The Sun – lifecycle

3 03 2009

Over the course of a human lifetime, the Sun is an unchanging, constant companion – bringer of light and heat and champion of the day. It is often tempting to believe that it will always be here and always has been here. The truth, however, is quite different.

Birth
We have already seen and discussed nebulae, the different types (reflection, emission & planetary) and the sheer sizes of them. We have also discussed how these large nebulae (also known as Giant Molecular Clouds, or GMCs) contain the remnants of long-dead stars to feed the next generations of star birth.

Big Bang Nucleosynthesis
Within 3 minutes of the Big Bang, protons and electrons were flying around, bumping into each other, neutrons were still stable enough to exist freely and together, with the incredible temperatures and pressures still existing, nuclear reactions occurred. This process, during the infancy of the universe, could only give rise to very basic elements – hydrogen, helium, deuterium (hydrogen with two protons) and a trace amount of lithium. The universes‘ expansion cooled the temperatures quickly enough to freeze the reactions at this point.

The expansion of the universe carried these elements with it and spread them around evenly – until a force (which has been called Dark Matter) began to condense the diffuse gasses into distinct clouds.

These clouds would have been 85% hydrogen, 14% helium, with the remainder made of lithium and deuterium. Even though they were now distinct clouds, rather than diffuse gasses, it took between 380,000 and 1 billion years for the first stars to condense out of these clouds.

The Populations
These stars (known as Population III stars) were metal poor, giant stars with many times the size and mass of stars we see today. Such massive stars, whilst impressive, are destined to die young, as their immense size means that they burn up their nuclear fuel very quickly and explode violently, expelling their debris across the cosmos.

Population III stars first seeded the universe with heavier elements, oxygen, carbon, iron, silicon, etc, that was incorporated into the next generation of stars (Population II).

Population II stars, the “children” of Population III, were smaller, less massive and seeded with metals. These stars burned longer, but not as brightly, and would be destined to explode violently, seeding even more complex elements throughout the cosmos.

The heavier elements (or metals) would coalesce into large clouds, where the radiation from old stars and new stars would ionise and mix them together to form molecules, which we can spectroscopically identify today (we have found complex amino acids and long-chain carbon based molecules in these clouds – the very building blocks of life, ready to assemble).

Eventually, the debris from Population II stars would combine to form a new generation of stars (Population I), which were metal-rich, smaller and some would be accompanied by a flattened disc of material around them.

Starbursts
GMCs can be many hundreds of light years across, just as wide and as tall, containing enough material to build millions or billions of stars. The exact process is still mostly a mystery, as GMCs, being comprised of gas and dust, obscure our view of the processes that happen inside, although in recent years, infra-red, x-ray and ultra-violet astronomy has begun to shed some light on the process of star birth.

A dense region of gas and dust begins to collapse – perhaps due to a nearby supernova, a passing black hole, or other phenomena. Small clumps of gas and dust attract others, due to gravity and eventually; a small knot of material is formed that slowly rotates about itself due to conservation of angular momentum. This disturbs the cloud nearby, allowing more material to be pulled in. Eventually, over a period of millions of years, enough material is collected to allow this protostar to begin to radiate heat – not through nuclear processes, but because the core of the protostar is beginning to experience increased pressure from the infalling material. Gravity makes this material shrink further, increasing the spin (think of an ice-skater bringing their arms in) and adding to the attraction – an accretion disk forms, like the disk of material around black holes, slowly sucking material down into the protostar, slowing the spin (otherwise, it would be in danger of flying apart), and sucking in yet more gas and dust. Whilst this disc forms, jets of ionised material are flung out along the protostars poles – several of these jets have been observed around protostars. After around 100 million years, the disk and jets have cleared away the surrounding material in the GMC, stopping the star from getting any bigger.

