evolution, as it gradually warmed, it was
a strong emitter of heat radiation or infrared. Only an infrared telescope, and
possibly a radio telescope, would have been able to penetrate the gas and dust and
home in on the low-energy radiation coming from the globule’s gently warming
core, and see the first, feeble stirrings of the yellow star that the globule
would one day become.
2 030 000 years
Protosun
Over tens of thousands of years, the gases inside the globule
continued to fall away from the inside edge of the cocoon, pulled inexorably
towards that dense core at the centre. By now, the core of the globule was
taking on a definite shape – a gargantuan ball, about the size of the
present-day Solar System out to Pluto. Its surface was still too cold to glow
optically. But, at last, its central regions had warmed up significantly – to
about 10 000 Celsius – and the molecules there had split into atoms of
hydrogen.
This marked an important point in the development of the Sun. At
this temperature, the cloud core was now hot enough for the radiation it emitted
to carry a significant punch. Radiation is composed of tiny packets of energy
called photons, each of which can be likened to a subatomic particle. If there
are enough of these photons emitted every second they can hit like a hail of
bullets, a barrage of electromagnetic force known as radiation pressure. Before
this point the core of the globule had been emitting too few photons to exert a
noticeable force. Now, though, as the growing waves of radiation streamed away
from the warming core they slammed into the outermost regions of the globule
where the gases were less dense, and slightly hindered their inbound journey.
Thus the contraction of the core slowed, but it did not stop, so overwhelming
was the inward pull of gravity. The very centre of the core was also dense
enough now that it was beginning to become opaque to the heat radiation
generated inside it. The energy could no longer escape so easily, so from here
on the nucleus heated up much faster as it shrank. The build-up of heat thus
slowed the contraction ever more, and the core grew at a much slower pace. It
had reached a configuration that astronomers ennoble with the term ‘protostar’.
By this time, the protostar – ‘protosun’ in this case – had
developed a marked rotation. Just as water being sucked down a plug hole
spirals around before it falls in, so the gases that had fallen into the
protosun had begun to swirl about. And in the same way that a yo-yo spun around
on its string spins faster as the string winds around a finger – owing to a
concept known as the conservation of angular momentum – so the infalling gases had
increased their angular speed as their long journey inwards had progressed. As
the protosun grew smaller and hotter, therefore, it began to spin faster and
faster.
2 130 000 years
Solar Nebula
The protosun’s collapse continued. Within 100 000 years or so it
had become a swollen semi-spherical mass, flattened at the poles by rotation. Its
surface temperature of the order of a few thousand degrees, the protosun was at
last glowing visibly for the first time. And its diameter was by now roughly
equal to that of the present orbit of Mercury – about 100 million kilometres.
But the newly forming star was no longer alone. Over the aeons the rapid
rotation of the infalling matter had flattened out the gases like a pizza dough
spun in the air. Now, a huge pancake of turbulent, swirling gas and dust
surrounded the protosun right down to its surface. Thinner near the centre,
flared vertically at the edges, this structure is
known as the Solar
Nebula.
The Solar Nebula measured about 100 to 200 astronomical units
(AU) across, where 1 AU is defined as the current distance from the Earth to
the Sun, 150 million kilometres. The disc would have contained about 1–10 per
cent of the current mass of the Sun – most of it in the form of gas, with about
0.1 per cent of a solar mass locked up inside particles of dust. Near the
centre of the disc, close to the seething protosun, the temperature may have
exceeded 2000 Celsius. Here, where things were hot and important, the disc may
have been hot enough to emit its own visible radiation – in any case it would
have shined optically by virtue of the light it reflected from the protosun.
Further out in the disc the temperature dropped rapidly with distance, though,
and it would have shone only in the infrared. At about 5 AU, the current
location of the planet Jupiter, the temperature dipped below _70 Celsius. And on the
outside edges, where the material was more rarefied and the disc vertically
flared, it was even colder. This
vast reservoir of
material was the raw substance out of which the planets would soon begin to
condense, as will become evident in Part 2. It is thus known as a
protoplanetary disc, or proplyd for short.
By now, much of the original globule had been consumed. Most of
it had fallen into the protosun, and the rest into the disc. At last, with the globule
eaten away, the newly forming star was revealed to the exterior cosmos for the
first time, as it prepared itself for the next – and most violent – stage in
its formation: the T-Tauri phase.
3 million years
T-Tauri Phase
By 3 million years or thereabouts – about 1 million years after
the initial collapse of the globule – the protosun had shrunk to a few solar
radii. Its temperature at the centre was now around 5 million degrees Celsius,
while the surface seethed and bubbled at around 4500 Celsius. At last the
object had crossed the line that separates protostars from true stellar
objects. It joined the ranks as what astronomers call a T-Tauri star.
Named after a prototypical young stellar object in the
constellation Taurus, the T-Tauri phase is one of extreme fury. And as with all
T-Tauri stars, this earliest form of solar activity would have been driven – at
least in part – by a powerful magnetic field. Because the gases inside the
young star were by now fully ionised – a soup of positively and negatively
charged elements – their movement as the star rotated effectively amounted to a
series of gigantic electric currents. Thus the spinning star developed a global
magnetic field in the same way that a wire carrying an electric current does –
just as the Sun generates its field even today. During the Sun’s T-Tauri phase,
though, the star would have been spinning very quickly – once in 8 days
compared with once in 30 days – spun up by the swirling
gases that had
ploughed into it earlier. This means that the T-Tauri Sun’s magnetic field was
much mightier than at present, and this is what made this phase in the Sun’s
formation so violent. The Sun was still surrounded by its protoplanetary disc.
