Search This Blog

Friday, 30 July 2010

How was our solar system born?

Image above: The shockwave from supernova 1987a ploughs into the surrounding interstellar gas and dust, producing a spectactular lightshow. Such events can be sources of creation as well as destruction: Image courtesy of NASA.

Where did our solar system come from? It begins in a vast cloud of dust and gas called the pre-solar nebula. This cloud gives birth to many stars, and a  blue giant star has exploded in a supernova: The shock wave has broken the nebula into fragments dense enough to begin contracting under their own gravity.

One of these fragments is us - everything you experience day to day, except the stars in the night sky, and all of humanity, will eventually emerge from that shrinking fragment of cloud.

This fragment of the pre-solar nebula has internal currents and eddies (this link takes some reading through to get to the relavent bit, but its well worth it) - slow and cold by our terrestrial standards, but important for the next stage of its journey from cloud to star system. As the fragment contracts these eddies begin to change; as some cancel out and some reinforce each other the overall motion of the cloud averages out into a rotation in one direction. This spin is about to be one of the main forces shaping our solar systems formation.
When the nebula fragment is huge - 40,000,000,000,000, km or about the same distance to the suns nearest stellar neighbour proxima centauri- the overall rotation is barely perceptible, the temperature barely above absolute zero. As it contracts a little and begins to glow dully in the infrared the cloud is known as a Bok globule. It is denser and slightly less frigid, yet otherwise seems little different than the pre-solar nebula that existed before - but already its fate is sealed.

Image above: Vast Bok globules, believed to shelter nascent star systems, float against the background of the Orion Nebula. Image courtesy of NASA.

As it contracts it begins to spin faster. If you’ve ever spun yourself around on an office chair to make yourself dizzy you may have encountered the same effect; spin with your legs sticking straight out then bring them up to your chest; your spin gets faster. This is known as conservation of angular momentum.

At the same time the cloud is becoming much denser, and collisions between dust particles and gas particles are turning the momentum of the disk, both its rotation and collapse, into energy, pumping heat into the cloud. The dust is also feeling friction from the gas, which heats it in the same process as a space-shuttle re-entering earth atmosphere goes through.

This heat is slowing the contraction; the core of the cloud is now dense enough to be opaque to infra red radiation. The heat being generated inside it is given off as infra red photons, which are trapped within the layers of gas and exert an outwards pressure that slows the inwards crush of gravity, and growth of the dark central knot of matter. The cloud reaches a temporary, slowly decaying equilibrium; self-gravity balanced imperfectly against thermal energy, with gravity prevailing little by little. This comparative quiet may last for a million years as the cloud slowly contracts. Astronomers know this stage as a proto-stellar cloud.

But by the time the cloud has contracted to 200 astronomical units (1AU is the average distance between the sun and Earth, or 149 598 000 km) across it is whirling madly. Fast enough, in fact, that the cloud isn’t a cloud anymore; it’s a flat disk of spinning gas and dust, known as a proto-planetary disk, or propyld for short. As the temperature of the central regions reaches 1000 degrees Kelvin the dust there begins to vaporize. The dust was making the cloud there opaque to radiation, which was slowing the collapse, so without it the radiation dissipates and the central regions collapse faster. Ironically this makes them heat more quickly!

Image above: Four protoplanetary disks (propylds) snapped by the Hubble. The coloured background is caused by the glow from the surrounding  Orion Nebula. Image courtesy of NASA.
 Now in the centre of this cosmic maelstrom a knot of collapsing matter, still as wide across as the orbit of Jupiter, with a temperature of thousands of degrees kelvin, is gorging itself of the yolk of the Bok globule, and will consume around 98%of the original cloud fragments weight. This the proto-star phase, and it will endure for a hundred thousand years as the core temperature climbs steadily.

A collapse to glory:
The core temperature hits a catastrophic point : 10,000 degrees Kelvin. Events begin to unfold more rapidly: At this temperature the hydrogen gas converts to plasma: the hydrogen atoms themselves are being broken apart. The heat energy that was slowing the contraction is suddenly being absorbed by this process. As with the supernova described in the first chapter, with no thermal energy supporting the cloud it implodes: the contraction becomes a runaway collapse, incredible amounts of hydrogen and helium come crashing down, and once the last of the hydrogen is ionised to plasma the temperature and pressure can climb as much in a few years as it would otherwise have done in eons. At the end of the collapse the core temperature exceeds 100,000 degrees kelvin, and naked hydrogen and helium nuclei seethe through a sea of free floating electrons. In just a few years, a mosquito's heartbeat astronomically, the cloud collapses from nearly a thousand million km diameter to ‘just’ a hundred million kilometres.

