Wonders of the Universe - Brian Cox [96]
The Sun was formed 4.57 billion years ago from a collapsing cloud of hydrogen and helium and a sprinkling of heavier elements. For the tiniest fraction of this time, humans have marked the passing of the days as it rose and set, and surely considered it to be an eternal presence. It was only during the twentieth century that we discovered the Sun’s fires must one day dim
The computer-generated image shows how dramatically different the Sun will look in our heavens as it dies and dims.
THE DEMISE OF OUR UNIVERSE
At the moment the Sun is in the middle of its life, fusing hydrogen into helium at a rate of around 600 million tonnes every second. It will continue to do this for another five billion years; but eventually, perhaps fittingly given the grandeur and beauty it has nurtured in its empire, it won’t simply fade away. As the stores of hydrogen run dry, the Sun’s core will collapse and momentarily, as helium begins to fuse into oxygen and carbon, a last release of energy will cause its outer layers to expand. Imperceptibly at first, the extra heat of the Sun will extend towards us as its diameter increases by around 250 times. The fiery surface of our star will move beyond Mercury, towards Venus and onwards to our fragile world.
The effects on our planet will be as catastrophic as they are certain. Gradually, the Earth will become hotter. In the distant future, if any of our descendants still remain, someone will experience the last perfect day on Earth. As the surface of the Sun encroaches, our oceans will boil away, the molecules in our atmosphere will be agitated off into space, and the memory of life on Earth will fade into someone’s history; or perhaps no one’s history if we have steadfastly remained at home.
Long after life has disappeared, the Sun will fill the horizon; it may extend beyond Earth itself. This swollen stage in a star’s life is known as the Red Giant phase, marked by the final release of energy and the beginning of a long, long decline. In six billion years’ time, in a most beautiful display of light and colour, our sun will shed its outer layers into space to form a planetary nebula. We know this because we have seen this sequence of events unfold in the final breath of distant stars – on someone else’s sun? Written across the night sky in filamentary patches of colour are the echoes of our future.
If in the far future, somewhere in the Universe, astronomers on a world not yet formed gaze through a telescope at our planetary nebula and reflect on its beauty, they may glimpse at its heart a faintly glowing ember; all that remains of a star we once thought of as magnificent. She will be smaller than the size of Earth, less than a millionth of her current volume and a fraction of her brightness. Our sun will have become a white dwarf – the destiny of almost all the stars in our galaxy – a fading, dense remnant, momentarily masked by a colourful cloud.
If our planet survives, little more than a scorched and barren rock will remain, silhouetted darkly against the fading embers of a star.
Sirius, the brightest star in our sky, sits at just over eight light years away, which makes it one of our nearest neighbours. It is so bright that on occasion it can be observed during bright twilight, partly because of its proximity but also because it is twice as big as our sun and twenty-five times as bright. It is therefore not surprising that observations of Sirius have been recorded in the oldest of astronomical records.
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