Eventually, after around 100,000 years, the protostar is large enough for gravity to shrink it to a size and pressure where nuclear processes begin in its core. Hydrogen begins to fuse into helium and the star “ignites”, blowing away most the surrounding material, its jets fade away and the new star begins its long, slow climb onto the main sequence. These baby stars are not stable, however, and it is a long time before the star settles down – it is prone to violent outbursts of energy and grows and shrinks as its core settles into a steady rhythm of nuclear fusion.

If there is any leftover material it the accretion disk, it eventually clumps together to form planets.

How big?
How big does a protostar have to be in order to become a star? Is there a lower size limit? Is Jupiter a “failed star”?

According to nuclear fusion calculations, the smallest a star can be in order to sustain fusion in its core is 0.1 solar masses (an object approximately the size and mass of 100 Jupiters). Anything smaller than this is known as a Brown Dwarf – an object that generates heat and light due to gravitational collapse, but does not have enough mass to initiate fusion. These objects will slowly contract over billions (even trillions) of years until they reach a stable equilibrium between pressure and gravity and slowly cool down.

It has been estimated that Population III stars may have been as massive as 100 – 200 solar masses.

Death

Slow decline
Our Sun is approximately 4.5 billion (4,500,000,000) years old. It is around halfway through its lifecycle – steadily burning hydrogen into helium to provide heat and light. However, there will come a time, when it stops behaving so well and life on Earth will become a lot harder than it is today.

The sun is a delicate balancing act between gravity and pressure. Gravity wants to contract it ever smaller, whilst the pressure from inside wants to push it outwards. In this way the Sun regulates itself well – if the core heats up, it expands, cooling it down. If the core cools, gravity shrinks the Sun down, helping to generate more heat and stabilises the Sun.

However, this happy equilibrium comes at a price, every second, 4.26 million tonnes of hydrogen are converted into energy and helium. This will eventually take its toll, as the core becomes more helium than hydrogen, choking the core of fuel. In order to regulate itself, once the fuel available reaches a critical level, several drastic things begin to happen.

The core, filling slowly with helium, becomes denser, requiring greater temperatures, so the Sun begins to grow. This growth causes temperatures to rise – over the next 1.2 billion years, the sun will become 10% brighter and the surface will get 150°C hotter. This will have the unfortunate effect of heating the Earth to the point where even the poles will be as hot and moist as the tropics. Eventually, all the water will boil away into space, so Earth will be uninhabitable long before the sun dies.

Around 3 billion years from now, as the Suns fuel runs low, it will begin to swell, increasing in brightness and size slowly until it is over twice as bright as it is today and looks around 10 times its current size in the sky. It is at this point things go from bad to worse for Earth, as the now dry continents and seabed begin to melt under the searing heat. It has been estimated that Mars will be receiving as much solar energy then as Earth does now.

The End..?
By 7.7 billion (Sun age), the sun will have begun to swell again; it will engulf Mercury, and begin to threaten Venus and Earth. Luckily for us, this increase in size also leads to a lessening of the surface gravity, so it will lose a great deal of surface material, blowing it off into space in a strong solar wind, slowly forming a beautiful planetary nebula.

The sun will swell to almost 200 times its present size (becoming a red giant), engulfing Venus but, thanks to the loss of mass thanks to the increased solar wind, Earth will escape (barely) but shifting its orbit out to near that of Mars (and Mars will shift outwards and so on), it will not be a happy planet – no atmosphere, the surface melted and scarred with a noon temperature of 600°C.

Finally, the sun will run out of hydrogen in its core – the sun will shrink again, increasing the core temperature enough to burn the helium This brief hiatus in size will dim the Sun to around 100th of it’s brightness. Finally, after only a few million years, the helium burning will end; the sun, having loss a lot of its mass, will not have enough left over to go any further than carbon fusion. At this point, the nuclear processes will cease, but its outer atmosphere will expand outwards in a second red giant phase, with brief moments where some of the remaining hydrogen and helium will spontaneously ignite and burn. Finally, after 100 million years of this phase, the Sun will throw off these layers of atmosphere to form a planetary nebula and a tiny core, no larger than Earth, made of carbon, slowly cooling off over billions of years.

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