So, as the Sun whirled around, it dragged its magnetic field through this disc.
Where the field and disc connected, vast globs of gas were wrenched out of the
surface of the disc and sucked along the field lines, right into the young Sun.
And where these packets of gas hit, the troubled star responded with the
violent flares that are the hallmarks of the T-Tauri phase of star formation.
Thus the adolescent Sun was very much more violent than the star
we know today. It looked the part too. Its larger, cooler surface meant it glowed
an angry red, not a soft yellow. And the sunspots that dotted the solar surface
then were very much larger than their modern counterparts. Sunspots are
generated when the Sun’s rotation tangles its magnetic field and creates
localised regions of enhanced magnetic field strength. Where these
entanglements are greatest, the increased magnetism hinders the flow of gases
on the surface and cools those regions down – and they appear as dark patches.
Today, the Sun’s spots cover less than 1 per cent of its surface. But the
T-Tauri Sun would have had sunspot ‘continents’ covering great stretches of its
bloated face.
Perhaps the most awesome aspect of the T-Tauri phase, however,
was the molecular outflow. This would come next.
3 million years
Outflow and Post-T-Tauri
Phase
Almost as soon as the Sun hit the T-Tauri stage – possibly even
slightly before – it developed what astronomers call a stellar wind. The modern
Sun also has one: a sea of charged particles that streams away from the solar surface,
out into the depths of the Solar System. But T-Tauri winds are much more
furious and contain more mass, moving at speeds of up to 200 kilometres per
second.
How T-Tauri winds are generated is still very poorly understood.
A possible cause is again the rapid rotation. Some of the gas dragged out of the
Solar Nebula disc would have plummeted towards the star’s surface. But not all
of it. Because the Sun was by now spinning very quickly, some of the gas pulled
out of the disc plane was hurled radially outwards, much as water is spun out
of the wet clothes in a spin dryer. The result was a steady flow of gas away
from the star’s surface. However the Sun’s T-Tauri wind arose, its effects
would have been quite dramatic. As the wind blasted away from the young Sun’s
surface it banged into the disc and was deflected through a sharp angle out of
the disc’s orbital plane. The disc might have been threaded by magnetic field
lines as well, and these would have channelled the flowing gas further away
from the disc and ‘up’ and ‘down’ into space. The result was a ‘beam’ of
charged particles blustering away from the young Sun in two opposed directions,
perpendicular to the protoplanetary disc. Astronomers call this a bipolar
molecular outflow. By now the Sun had stopped amassing material and in fact
would lose a significant fraction of its original mass, throughout the life of
the wind, via the outflow.
By the time the wind had ceased, a fleeting 10 000 years since
it had started, the Sun’s mass had begun to stabilise. However, it continued to
shrink under gravity because the pressure at its core, though great, was not yet
adequate to stop the contraction. All the time the Sun was slowly contracting it
was also gradually approaching its modern temperature and luminosity. This was
by far the slowest period in the Sun’s formation. Even as the Sun’s violence
ended and it entered the relative calm of the post- T-Tauri phase, a couple of
million years after it had started, the Sun still had tens of millions of years
to go before reaching full maturity.
30–50 million years
The Main Sequence
At last, after a period of perhaps 30 to 50 million years –
astronomers still cannot agree on their numbers – the Sun’s contraction finally
came to an end. Why? Because the Sun’s internal temperature had reached an
all-time high of 15 000 000 Celsius – and something had begun to happen to its
supply of hydrogen.
Hydrogen is the simplest of all elements. Each atom contains
just a single subatomic particle called a proton in its nucleus, positively
charged. Orbiting this, meanwhile, is a single much smaller particle with
exactly the opposite electric charge: an electron. Inside the Sun, these atoms
are ionised: the electrons are detached and roam freely in the sea of hydrogen nuclei
or protons. Very often, two of these hydrogen nuclei come together. Just as two
magnetic poles of like polarity repel each other, so too do two protons. But
not if they are brought together with sufficient speed. The speed of particles
in a gas can be measured by the gas’s temperature. And at 15 000 000 Celsius,
the positively charged hydrogen nuclei at the Sun’s core were now moving so
quickly that when they smashed together they overcame their electrostatic
repulsion, and fused as stronger nuclear forces took over. At last, the
hydrogen was being consumed, gradually converted
into helium in the
Sun’s core via a chain of nuclear reactions. Energy is a by-product of these
reactions. And so the Sun now began to generate a significant amount of power
in its core. The pressure of this virgin radiation was so intense that for the
first time since the original gas cloud had started to contract, tens of
millions of years earlier, the force of gravity had finally met its match.
Exactly balanced against further contraction, and slowly metamorphosing
hydrogen into helium in its core, the Sun at last got its first taste of the
so-called main sequence. It had become a stable star, in a state that
astronomers call hydrostatic equilibrium.
This, the ignition of core hydrogen, was the point at which the
Sun as we know it was truly born. Called the main sequence, or hydrogen-burning
phase, this is by far the longest-lived stage in the life of a star. It took
tens of millions of years for the Sun to get to this point – yet so far it has
existed for about 100 times longer than that with little change. About 4600
million years later, it is not quite halfway through its main-sequence journey.
It still has a long life ahead.
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