If you were hanging in space above the disk, around this time, you would see a huge circular ‘hole’ in the background stars, ill defined around its edges, and with a hellish red-maroon glow growing  in its center. And if you were to fly across the radius of the maelstrom of gas and dust towards its edge you would find the edges flared, blocking the ember-red glow of the core as you crossed from one side of the disk to the other. Towering above the disk on both sides are beams of light. These are stellar, or Herbig-Haro jets. These colossal beams are fountains of matter, weighing as much as twenty planet Earths, that has been broken down into a plasma, and then spat violently away from the poles of the growing star. Where the plasma cuts into the surrounding gas it excites it, forming vast glowing shockwaves.

Above: Images of four Herbig - Haro objects. In several of these both the jets and the protoplanetary disks of the nascent star systems can be seen. Image courtesy of

That growing glow where the beams meet, at the center of the disk, that marks the spot where an immense power for both creation and destruction is about to give it’s first bellow. Our sun is beginning to shine:

The proto-sun has shrunk down to roughly the size of Mercury's orbit today. Within it the contraction- slower again now- is still turning gravitational potential energy into heat, and crushing pressure. As a result the ‘star’ has begun to shine, despite it’s core still being too cool to produce fusion energy. This is known as the T-Tauri phase, after the first of its kind to be discovered. The star is rarefied titan compared to the modern sun, it would fill the orbit of the planet mercury, but weighs the same and so is much less dense.

It is an immature, ill tempered, monster too; T-Tauri stars are large, fast spinning, and have immensely strong magnetic fields. The electrically conductive plasma, together with the stars fast rotation gives it a field that dwarfs even the immense magnetic power of the modern sun.
Gigantic sunspots blotch the surface, and herald the activity of immense flares that can increase the suns intensity tenfold for days at a time. The frigidly calm cathedral of gas and dust that was the pre-solar nebula has become an altar of unpredictable violence.
The magnetic field is what is feeding the growing sun now; the innermost material of the disk is carved off and funnelled down to the surface in great globules by the gargantuan field lines. But the T-Tauri sun has numbered its own days of growth: Immensely strong solar winds roar out from the surface, scorching and stripping the disk of lighter dust and gas, and chasing the remnants of the Bok globule into interstellar space. This wind is believed to be the cause of the jets; the disk of dust and gas around the star squeezes the wind out from the gaps over the poles.

Only one thing remains for our new sun: The T-Tauri sun is shining by heat from contraction- so to continue to shine it must still be contracting. But that will change. The core temperature is still rising, and when it eventually reaches ten million degrees Kelvin something magical will start to happen....

How do we know all this?

All the stages of stellar life are laid out in the night sky for us, but actually making sense of them is hampered by the timescale over which they live. For example, the smallest slowest burning red dwarfs have lifespans so long that it is impossible to tell how they die, as the universe hasn't been in existence long enough for one to pop its clogs!

The precise mechanisms behind the origins of stars like our sun are especially hard to investigate, as they hide themselves within the protective cocoon of the remaining bok globule material. This isn't just the cosmos inconveniencing us; the globule cocoon shields the growing proto-star from being disrupted by any more supernova explosions, or the intense winds from unstable nearby stars.
However there are some ways to probe inside the protective husks of gas and dust: infrared telescopes can percieve the embryonic stars, as most of the radiation emitted by them until the hydrogen ionisation catastrophy is in these wavelengths. Useing infrared spectroscopy we can begin to build up an idea of what these objects are made of. Radio and microwave telescopes can probe the gasses surrounding them. Using this data, and with the help of computer modelling, astronomers can then piece together the timeline of a stars birth and where the various objects seen in star forming regions fit into it. Observatories in space, like the Planck and Herschel space telescopes, are a great help in this, as they are above the layers of earths atmosphere, that absorb much of the radiation we are interested in. We can also learn from samples of pre-solar and early solar nebular bought back to earth by missions like stardust. But as with all science, what we 'know' is simply a theory that happens to fit the facts and evidence we currentl have. Tomorrow something could be found in the sky that turns all our ideas on their heads....

No comments:

Post a Comment