The Meerkat of Science: The Sun
The Sun is the single most important driving factor behind the processes that give life to us on earth. It gives us light, and provides us with heat, and the energy from the Sun also affects our weather. Ancient peoples used to worship the Sun as a deity, and given how important it is to us, that is understandable!
The Birth of our Star
The Sun first took shape 4.6 billion years ago, when a large cloud of gas and dust contracted under the influence of an outside force (which could have been a passing star, or a shockwave from a nearby supernova). This cloud collapsed, and compressed, and via processes far too complicated for this meerkat to coherently explain, the centre reached a point of pressure and heat where the fusion of hydrogen into helium could begin. At this moment the Sun reached the main sequence of its life, known as… the main sequence. As it did so, the Sun pushed away lighter elements from its birthing cloud, whilst heavier elements stayed closer to the star.
A Beacon of Stability
Since the Sun reached the main sequence, it has been virtually unchanged for four billion years. It is thought it will remain unchanged for around five billion more. This stability is critical to the survival of the solar system as a whole; a star inclined to belch out material in powerful outbursts would fry any nearby planets, and some stars are prone to such erratic behaviour. What makes the Sun so stable during this phase of its life is a balancing act between gravity, and the fusion reaction at its core. These two forces have reached an equilibrium, and as long as the Sun has hydrogen to burn, this delicate balance will remain, providing the solar system with a beacon of light and heat at its centre.
The trouble is, the Sun’s supply of hydrogen is finite. One day, there won’t be any more fuel to burn. At that point, the Sun, and the solar system, will become very different…
A Cycle of Destruction
Over the course of billions of years, the Sun will burn up its supply of hydrogen at its core, and the powerful nuclear reaction at its core will cease. At this stage, there will be nothing to counter gravity’s squeezing grip, and the Sun will compress. The temperatures at the core will increase, and the release of gravitational potential energy will cause the Sun to both expand, and become more luminous. At some point a shell of hydrogen around the core will undergo nuclear fusion, further adding to the Sun’s luminescence. The Sun will become a red giant, and it will ebb and flow, but ultimately expand to a radius greater than that of earth’s orbit.
In other words, the earth (along with Mercury and Venus) is doomed.
Whilst this process plays out, the Sun will build up a lot of degenerate helium at its core. Once the hydrogen fusion ceases, the Sun will once again contract, until this time conditions evolve to allow for helium fusion. The entire process of expansion will happen all over again once the helium fuel is exhausted, only this will switch between hydrogen and helium for a time. Thermal pulses (the result of increased instability) will see our star lose a lot of mass, and increase and decrease in radius.
The end result will be an exposed core, a dense body of hydrogen, helium, carbon and oxygen. As the Sun expels its outer layers, this core, a white dwarf, will be a lasting remnant, cooling over the course of trillions of years. This remnant will hold around half the Sun’s mass, in a body similar in size to the earth.
It seems sort of sad that one day, the beautiful, and vital beacon of light and warmth that fills our skies, and gives us gorgeous sunrises and sunsets, will be gone. You need not worry, for that day is several billion years away, so for us, here and now, we can appreciate what we have!
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