Wednesday, 25 January 2012

Genesis of the Sun and Solar Nebula

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

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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